A very simple low power three stage transistor amplifier circuit is shown below. First and second stages are pre amplifiers and third is the output stage. Collector to base biasing is employed for each stages and it is enough for circuits like this.
Collector to base biasing: Consider the first stage.R1 is the collector resistor and R2 is the base resistor. This type of biasing provides some negative feedback, avoids thermal runaway and stabilizes the operating point. When the temperature increases the collector current also increases. This increases the voltage drop across the collector resistance R1 and this decreases the voltage drop across base resistor R2. As a result, base current drops which in turn drops the collector current and thermal runaway is avoided. Stabilization of operating point is achieved by the prevention of variation in collector current with temperature.
When a transistor amplifier is operating, the collector voltage will be phase opposite to the input signal. Since base resistor R2 is connected between collector and base, some fraction of the phase opposite collector voltage is fed back to the input (base) through the base resistor R2 and thus negative feedback is achieved. Negative feed back reduces the voltage gain of the amplifier but it improves the stability.
In the circuit collector of Q1 is coupled to the base of Q2 using the capacitor C2. R3 is the biasing resistor for Q2 and R4 is its collector resistor.Collector of Q2 is coupled to the base of Q3 using the capacitor C3. R5 is the biasing resistor for Q3 and speaker is connected as the collector load. Capacitors C4 and C5 are filters. C1 is the input DC decoupling capacitor.
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