# Amplifier's frequency compensation and phase shifts...?

#### Himanshoo

Joined Apr 3, 2015
265

Discussing compensation of amplifiers we find articles which says that --- "the amplifier requires compensation because its open loop gain is still higher at frequencies where the internal phase shift are reaching 180 degrees."

So the deal is to reduce the open loop gain lower than unity (in order to prevent sustained oscillation) before the accumulated phase shift reaches 180 degree at higher frequencies..

The thing which I am not getting is how exactly phase shift grows larger with the increase in frequency....

Thanks.....

#### crutschow

Joined Mar 14, 2008
29,765
The various parasitic capacitances and inductances in a circuit have an increasing effect on the output waveform as the frequency increases, since their impedance is frequency dependent, eventually generating sufficient phase-shift to cause instability through any negative feedback loop.

Joined Dec 4, 2009
328
For the sake of understanding and remembering, let us assume the amp introduces a certain fixed delay. Now the phase shift is this delay in relation to the period. Since the period keeps reducing as the frequency goes up, this ratio keeps going up.

#### Himanshoo

Joined Apr 3, 2015
265
The various parasitic capacitances and inductances in a circuit have an increasing effect on the output waveform as the frequency increases, since their impedance is frequency dependent, eventually generating sufficient phase-shift to cause instability through any negative feedback loop.
As the text state to tailor the open loop gain below unity before phase shift build up at a verge of causing oscillation at higher frequencies
Why it says to tailor the open loop gain…rather than it should be closed loop gain because…
First open loop gain(gain of op-amp without feedback) is constant..can’t be change.

Second… though parasitic capacitance and inductance may be present in the internal circuitry of op-amp ….but they are also present outside as well like a capacitive load and RC filter(delay) formed by feedback resistance and input capacitance ..and so on.....