Op amp feedback system stability

Thread Starter

Nemanja_95

Joined Nov 14, 2017
24
I have general question about analyzing stability with op amp:

There is some bode plot of A (op amp) and feedback factor (1/Beta) of some circuit that I am trying to analyze. Below the critical frequency (fc, where A*Beta = 1), there is no stability problem, but what is happening if my input signal is at frequency that is higher then critical? Do we analyze stability always with step impulse at input or we can put some sin or cos signal and see what is happening at output?

The circuit is simple buffer with capacitive load, but as I sad at the beggining, my question relates on general situation.

za ac.jpg
 

crutschow

Joined Mar 14, 2008
34,455
You always analyze stability over the complete frequency range.

A step input generates many frequencies, as shown by its Fourier transform, will will show instability by overshoot or ringing in the circuit output.
The frequency of any ringing gives an indication of where the instability would be seen in an AC analysis.
 

MisterBill2

Joined Jan 23, 2018
18,539
For an op-amp circuit to be reasonably stable the forward gain must drop below unity below the loop phase shift makes the feedback positive. that is why phase-frequency plots are handy. Of course, this same condition applies to almost all feedback systems, electrical, hydraulic, or whatever.
 
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