Open Loop Responce
– 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 (fcu) minus the lower critical frequency (fcl).
– The lower critical frequency is zero for an op-amp. Thus:
BW = fcu
– The open loop gain at any given frequency can be found with the following formula:
Aol = Aol(mid)/?(1 + f2/fcu2)
– 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, Aol, 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/fc)
– 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.