Open-Loop Response

– The frequency response of an op-amp varies with the frequency of the input signal.

– So far we have assumed mid-range frequencies (0 Hz up to a critical frequency at which the gain is 3 dB less than the midrange value).

– The open-loop gain of an op-amp is determined by its internal design.

– It is very high.

– The high frequency cutoff frequency of an open-loop op-amp is about 10Hz.

– Curve rolls off at –20 dB per decade.

– Midrange gain is 200000 (106 dB).

– The bandwidth of an amplifier is the frequency range between the points where the gain is 3 dB less than the midrange gain.

– In general, bandwidth equals the upper critical frequency (f_cu) minus the lower critical frequency (f_cl).

– The lower critical frequency is zero for an op-amp. Thus:

BW = f_cu

– The open loop gain at any given frequency can be found with the following formula:

A_ol = A_ol(mid)/?(1 + f²/f_cu²)

– It is apparent that the open loop gain equals the midrange gain when the signal frequency is much less than the critical frequency.

– The open loop gain, A_ol, drops off as the frequency increases.

– There is also a phase shift from the input signal to the output signal.

– The phase shift is given by:

q = -tan^-1(f/f_c)

– Note that we have the following circuit (internal to an op-amp).

– If we have multistage amplifiers, each amplifier will contribute to the drop off of the frequency response.