Amplifier Gain Circuit schematic with explanation

Amplifier Gain

This is one the main characteristics to determine when designing or choosing an amplifier. This is a measure of the increase (or decrease in case of negative gain) the amplitude of the input signal.

Representation:

There are a few different ways to represent amplifier gain. One of the more common way among beginners and hobbyists, specially for DC or small signals, is to describe gain as the ratio of input vs output amplitude:

Gain = Vout/Vin, Where both input and output are either voltage or current (amperage).

Another way to represent amplifier gain is using a logarithmic decibel scale (dB). This representation is calculated using the ratio of input/output powers using the formula:

Gain = 10log(Pout/Pin)

Utility of gain:

There are countless applications and uses for amplifiers, since in the electronics world most signals we get from sensors or transmission lines is very small. There are also other times when not the amplitude of the signal is required but its power to transform into useful work, like when powering a motor, transmitting a radio signal and displaying an image on a screen.

How to calculate:

The specifics of how much gain can an amplifier have depend heavily on the components or circuits used, as well as the topology (configuration) of the amplifier. You can have a better understanding of the formulas used for each component and configuration by going to the specific page.

Opamp Configurations - Integrator Circuit schematic with explanation

Opamp Configurations - Integrator

If you replace the feedback resistor with a capacitor, you get an integrating amplifier.

In math, an integration operation is basically the area under a curve. If we have a voltage vs time graph, and the voltage remains constant, the integral of that will be the voltage times the time it stays at that level. As you can see, the longer the time the voltage remains constant, the higher the integral will be.

Back to our integrator, as the input voltage is applied to the inverting input via an input resistor that creates an input current. The Opamp will try to compensate the current by creating a voltage across the feedback element enough to make a current flow equal to that at the input to conform to the current rule: the inputs draw virtually no current.

In the simple inverting amplifier, the feedback resistor developed a constant current at a constant voltage at the output with respect to the inverting input, tied to ground. This time however, the feedback element is a capacitor; an element that can store charge, charge that eventually develops a voltage across it as it gets more and more charged.

If we apply a constant voltage at the input, a current flows through the input resistor. This current the opamp tries to compensate by creating a voltage at the capacitor to induce a current equal to that of the input. If the capacitor is initially completely discharged, the voltage across it is 0v, and its "resistance" is infinite since it is effectively insulating both sides so no current flows.

The gain is initially infinite, since Rfb/Rin tends to infinity by action of Rfb being infinity. This makes the output voltage go down quickly in a small amount of time (remember that the opamp is acting in an inverting configuration). As the capacitor starts charging, the charges entering the out plate of the capacitor push the charges on the other side, effectively creating a current across the capacitor, enough to counteract the input current.

As the charges build up inside the capacitor, a voltage develops across it in opposition of the output voltage, making it seem as if less voltage is applied to it, slowing down the amount of charges getting into the capacitor.

Less new charges going into the capacitor causes less charges being pushed out at the other plate. The Opamp tries to compensate by further lowering the voltage.

As you can see, the charges keep building up and the opamp is always trying to compensate by lowering the output voltage. At one point, the opamp will not be able to lower the output voltage, at which point it is said to be saturated.

The rate of charge of the capacitor depends on the current that is applied to it, and the current depends on the voltage and resistor at the input by ohms law I = V/R. The higher the voltage, the faster the capacitor charges and the output going lower, and the lower the input resistor the more current flows, charging the capacitor faster and resulting in the same faster lower output.

This action is the same as in the integration operation: the higher the value of the graph the higher the integral will be in the same amount of time.

Also if the input goes negative, the capacitor starts discharging and the output will go higher to compensate. If at any point the input goes to 0, the current through the input resistor will be zero, and the opamp will compensate by setting the output voltage at the same level as the capacitor voltage, in order to stop it from being charged or discharged.

Similar to what happens in an integration: if the graph crosses 0 and stays there, the integral will be the sum of areas up until that point and stay there for as long as the graph stays at zero. Also, if the graph goes lower than 0 then the integral will go lower because the area will be negative relative to 0.

Opamp Configurations - Differentiator Circuit schematic with explanation

Opamp Configurations - Differentiator

The inverse function to integration is differentiation, in other words finding the derivative, which the opamp can also perform. The derivative is defined as the rate at which the function changes.

By using an input capacitor instead of a resistor, we can accomplish the same thing. If you remember, a capacitor stores charges in its plates, when one of them starts accumulating charges, the same charges will be pushed out from the other plate, as if current was flowing through the capacitor despite the intrinsic insulating layer.

The capacitor's charges start building up and creating a voltage across itself in opposition to the charging voltage, thus slowing down the incoming charges, slowing down the charging process in general. When enough charges have accumulated, the charges inside the capacitor completely push away the charges coming from the source, no more charges enter the capacitor, and because of this no more charges are pushed out on the other side of the capacitor, so no more apparent flow of current across the capacitor.

When used as input for a signal, if the signal does not change (like a DC input), the capacitor will have an initial apparent current through it as the voltage across it builds up due to incoming charges, and since the input of the amplifier tries to not draw any current, it will create a voltage at its output so that the current through the feedback resistor is the same as the apparent current through the capacitor.

Since the capacitor charges very quickly due to the voltage applied to it and the fact that there's no current limiting component like a resistor, the apparent current through the capacitor falls very quickly as the voltage across it in opposition rises as quickly; the falling current is also causes the opamp to drive the output voltage less, since there's less current to compensate for.

Applying a DC input to the differentiator thus creates a spike in input as well as in output as the capacitor's initial charge is developed, and then goes back to 0v as there's no more apparent current to compensate for; Similar to the operation of finding a constant's derivative, which is always 0.

The fact that there's an initial spike can be mathematically modeled as a period in which there's a function that rises at a very high rate (which actually happens, the voltage doesn't just jump from 0v to the DC input voltage, it rises very rapidly towards it), so its rate of change is very high for a brief period of time; hence the spike.

As the input voltage stabilizes, its rate of change slows down very rapidly as well, going towards zero when fully stabilized; this is reflected in the opamp's output by the fact that as the voltage stabilizes, the output spike goes down very rapidly towards zero and stays there.

Now instead of applying a constant input, you can replace it with a constantly changing input.

If the input is increasing at a constant rate, there will be a constant apparent flow of current through the capacitor, since the voltage buildup across the capacitor is compensated by the increase in input signal. Since there's a constant apparent flow of current through the capacitor, the opamp compensated by setting the output voltage at a level that will make the feedback resistor draw the same amount of current, so that the opamp input does not draw it.

Since the amount of apparent current is constant, a constant output voltage is enough to keep the feedback resistor drawing the current, and the opamp keeps a constant output at the output.

This mode is very similar to using a resistor with constant dc as the input.

The same is true for a constantly decreasing input voltage; the output will just be of reversed polarity. To compare with the mathematical definition of the derivative of a linear variable, the derivative will be a constant.

This can be expanded to other functions, one of the most widely used being the sine function. Since the mathematical derivative of the sin(x) function is cos(x), which is a shifted version of sin(x) by 90 degrees, when you input a sine input at the differentiator amplifier, the output will be the same function shifted 90 degrees, in essence, a cosine function.

Amplifiers Intronduction Explanation Circuit schematic with explanation

Amplifiers Intronduction Explanation

Amplifiers increase either the amplitude (voltage) or power (Amperage/Current)
applied to its input.

Components of an amplifier:

Gain component: The main component of the amplifier, defines many of its characteristics like noise, bandwidth, gain, input and output impedance, and others.

Bias: Some types of components need a bias point in order to operate correctly. The bias point is a dc voltage applied to the input of the amplifier. There are many ways to set the bias point,
depending on the gain component used.

Accessories: These are many kinds of sub-circuits used to fine tune the operation of the amplifier, including preamplifiers, buffers, stabilizers, filters, limiters, etc..

Stages of Amplifiers:

Input: This stage consists of a signal from another subsystem outside the amplifier, or a sensor like a microphone, photodiode or any other component that delivers a small signal. Depending on
the intended purpose and input signal, this stage may contain a preamplifier, which is a signal (voltage) amplification before the main power (current) amplification stage, and a filter to
limit incoming frequencies.

Amplification: Main stage of any amplifier, most of the times it is a power amplification process, sometimes with signal amplification as well. This stage is where the gain component and many of the accessories like stabilizers and limiters are located.

Output: Last stage, sometimes consists of a buffer and/or filter to remove any noise generated in the main amplification stage. The buffer sometimes added to deliver more current (lower output impedance).


Block Diagram of a Amplifiers
(Click to enlarge)

Description of Amplifier accessories:

Coupling: This is usually done with a capacitor. The purpose of the coupling capacitor is to prevent any DC voltage from modifying the bias point of the amplifier, to prevent clipping (driving the signal to the max voltage, distorting it) from a high or low bias point.

Another coupling method is using transformers. This is done on lower frequency signals where the reactance (resistance-like behavior when a component is applied an AC voltage) of capacitors is so high to the point the signal is practically lost.

A third choice is using tuned transformers, by using a capacitor in parallel with the transformer. This creates a tuned circuit that has a very narrow bandwidth, useful in some special interest amplifiers.

Filters: This topic is so extensive it deserves its own article. Amplifiers have uses for filters to limit noise and reject unwanted signals from its input. Combining a filter and an amplifier creates an active filter (filter that has gain).

Most filters use RC networks to create the filter, although RL or RLC are also used in some designs.

Stabilizers: This is usually some kind of feedback used to prevent clipping or other circuitry to keep the frequency within a certain range (stop frequency drifting).

Limiters: Sometimes only voltages up to a certain point are needed or desired, here limiters come into use. They limit or sometimes clip a signal if it goes above a certain voltage, other kind of limiters use feedback to control the gain of the amplifier so as to keep the output signal within the specified voltage range.

Buffers: Also called voltage followers, this is just another name for another stage of amplification with a gain of 1. This is to provide more current and avoid overloading the main amplifier, as doing so can reduce either the gain or bandwidth.

If you need a specific implementation of an amplifier circuit, you may want to consider learning all the abstract theory first and then moving on to the components page, where all component-specific circuits and modes of operation are listed.

Opamp Configurations - Comparator circuit Circuit schematic with explanation

One of the main reasons for using opamps as active devices in circuits is that their internal gain is so high, that even if we reduce it to a tiny fraction, it will still be enough for practical purposes. This particular configuration depends on the very high gain of the opamp to swing the output to one of the extremes; the sign of which tells us which input is more positive than the other.

By connecting the non inverting input to a voltage source, we are setting the reference point of the comparator. Remember that since there's no feedback, and because internally the opamp is just a very high gain difference amplifier, the output will be the non inverting input voltage minus the inverting input voltage, multiplied by the internal gain (in the 100k's).

This means that a difference of just millivolts will drive the output into saturation; if the difference is positive it will swing to full positive, limited by the supply. If the difference is negative, it will swing to full negative, again limited only by the supply.

On most amplifier circuits it is not advisable to drive the opamp into saturation because it clips the signal from going any further on both ends, but in this case we are not so much interested in the signal itself but on the relationship between the signal and a reference, so this circuit serves its purpose.

Amplifier Gain Circuit schematic with explanation

Amplifier Gain

This is one the main characteristics to determine when designing or choosing an amplifier. This is a measure of the increase (or decrease in case of negative gain) the amplitude of the input signal.

Representation:

There are a few different ways to represent amplifier gain. One of the more common way among beginners and hobbyists, specially for DC or small signals, is to describe gain as the ratio of input vs output amplitude:

Gain = Vout/Vin, Where both input and output are either voltage or current (amperage).

Another way to represent amplifier gain is using a logarithmic decibel scale (dB). This representation is calculated using the ratio of input/output powers using the formula:

Gain = 10log(Pout/Pin)

Utility of gain:

There are countless applications and uses for amplifiers, since in the electronics world most signals we get from sensors or transmission lines is very small. There are also other times when not the amplitude of the signal is required but its power to transform into useful work, like when powering a motor, transmitting a radio signal and displaying an image on a screen.

How to calculate:

The specifics of how much gain can an amplifier have depend heavily on the components or circuits used, as well as the topology (configuration) of the amplifier. You can have a better understanding of the formulas used for each component and configuration by going to the specific page.

The Operational Amplifier OpAmp

The Operational Amplifier OpAmp

The operational amplifier is perhaps the most versatile of amplifier circuits, used many different applications as a gain component due to high stability, gain and input impedance, as well as the fact that very little external components are needed for operation.

Internally, the OpAmp is based around a transistorized differential amplifier; two transistors connected to the same emitter resistor, where one of the inputs is inverted and added to the other to essentially subtract one from the other, the difference amplified by a certain factor and fed as the output.

The basic opamp is a simple differential amplifier. Most commercially available opamps have extra internal circuitry to compensate for temperature change, different voltage source values and compensation to get an exact 0v when both inputs are disconnected.

There are two characteristics that make opamps so versatile: The voltage difference across its inputs will be very close to 0v, and its inputs draw virtually no current. This characteristics are only valid only under Negative Feedback.

Opamp Configurations - Window comparator

Opamp Configurations - Window comparator

The simple comparator circuit has one inherent problem: it can only tell us if one of the input voltages is higher than the other.

But what if you needed a circuit that tells us if a signal is within a range of values? you would need a circuit that tells you if the signal is higher than a minimum and if it also is lower than the maximum. The problem itself hints at the solution.

For a window comparator, we need one simple comparator set up just like the previous circuit: use the non inverting as reference and the inverting input as the signal entry. This comparator will set the maximum; if the signal goes higher than the reference the output will go negative, signaling an out of range (if we consider positive to be in range).

Another comparator is set by switching the reference and signal inputs, connecting the reference to the inverting input and the signal to the non inverting. If the signal is lower than the reference, the output will go negative, again indicating an out of range; this comparator sets the minimum.

When both opamp outputs go positive, it means that the signal is below the maximum and above the minimum, in other words, the signal is within the window of voltages you have defined.

There's one thing to consider with this configuration, when the signal is out of range, one of the opamps will go full negative (virtual connection to negative supply) and the other will be full positive (virtual connection to positive supply). This causes a short circuit condition that needs to be avoided as it could cause damage to the circuit or the supplies.

One way to protect from this condition is use diodes configured as the logic AND gate. This simply means to connect two diodes at the opamp outputs, connect both their anodes together and to the positive supply via a high value resistor.

What this does is that only when both opamps are at full positive (diodes' conduction blocked, basically disconnecting the opamps from the rest of the circuit) will the output be positive, held by the high value resistor.

When either opamp goes negative, the diode connected to it will be forward biased, basically connecting the output to ground; the other opamp is blocked from connecting to the output by the reverse biased diode (positive opamp output connected to cathode, negative to anode) and no short circuit condition occurs.

Transistor

A transistor is a semiconductor active device that amplifies or switches and electrical signal. It is used in amplifiers, digital electronics and buffer circuits.

Theory checklist:

• Gain
• Bias point
• Saturation and cutoff

Modes of operation

• Common base, emitter and collector configurations
• Logic gates

Simple FM Transmitter Circuit Schematic With explanation

Simple FM Transmitter Circuit Schematic With explanation

This circuit uses a small microphone to capture the sound and some transistors to generate radio waves that can be picked up by a FM receiver like a car stereo.How it works:
From left to right, the first part is the microphone and some resistors to get it working. Next we have a capacitor and the first transistor, this amplifies the sound from the microphone so that it can be loud enough to work with. The last part, there is a transistor, a coil and some capacitors. This part generates the radio waves and combines them with the sound from the mic to transmit it thru the antenna.
The coil is made with about 9 turns of wire, use a pencil to get the right diameter for the coil. The capacitor with the arrow is called a trimmer capacitor, it has a small screw to adjust the value, we'll use it to tune a certain frequency or station to transmit on.

Simple FM Transmitter Circuit Schematic With explanation

(click to enlarge)

Opamp Configurations - The non inverting amplifier

Opamp Configurations - The non inverting amplifier

For a non inverting action, a simple way to obtain it is to keep the feedback loop in place and connecting the terminal where the input used to be connected, to ground, while feeding the input signal to the non inverting input.

This makes the opamp create an output voltage so that the current flowing through the feedback resistor network will be the necessary to develop a voltage at the inverting input that is the same as the non inverting input.

Since we know that the inputs draw virtually no current, then the voltage at the inverting terminal will be defined by the voltage divider created with by the feedback network.
    Vinv = VoutR2 / (R1 + R2)
Since Vinv, the inverting input, is at the same potential as the non inverting input, then
    Vin = VoutR2/(R1+R2)
The gain is the ratio of output voltage to input voltage
    gain = Vout/Vin
A rewrite of the Vin equation gives you
    Vin/Vout = R2/(R1+R2)
This las equation is the inverse of what we need, so lets get it straight
    Vin = Vout R2/(R1+R2)
    Vin (R1+R2) = Vout R2
    (R1+R2) = R2 (Vout/Vin)
    (R1+R2)/R2 = Vout/Vin
That's an equation for gain, which can be further simplified by separating the terms
    (R1/R2) + (R2/R2) = Vout/Vin
    (R1/R2) + 1 = Vout/Vin
As you can see, the gain is similar to the inverting amplifier, set by the ratio of the feedback resistors. In this case however, the gain will always be higher than 1. You can think of it as if the amplifier is adding the amplified signal to the non inverting reference voltage, which in fact is the same as the inverting, just that in this case the reference is not ground (0v).

Grouping Flip flops: Registers

Any number of flip flops can be grouped together that share the same clock signal and work as a single unit. Common numbers of flip flops grouped together in a register are 4, 8, 16, and 32 (corresponding to 2^2, 2^3, 2^4 and 2^5).

A register functions as a more complete unit of memory within a circuit, grouping together data with a similar meaning in most cases, or just a more compact way of storing a bunch of bits. Most registers are made out of D flip flops due to the lower pin count needed for signals (JK would need much larger IC's because of the need for more control pins space).

There are four main kinds of registers, categorized by the way in which data is put in and taken out.

Parallel In, Parallel Out Registers

This kind is the simplest of registers. A parallel in/ parallel out is just a collection of flip flops that share a common clock signal but have independent data signals and outputs.

Their main application is storing data or state information (represented in binary digits) for use in later steps in sequential circuits.

Serial In, Parallel out Registers

Serial in/ parallel out registers get their input from a single data line. The output of the flip flop is used as one of the outputs, as well as connecting it to the data line of the next flip flop. What this accomplishes is that for every clock signal, the bits stored move one place and a new bit is captured at the first flip flop; the last flip flop is used just as another output, so its data is rewritten after every clock pulse.

This type of registers are used as buffers in digital data lines, where data is sent using only one wire but each bit is needed separately for further use.

Parallel in, Serial Out Register

Parallel in/ serial out registers have special control circuitry (sometimes a simple multiplexor suffices) that can select whether to use an external set of bits or the previous flip flop's output as input.

This is so that there's a possibility to get an external set of bits all at one (in parallel), and send them along one by one. Since each clock pulse the data moves to the next flip flop, the last can be used as the register's serial output, sending the data it stores one bit at a time.

In contrast to the receiving and "semimultiplexing" action of the serial in/ parallel out, the parallel in/ serial out combines number of data lines into a single one, most likely for transmitting over a digital line.

Serial in, Serial out Register

Kind of a special purpose register. This is always wired so that the first flip flop gets external data, and all internal flip flop's outputs are connected to the input of the next, the last one being used as output.

This register's main purpose is to delay the transmission of data in a digital data line.

Diode explanation

A diode is a semiconductor device that allow current to flow in only one direction. Depending on the type of diode, it has uses as voltage rectifier, indicator and regulator.

Theory checklist:

• The P-N junction
• Forward and reverse bias

Modes of operation:

• Rectifier
• Diode as reference voltage
• The zener voltage regulator
• The led diode indicator
• The varicap variable capacitor
• The photodiode light sensor

Resistor Explanation and kinds of resistors

Resistor Explanation and kinds of resistors

Resistors are electronic and electric components that oppose the flow of current in a circuit. They are made from relatively poor conductors but that don't stop current from flowing altogether.

There are many kinds of resistor constructions, each suited for many purposes that overlap.

The simplest form is a cylinder of carbon material with two connection leads attached at both sides. The diameter and length of the cylinder, as well as the carbon composition of the filling determine the resistance. In general, a longer cylinder has more resistance than a shorter one, and a thicker one will have less resistance than a thin cylinder.

The apparent counter intuitive nature of a thick resistor having less resistance lies in how current flows in a circuit: it will always look for an easier path, and with a thick resistor with more overlapping paths, current has a higher chance of finding an easier path than in a limited and crowded thin resistor.

Another construction method is to coat a ceramic core with a resistor material and shape it in the form of a spiral by removing some of the material along the edge of the spiral. Since this method effectively increases or decreases the length of the resistor material, resistance can be carefully selected and determined.

High power resistances use that same method but instead of resistor material covering a core, resistive wire is used to allow for better heat handling.

Resistors have a standard color code that reflects the value of the resistance of the component. It consists of four color bands, the first two represent numbers and the third represents the number of zeros to add at the end of such number (more on the color code).

Series and parallel resistors

Series when only two components, in this case resistors, share only one of their connections; It could also be described as connecting one resistor after the other forming a chain.

From the construction characteristics of resistors, we can see that when we connect resistors in series, we are effectively creating a single, longer resistor, so what happens with the total resistance?

Simple, they are added together.

For example, we have a square tube we will fill with water. If we wanted to know the volume, we multiply base times height of the water in it to get the volume of water we put in. We measure separately the volume of a one by one cube of water and another of one by two, and get 1 and 2 respectively. We then fill tube with both, how much volume is the water in the tube?

We only put in 3 units of volume, and if we know that none leaked out of the tube, there can be no less than 3 units. So in effect the volumes add together.

Now the volume can be thought as the resistance, put two resistors into a single line and their resistances add up. No math involved, although there's a math proof of this derived from ohm's law.

Parallel is when two or more components share both of their connections together.

What happens with the resistance in parallel circuits? It happens something similar as having a thicker resistor, but not for the same reasons.

Imagine a circuit with one voltage source and two resistors in parallel, both resistors draw current from the source. From the point of view of the source, providing more current to the circuit is the same as providing current to a lower valued resistance, following I = V/R. To know exactly how much resistance the source 'sees' we have to do some math.

It = V/R1 + V/R2 : where It is the total current supplied by the source, R1 and R2 the respective resistances.
V/Rt = V/R1 + V/R2 : We replace It with V/Rt, since we want to know the total resistance the source 'sees'
1/Rt = 1/R1 + 1/R2 : Divide both sides by V

From this last formula we see that the inverse of the resistance is what's added thogether. The formula can be further worked to result in a simple, easy to remember formula.

1/Rt = (R1+R2)/(R1*R2)
Rt = (R1*R2)/(R1+R2)

Note that this only works for two resistors.

H bridge circuit Schematic With explaation

H bridge circuit Schematic With explaation

An H bridge is a kind of circuit you use to control the direction (and sometimes speed) of an electric motor, using only a single polarity voltage (you need to reverse the way current flows in order to reverse the way the motor rolls).

How it works:

You have 4 transistors, wired as ON OFF switches. Two signal lines allow you to run the motor in one direction, when reversed, the motor runs in the other direction. It's very straightforward to use and build, but be careful to use only small motors, as the currents drawn from the bigger types can burn your components.

There are 3 modes of operation:
Both equal ( on or off ): motor doesn't run, as it's shorted or not connected
S1 on, S2 off: motor runs in reverse ( from negative [blue] to positive [red] )
S1 off, S2 on: motor runs normal.

Also note, unless you use power transistors, you need to connect diodes across the transistors in order to protect from overvoltages.

Simplified schematic:

Tone generator circuit Schematic With Explanation

Tone generator circuit Schematic With Explanation

Simple, low component count tone generator. It can be adapted to create a morse code circuit, by adding a switch to the output.

How it works:

This circuit is based around the 555 timer circuit, used as an astable (free running) oscillator. The frequency (pitch) of the tone is set by the resistors and capacitors in the left side of the circuit. The first one is a potentiometer (variable resistor), this is our pitch control, which is basically all the external components you need. The capacitor to the far left is to reduce as much noise or undesired operation of the potentiometer, getting a smooth pitch change when adjusting.

You can find the timer's datasheet by following the link: 555 timer

Led Chaser

This Led chaser is built using some very common digital logic circuits. Easy to build, easy to work with, and looks amazing in the dark. This circuit will light each led in sequence, creating a moving light illusion.

How it works:

From left to right, the first IC is a binary counter, 74ls163, that is used to generate the address numbers we are going to use in the second part of the circuit, the demultiplexer 74ls138. This demux is the core of the circuit, as this IC pulls low the pin selected in the address inputs. Note that you need an external clock for the counter to work (count).

This is a very simple circuit to build, just take note of the control pins on each IC, since both have enable inputs for counting or output, but once set you don't need to worry about them. It can also be extended to 16 leds by using the Q3 output of the counter to control the enable inputs of a second demultiplexer, or just add a second one to generate a double pattern.

You can find the datasheets for both IC's by following the links: 74ls138 | | 74ls163

Capacitor

Capacitor

Storing charge - the capacitor

To understand how another component in electronic circuits works, imagine the following:

Imagine that we have two 'boxes' to put charged particles separated by a piece of plastic. We fill the top one with positive charges and the bottom one with negative charges, only the positive charges are allowed to move. We know that they will try to come together because of the forces generated by opposite charges, but since they cannot get out of the box, the are just stored there not doing any work.

Now imagine that we connect the two boxes with a pipe through where the charges can move. The positive charges will move to meet with the negatives and be in equilibrium. Now that they are moving, there's work and energy being expended that can be put to use.

The device that accomplishes this is called a capacitor. Basically they are two conductor plates separated by an insulator layer, in effect creating the two boxes mentioned above.

When we connect a voltage source to a capacitor, the capacitor is 'empty', with no charges, then charges from the source will start filling it up. As more and more charges reach the capacitor, they will start exerting a force on the charges trying to come in from the source, so it will start filling slower and slower.

Once the capacitor is filled, no more charges flow from the source to the capacitor. If the voltage source is removed, the charges the capacitor has remain there, waiting for something to allow them to meet with opposite charges to reach equilibrium.

If we connect a resistor across, the potential difference created by the separated charges in the capacitor allow it to function as a voltage source, so these charges start flowing through the resistor. As more charges flow, the capacitor starts emptying, causing less potential difference over time, until it can no longer provide charges and the current flow stops.

One way to picture the charging and discharging of a capacitor is to think of a balloon with two mouths, one connected to an air pump and the other left open simulating the resistor through which charges escape. The pump will inflate the balloon to a certain pressure that will be kept constant by the air that escapes the balloon through the other mouth.

If a sudden increase in pressure from the pump occurs, the balloon will inflate more but the air coming out will remain at about the same level, increasing until the air that comes in is the same air that comes out. If the increase in pressure is short, the balloon will inflate and deflate quickly, and the air coming out would remain almost the same throughout.

Same happens with capacitors, when a sudden spike in voltage occurs, the capacitor stores the charges and the voltage in it rises slowly, outputting about the same current throughout the process. This property gives capacitors most of its uses with direct current circuits.

Opamp Configurations - Difference Amplifier

Opamp Configurations - Difference Amplifier

Difference Amplifier

So far you've learned about how to make an opamp add an inverted (negative) voltage to a reference, and to add a positive voltage by setting the reference.

Since the opamp has two inputs, one inverting and one non inverting, it should be possible to use both at the same time to add them to one another, and since one will be inverted, the effect will be a difference of voltages.

This one is a bit trickier to derive equations for, since, as you already know, the voltage that will be applied to the non inverting input will also appear at the inverting input via the opamp trying to compensate.

Since we are using resistor ratios in the voltage divider to set the voltage at the non inverting input, the voltage at the inverting one will be in terms of those resistors as well, otherwise the equations are derived the same as for the inverting amplifier.

Lets start with the inverting amplifier equations
    Vrin = Vin - Vinv
    IinRin = Vin - Vinv => Iin = (Vin - Vinv)/Rin
Same as last time, except Vinv is non zero, set by the voltage divider. Applying the current rule:
    Iin = Ifb, Ifb is the feedback current.
    Ifb = (Vinv - Vout)/Rfb
Vinv is not tied to ground, so it can't be simplified more at this point. We also have
    Iin = Ifb => (Vin - Vinv)/Rin = (Vinv - Vout)/Rfb
Expressed in terms of Vout, this becomes
    (Vin - Vinv) (Rfb/Rin) = Vinv - Vout
    (Vin - Vinv) (Rfb/Rin) - Vinv = - Vout
Multiply both sides by -1
    (-1)[(Vin - Vinv) (Rfb/Rin) - Vinv] = (-1)(- Vout)
    - (Vin - Vinv) (Rfb/Rin) + Vinv = Vout
    Vinv - (Vin - Vinv) (Rfb/Rin) = Vout
Now, since Vinv is in terms of the non inverting voltage, we have
    Vninv = Vin2 R2 / (R1+R2)
And
    Vinv  = Vninv => Vinv = Vin2 R2 / (R1+R2)
So we can rewrite our Vout equation now in terms of both input voltages
    Vinv - (Vin - Vinv) (Rfb/Rin) = Vout
    [Vin2 R2 / (R1+R2)] - (Vin - [Vin2 R2 / (R1+R2)]) [Rfb/Rin] = Vout
This seems complicated enough as it is, so from here we are going to simplify by making some assumptions. Lets make all resistors equal.
    R = R1 = R2 = Rfb = Rin
The equation then becomes
    [Vin2 R/2R] - (Vin - [Vin2 R/2R] [R/R] = Vout
    Vin2 (1/2) - (Vin - [Vin2 (1/2)] [ 1/1 ] = Vout
    Vin2 (1/2) - (Vin - [Vin2 (1/2)] = Vout
    Vin2 (1/2) - (Vin - [Vin2 (1/2)] = Vout
    Vin2 (1/2) - Vin + Vin2 (1/2) = Vout
    Vin2 - Vin = Vout
As you can see, with our assumption of equal resistors, the output will be equal to the difference of voltages applied, the applied at the non inverting minus the one applied at the inverting. In practice if you use the same ratios of resistors, the relation holds. You could also use equal ratios (not precisely 1:1) to set the gain; if you use different ratios you will get a weighted difference.

Simple power supply Circuit Schematic

Simple power supply Circuit Schematic

This circuit is very useful in beginners circuits, since most will work on 5v, which is the voltage of this easy little circuit.

How it works:

This circuit is just an implementation of the 7805 integrated voltage regulator. What this little component does is to lower a voltage and stabilize it by reducing noise and ripple, in order for circuits to have the constant voltage needed to work correctly

Simple power supply Circuit Schematic

You can find the datasheet by following the link: 7805, take note of the typical applications notes, you can find some more uses for this versatile little regulator

Tone generator circuit Schematic With Explanation

Tone generator circuit Schematic With Explanation

Simple, low component count tone generator. It can be adapted to create a morse code circuit, by adding a switch to the output.

How it works:

This circuit is based around the 555 timer circuit, used as an astable (free running) oscillator. The frequency (pitch) of the tone is set by the resistors and capacitors in the left side of the circuit. The first one is a potentiometer (variable resistor), this is our pitch control, which is basically all the external components you need. The capacitor to the far left is to reduce as much noise or undesired operation of the potentiometer, getting a smooth pitch change when adjusting.

Tone generator circuit Schematic With Explanation

You can find the timer's datasheet by following the link: 555 timer

Audio Amplifier Circuit Schematic With explanation

Audio Amplifier Circuit Schematic With explanation

Here is a simple audio amplifier circuit that is easy to build and has few components. This circuit is built around the LM386 (click for datasheet) audio amplifier integrated circuit, useful when you need to power medium sized speakers from a music player that can only drive earphones.

How it works:
From left to right, the first part is the input stage, here is the connector to the audio source connected too the circuit using a capacitor. This capacitor passes only the audio, and blocks any direct current that may affect the function of the amplifier. Next to the capacitor is a variable transistor (potenciometer), this is used as a volume control.

Next is the LM386 itself, this amplifies the audio input using energy from the battery it is connected to. You'll notice there are two capacitors connected to it, one above and one below in the schematic. The top one is connected from pin 1 (positive side of capacitor) to pin 8 (negative side), this is to get the maximum amplification this IC can generate. The bottom one is also there to help get maximum amplification, this one goes connected from pin 7 (positive) to ground.

Last is the output stage, it is made with two capacitors, one resistor and the speaker. The resistor and capacitor that are connected before the speaker form a filter, that attenuates high frequency signals coming from the amplifier, most likely noise picked up or generated in the amplifying process. The capacitor connected to the speaker is there for the same reason we used a capacitor in the input stage, to prevent direct current from causing undesired operation of the speaker.


(click to enlarge)

Led Flasher using 555 SChematic Circuit With Explanation

Led Flasher using 555 SChematic Circuit With Explanation

This circuit is built around one of the most popular timer integrated circuits, the 555 timer.This circuit will flash the led on and of at regular intervals.
How it works:
From left to right, the two resistors and the capacitor set the time it takes to turn the led on or off, by changing the time it takes to charge the capacitor to trigger the timer. Next is the 555 timer, this is where all the work gets done to determine the time the led stays on and off. It contains a complicated circuit inside, but since it is packaged in the IC it can be used as a simple component.
The two capacitors that are right of the timer are just accessories so to speak, but are needed for the timer to work correctly. The last part is the resistor and the led, the resistor is there to limit the current on the led so that it won't burn.


(click to enlarge)

(click to enlarge)
(pin numbers on actual IC)

Circuit For Tone generator Schematic with explanation

Circuit For Tone generator Schematic with explanation

Simple, low component count tone generator. It can be adapted to create a morse code circuit, by adding a switch to the output.

How it works:

This circuit is based around the 555 timer circuit, used as an astable (free running) oscillator. The frequency (pitch) of the tone is set by the resistors and capacitors in the left side of the circuit. The first one is a potentiometer (variable resistor), this is our pitch control, which is basically all the external components you need. The capacitor to the far left is to reduce as much noise or undesired operation of the potentiometer, getting a smooth pitch change when adjusting.

Circuit For Tone generator Schematic with explanation

You can find the timer's datasheet by following the link: 555 timer

Inductor

An inductor is a loop of wire that stores energy in the form of a magnetic field. It has uses in oscillators, filters, voltage sources and converters.

Theory checklist:

• Inductive Reactance
• Back EMF
• Q factor

Modes of operation:

• LC filters and the tank circuit
• Autotransformer
• Transformer

Logic Gates explanation article

Logic Gates explanation article

Logic gates are the basic building blocks of digital electronics. These are circuits made out of transistors that perform a a logical operation (see Boolean algebra).

Digital electronics represent data (called bits) with only two states. Since in electronics we work with voltages, these two states are most times represented by a presence or lack of voltage. One (high state) in TTL logic familiy is represented by 5v, zero (low state) is represented by 0v (ground).

There are three basic gates: AND, OR, and NOT (Inverter).
Other common gates are NAND, NOR, XOR, XNOR (Equivalence). These gates are made with combinations of the basic logic gates. Its functions can be represented using a truth table, which lists every combination of inputs (A, B) and the resulting output (Z).

AND gate: two input gate, will output 1 when both inputs are 1. It is a one bit multiplication in Boolean algebra.

A B | Z
--------
0 0 | 0
0 1 | 0
1 0 | 0
1 1 | 1

OR gate: two input gate, will output 1 when one or both inputs are 1. It is a one bit addition.

A B | Z
--------
0 0 | 0
0 1 | 1
1 0 | 1
1 1 | 1

NOT gate or Inverter: one input gate, will output 1 when the input is 0 and viceversa.

A | Z
------
0 | 1
1 | 0

NAND gate: two input gate, same as AND gate but with a NOT at its output. Will output one as long as both its inputs are NOT 1. if none or one of the inputs is 0 it will output 1.

A B | Z
--------
0 0 | 1
0 1 | 1
1 0 | 1
1 1 | 0

NOR gate: two input gate, same as OR gate but with a NOT at its output. Will output one as long as none of its inputs are 1. if both inputs are 0 it will output 1.

A B | Z
--------
0 0 | 1
0 1 | 0
1 0 | 0
1 1 | 0

XOR gate: two input gate, will output 1 when one of its inputs is 1, but not both. This gate is actually a combination of gates, its boolean equation is A'B + AB'.

A B | Z
--------
0 0 | 0
0 1 | 1
1 0 | 1
1 1 | 0

XNOR gate or Equivalence: two input gate, will output 1 when both its inputs are the same, either 0 or 1. XOR gate with a NOT at its output, its boolean equation is A'B' + AB.

A B | Z
--------
0 0 | 1
0 1 | 0
1 0 | 0
1 1 | 1

Gate Diagrams:
 


Building other gates with NAND and NOR:

NAND and NOR gates have a remarkable characteristic, with enough of either one of them and connected in a certain way you can actually recreate the behavior of any other gate. This ability has made them very popular for large scale manufacturing of logic gates, since it is cheaper to build only one kind of device instead of having separate machines to create different logic gates for a single circuit.

Here are the circuit diagrams to create other gates with NAND and NOR.

AND gate:

OR gate:

NOT gate:

NAND gate:

NOR gate:

Since all digital electronic circuits are made with transistors, you can make all the above gates using them. When creating logic gates with transistors, the best option is to make them using NAND, NOR and simple NOT gates. The benefit of this is that any other gate can be constructed with a slight variation in the number and configuration of the transistors, instead of having several different circuits for each gate.

Logic gate's transistor diagrams:

NAND gate:

For this gate, the transistors are connected in series, so that the path from the output to ground is completed (thus giving 0 as output) only when both transistors are on (both inputs 1)

NOR gate:

For the NOR gate, the transistors are connected in parallel, so that the circuit from the output to ground is closed when either transistor is on.

NOT gate:

This gate is the simplest one to build with transistors, the NOT gate requires only one transistor. Here the transistor is configured so that when it is on (input 1), the circuit to ground is closed (output 0) and viceversa.

With these schematics and the above diagrams you can create a complete digital circuit using only transistors and resistors. Digital gates are very flexible, but up to a point. When creating a circuit that has more than three or four inputs, the circuit becomes too large to build using only logic gates, and that is where programmable devices come in handy, which we'll discuss in another article.

Types of circuits

From the smallest circuit to the largest electronics project, every circuit that performs a useful function has one or more of the same building blocks. I’m not talking about electronic components; I’m talking about sub-circuits that have a defined function.

These circuits are divided in digital and analog. In these pages you’ll learn how to design every type of circuit listed, with emphasis on a functionality level, instead of a component level, in order to be able to create any kind of amplifier as required by the project. Here’s the list of them:

Analog

• Amplifiers
• Filters
• Power sources
• Oscillators
• Rectifiers
• Timers
• Modulators
• Demodulators

Digital

• Logic gates
• Counters
• Encoders
• Decoders
• Flip-Flops
• Multiplexers
• Demultiplexers
• Analog to Digital Converter (ADC)
• Digital to Analog Converter (DAC)
• Microcontrollers
• Microprocessors

All of these sub-circuits have a defined function within a complete project, and some of them are even a project on their own. These categories are somewhat broad; every one of them has many different designs and implementations depending on the particular characteristics of the project, for example amplifiers.

There are transistor and OpAmp amplifiers. In transistor amplifiers there are common source, common base, common collector, there are Darlington amplifiers. Transistor amplifiers are further divided by the kind of transistor used: BJT, N-channel JFET, P-channel JFET, MosFET, Nmos, Pmos, Cmos; Each with its own set of configurations.

On OpAmp there are negative feedback, positive feedback, voltage follower and others.
As you can see there are a million different combinations of amplifier topologies as they are called, way too many to be familiar with all of them.

article on Amplifiers Component, stage and description

article on Amplifiers Component, stage and description

Amplifiers increase either the amplitude (voltage) or power (Amperage/Current)
applied to its input.

Components of an amplifier:

Gain component: The main component of the amplifier, defines many of its characteristics like noise, bandwidth, gain, input and output impedance, and others.

Bias: Some types of components need a bias point in order to operate correctly. The bias point is a dc voltage applied to the input of the amplifier. There are many ways to set the bias point,
depending on the gain component used.

Accessories: These are many kinds of sub-circuits used to fine tune the operation of the amplifier, including preamplifiers, buffers, stabilizers, filters, limiters, etc..

Stages of Amplifiers:

Input: This stage consists of a signal from another subsystem outside the amplifier, or a sensor like a microphone, photodiode or any other component that delivers a small signal. Depending on
the intended purpose and input signal, this stage may contain a preamplifier, which is a signal (voltage) amplification before the main power (current) amplification stage, and a filter to
limit incoming frequencies.

Amplification: Main stage of any amplifier, most of the times it is a power amplification process, sometimes with signal amplification as well. This stage is where the gain component and many of the accessories like stabilizers and limiters are located.

Output: Last stage, sometimes consists of a buffer and/or filter to remove any noise generated in the main amplification stage. The buffer sometimes added to deliver more current (lower output impedance).


Block Diagram of a Amplifiers
(Click to enlarge)

Description of Amplifier accessories:

Coupling: This is usually done with a capacitor. The purpose of the coupling capacitor is to prevent any DC voltage from modifying the bias point of the amplifier, to prevent clipping (driving the signal to the max voltage, distorting it) from a high or low bias point.

Another coupling method is using transformers. This is done on lower frequency signals where the reactance (resistance-like behavior when a component is applied an AC voltage) of capacitors is so high to the point the signal is practically lost.

A third choice is using tuned transformers, by using a capacitor in parallel with the transformer. This creates a tuned circuit that has a very narrow bandwidth, useful in some special interest amplifiers.

Filters: This topic is so extensive it deserves its own article. Amplifiers have uses for filters to limit noise and reject unwanted signals from its input. Combining a filter and an amplifier creates an active filter (filter that has gain).

Most filters use RC networks to create the filter, although RL or RLC are also used in some designs.

Stabilizers: This is usually some kind of feedback used to prevent clipping or other circuitry to keep the frequency within a certain range (stop frequency drifting).

Limiters: Sometimes only voltages up to a certain point are needed or desired, here limiters come into use. They limit or sometimes clip a signal if it goes above a certain voltage, other kind of limiters use feedback to control the gain of the amplifier so as to keep the output signal within the specified voltage range.

Buffers: Also called voltage followers, this is just another name for another stage of amplification with a gain of 1. This is to provide more current and avoid overloading the main amplifier, as doing so can reduce either the gain or bandwidth.

If you need a specific implementation of an amplifier circuit, you may want to consider learning all the abstract theory first and then moving on to the components page, where all component-specific circuits and modes of operation are listed.

Buffer amplifier

A buffer amplifier, or simply a buffer, is an electronic amplifier that is designed to have an amplifier gain of 1. Buffers are used in Impedance matching, the benefit of which is to maximize energy transfer between circuits or systems.

There are two main kinds of buffer circuits, Voltage buffers and Current buffers. The purposes of each is to isolate the mentioned characteristic to avoid loading the input circuit or source from the output stage.

Another name by which buffer amplifiers are known as is a voltage follower. The name is given because of the characteristic of the amplifier to output a signal of the same amplitude as the input (given the unity gain [gain of 1 or 0dB] ).

Examples of Buffer amplifiers:

The examples are too many to mention in this page, the most common being the transistor voltage follower and op amp version of it. The exact characteristics, formulas and construction instructions can be found on the specific component's page.

DISCRETE SEMICONDUCTOR CIRCUITS

Introduction

The invention of the bipolar transistor in 1948 ushered in a revolution in electronics. Technical feats previously requiring relatively large, mechanically fragile, power-hungry vacuum tubes were suddenly achievable with tiny, mechanically rugged, power-thrifty specks of crystalline silicon. This revolution made possible the design and manufacture of lightweight, inexpensive electronic devices that we now take for granted. Understanding how transistors function is of paramount importance to anyone interested in understanding modern electronics.
My intent here is to focus as exclusively as possible on the practical function and application of bipolar transistors, rather than to explore the quantum world of semiconductor theory. Discussions of holes and electrons are better left to another chapter in my opinion. Here I want to explore how to use these components, not analyze their intimate internal details. I don't mean to downplay the importance of understanding semiconductor physics, but sometimes an intense focus on solid-state physics detracts from understanding these devices' functions on a component level. In taking this approach, however, I assume that the reader possesses a certain minimum knowledge of semiconductors: the difference between "P" and "N" doped semiconductors, the functional characteristics of a PN (diode) junction, and the meanings of the terms "reverse biased" and "forward biased." If these concepts are unclear to you, it is best to refer to earlier chapters in this book before proceeding with this one.
A bipolar transistor consists of a three-layer "sandwich" of doped (extrinsic) semiconductor materials, either P-N-P or N-P-N. Each layer forming the transistor has a specific name, and each layer is provided with a wire contact for connection to a circuit. Shown here are schematic symbols and physical diagrams of these two transistor types:







The only functional difference between a PNP transistor and an NPN transistor is the proper biasing (polarity) of the junctions when operating. For any given state of operation, the current directions and voltage polarities for each type of transistor are exactly opposite each other.
Bipolar transistors work as current-controlled current regulators. In other words, they restrict the amount of current that can go through them according to a smaller, controlling current. The main current that is controlled goes from collector to emitter, or from emitter to collector, depending on the type of transistor it is (PNP or NPN, respectively). The small current that controls the main current goes from base to emitter, or from emitter to base, once again depending on the type of transistor it is (PNP or NPN, respectively). According to the confusing standards of semiconductor symbology, the arrow always points against the direction of electron flow:

Bipolar transistors are called bipolar because the main flow of electrons through them takes place in two types of semiconductor material: P and N, as the main current goes from emitter to collector (or vice versa). In other words, two types of charge carriers -- electrons and holes -- comprise this main current through the transistor.
As you can see, the controlling current and the controlled current always mesh together through the emitter wire, and their electrons always flow against the direction of the transistor's arrow. This is the first and foremost rule in the use of transistors: all currents must be going in the proper directions for the device to work as a current regulator. The small, controlling current is usually referred to simply as the base current because it is the only current that goes through the base wire of the transistor. Conversely, the large, controlled current is referred to as the collector current because it is the only current that goes through the collector wire. The emitter current is the sum of the base and collector currents, in compliance with Kirchhoff's Current Law.
If there is no current through the base of the transistor, it shuts off like an open switch and prevents current through the collector. If there is a base current, then the transistor turns on like a closed switch and allows a proportional amount of current through the collector. Collector current is primarily limited by the base current, regardless of the amount of voltage available to push it. The next section will explore in more detail the use of bipolar transistors as switching elements.

• REVIEW:
• Bipolar transistors are so named because the controlled current must go through two types of semiconductor material: P and N. The current consists of both electron and hole flow, in different parts of the transistor.
• Bipolar transistors consist of either a P-N-P or an N-P-N semiconductor "sandwich" structure.
• The three leads of a bipolar transistor are called the Emitter, Base, and Collector.
• Transistors function as current regulators by allowing a small current to control a larger current. The amount of current allowed between collector and emitter is primarily determined by the amount of current moving between base and emitter.
• In order for a transistor to properly function as a current regulator, the controlling (base) current and the controlled (collector) currents must be going in the proper directions: meshing additively at the emitter and going against the emitter arrow symbol.

The transistor as a switch

Because a transistor's collector current is proportionally limited by its base current, it can be used as a sort of current-controlled switch. A relatively small flow of electrons sent through the base of the transistor has the ability to exert control over a much larger flow of electrons through the collector.
Suppose we had a lamp that we wanted to turn on and off by means of a switch. Such a circuit would be extremely simple:

For the sake of illustration, let's insert a transistor in place of the switch to show how it can control the flow of electrons through the lamp. Remember that the controlled current through a transistor must go between collector and emitter. Since its the current through the lamp that we want to control, we must position the collector and emitter of our transistor where the two contacts of the switch are now. We must also make sure that the lamp's current will move against the direction of the emitter arrow symbol to ensure that the transistor's junction bias will be correct:

In this example I happened to choose an NPN transistor. A PNP transistor could also have been chosen for the job, and its application would look like this:

The choice between NPN and PNP is really arbitrary. All that matters is that the proper current directions are maintained for the sake of correct junction biasing (electron flow going against the transistor symbol's arrow).
Going back to the NPN transistor in our example circuit, we are faced with the need to add something more so that we can have base current. Without a connection to the base wire of the transistor, base current will be zero, and the transistor cannot turn on, resulting in a lamp that is always off. Remember that for an NPN transistor, base current must consist of electrons flowing from emitter to base (against the emitter arrow symbol, just like the lamp current). Perhaps the simplest thing to do would be to connect a switch between the base and collector wires of the transistor like this:

If the switch is open, the base wire of the transistor will be left "floating" (not connected to anything) and there will be no current through it. In this state, the transistor is said to be cutoff. If the switch is closed, however, electrons will be able to flow from the emitter through to the base of the transistor, through the switch and up to the left side of the lamp, back to the positive side of the battery. This base current will enable a much larger flow of electrons from the emitter through to the collector, thus lighting up the lamp. In this state of maximum circuit current, the transistor is said to be saturated.

Of course, it may seem pointless to use a transistor in this capacity to control the lamp. After all, we're still using a switch in the circuit, aren't we? If we're still using a switch to control the lamp -- if only indirectly -- then what's the point of having a transistor to control the current? Why not just go back to our original circuit and use the switch directly to control the lamp current?
There are a couple of points to be made here, actually. First is the fact that when used in this manner, the switch contacts need only handle what little base current is necessary to turn the transistor on, while the transistor itself handles the majority of the lamp's current. This may be an important advantage if the switch has a low current rating: a small switch may be used to control a relatively high-current load. Perhaps more importantly, though, is the fact that the current-controlling behavior of the transistor enables us to use something completely different to turn the lamp on or off. Consider this example, where a solar cell is used to control the transistor, which in turn controls the lamp:

Or, we could use a thermocouple to provide the necessary base current to turn the transistor on:

Even a microphone of sufficient voltage and current output could be used to turn the transistor on, provided its output is rectified from AC to DC so that the emitter-base PN junction within the transistor will always be forward-biased:

The point should be quite apparent by now: any sufficient source of DC current may be used to turn the transistor on, and that source of current need only be a fraction of the amount of current needed to energize the lamp. Here we see the transistor functioning not only as a switch, but as a true amplifier: using a relatively low-power signal to control a relatively large amount of power. Please note that the actual power for lighting up the lamp comes from the battery to the right of the schematic. It is not as though the small signal current from the solar cell, thermocouple, or microphone is being magically transformed into a greater amount of power. Rather, those small power sources are simply controlling the battery's power to light up the lamp.

• REVIEW:
• Transistors may be used as switching elements to control DC power to a load. The switched (controlled) current goes between emitter and collector, while the controlling current goes between emitter and base.
• When a transistor has zero current through it, it is said to be in a state of cutoff (fully nonconducting).
• When a transistor has maximum current through it, it is said to be in a state of saturation (fully conducting).

Meter check of a transistor

Bipolar transistors are constructed of a three-layer semiconductor "sandwich," either PNP or NPN. As such, they register as two diodes connected back-to-back when tested with a multimeter's "resistance" or "diode check" functions:

Here I'm assuming the use of a multimeter with only a single continuity range (resistance) function to check the PN junctions. Some multimeters are equipped with two separate continuity check functions: resistance and "diode check," each with its own purpose. If your meter has a designated "diode check" function, use that rather than the "resistance" range, and the meter will display the actual forward voltage of the PN junction and not just whether or not it conducts current.

Meter readings will be exactly opposite, of course, for an NPN transistor, with both PN junctions facing the other way. If a multimeter with a "diode check" function is used in this test, it will be found that the emitter-base junction possesses a slightly greater forward voltage drop than the collector-base junction. This forward voltage difference is due to the disparity in doping concentration between the emitter and collector regions of the transistor: the emitter is a much more heavily doped piece of semiconductor material than the collector, causing its junction with the base to produce a higher forward voltage drop.
Knowing this, it becomes possible to determine which wire is which on an unmarked transistor. This is important because transistor packaging, unfortunately, is not standardized. All bipolar transistors have three wires, of course, but the positions of the three wires on the actual physical package are not arranged in any universal, standardized order.
Suppose a technician finds a bipolar transistor and proceeds to measure continuity with a multimeter set in the "diode check" mode. Measuring between pairs of wires and recording the values displayed by the meter, the technician obtains the following data:

• Meter touching wire 1 (+) and 2 (-): "OL"
• Meter touching wire 1 (-) and 2 (+): "OL"
• Meter touching wire 1 (+) and 3 (-): 0.655 volts
• Meter touching wire 1 (-) and 3 (+): "OL"
• Meter touching wire 2 (+) and 3 (-): 0.621 volts
• Meter touching wire 2 (-) and 3 (+): "OL"

The only combinations of test points giving conducting meter readings are wires 1 and 3 (red test lead on 1 and black test lead on 3), and wires 2 and 3 (red test lead on 2 and black test lead on 3). These two readings must indicate forward biasing of the emitter-to-base junction (0.655 volts) and the collector-to-base junction (0.621 volts).
Now we look for the one wire common to both sets of conductive readings. It must be the base connection of the transistor, because the base is the only layer of the three-layer device common to both sets of PN junctions (emitter-base and collector-base). In this example, that wire is number 3, being common to both the 1-3 and the 2-3 test point combinations. In both those sets of meter readings, the black (-) meter test lead was touching wire 3, which tells us that the base of this transistor is made of N-type semiconductor material (black = negative). Thus, the transistor is an PNP type with base on wire 3, emitter on wire 1 and collector on wire 2:

Please note that the base wire in this example is not the middle lead of the transistor, as one might expect from the three-layer "sandwich" model of a bipolar transistor. This is quite often the case, and tends to confuse new students of electronics. The only way to be sure which lead is which is by a meter check, or by referencing the manufacturer's "data sheet" documentation on that particular part number of transistor.
Knowing that a bipolar transistor behaves as two back-to-back diodes when tested with a conductivity meter is helpful for identifying an unknown transistor purely by meter readings. It is also helpful for a quick functional check of the transistor. If the technician were to measure continuity in any more than two or any less than two of the six test lead combinations, he or she would immediately know that the transistor was defective (or else that it wasn't a bipolar transistor but rather something else -- a distinct possibility if no part numbers can be referenced for sure identification!). However, the "two diode" model of the transistor fails to explain how or why it acts as an amplifying device.
To better illustrate this paradox, let's examine one of the transistor switch circuits using the physical diagram rather than the schematic symbol to represent the transistor. This way the two PN junctions will be easier to see:

A grey-colored diagonal arrow shows the direction of electron flow through the emitter-base junction. This part makes sense, since the electrons are flowing from the N-type emitter to the P-type base: the junction is obviously forward-biased. However, the base-collector junction is another matter entirely. Notice how the grey-colored thick arrow is pointing in the direction of electron flow (upwards) from base to collector. With the base made of P-type material and the collector of N-type material, this direction of electron flow is clearly backwards to the direction normally associated with a PN junction! A normal PN junction wouldn't permit this "backward" direction of flow, at least not without offering significant opposition. However, when the transistor is saturated, there is very little opposition to electrons all the way from emitter to collector, as evidenced by the lamp's illumination!
Clearly then, something is going on here that defies the simple "two-diode" explanatory model of the bipolar transistor. When I was first learning about transistor operation, I tried to construct my own transistor from two back-to-back diodes, like this:

My circuit didn't work, and I was mystified. However useful the "two diode" description of a transistor might be for testing purposes, it doesn't explain how a transistor can behave as a controlled switch.
What happens in a transistor is this: the reverse bias of the base-collector junction prevents collector current when the transistor is in cutoff mode (that is, when there is no base current). However, when the base-emitter junction is forward biased by the controlling signal, the normally-blocking action of the base-collector junction is overridden and current is permitted through the collector, despite the fact that electrons are going the "wrong way" through that PN junction. This action is dependent on the quantum physics of semiconductor junctions, and can only take place when the two junctions are properly spaced and the doping concentrations of the three layers are properly proportioned. Two diodes wired in series fail to meet these criteria, and so the top diode can never "turn on" when it is reversed biased, no matter how much current goes through the bottom diode in the base wire loop.
That doping concentrations play a crucial part in the special abilities of the transistor is further evidenced by the fact that collector and emitter are not interchangeable. If the transistor is merely viewed as two back-to-back PN junctions, or merely as a plain N-P-N or P-N-P sandwich of materials, it may seem as though either end of the transistor could serve as collector or emitter. This, however, is not true. If connected "backwards" in a circuit, a base-collector current will fail to control current between collector and emitter. Despite the fact that both the emitter and collector layers of a bipolar transistor are of the same doping type (either N or P), they are definitely not identical!
So, current through the emitter-base junction allows current through the reverse-biased base-collector junction. The action of base current can be thought of as "opening a gate" for current through the collector. More specifically, any given amount of emitter-to-base current permits a limited amount of base-to-collector current. For every electron that passes through the emitter-base junction and on through the base wire, there is allowed a certain, restricted number of electrons to pass through the base-collector junction and no more.
In the next section, this current-limiting behavior of the transistor will be investigated in more detail.

• REVIEW:
• Tested with a multimeter in the "resistance" or "diode check" modes, a transistor behaves like two back-to-back PN (diode) junctions.
• The emitter-base PN junction has a slightly greater forward voltage drop than the collector-base PN junction, due to more concentrated doping of the emitter semiconductor layer.
• The reverse-biased base-collector junction normally blocks any current from going through the transistor between emitter and collector. However, that junction begins to conduct if current is drawn through the base wire. Base current can be thought of as "opening a gate" for a certain, limited amount of current through the collector.