A question about feedback

[holddreams]

When using feedback circuit, the stability should be measured.
As there are series-shunt, series-series, shunt-shunt,shunt-series feedback.

So for the stability analysis,the loop should be open. How to do ?Check the loop gain or open loop gain?
Just add a ac at the input ,and check the gain at the point we open the loop?

Thanks.

 

[sixth]

When using feedback circuit, the stability should be measured.
As there are series-shunt, series-series, shunt-shunt,shunt-series feedback.

So for the stability analysis,the loop should be open. How to do ?Check the loop gain or open loop gain?
Just add a ac at the input ,and check the gain at the point we open the loop?

Thanks.

For simulation, both of the open loop and close loop method are effective.
For open loop analysis, you should put a ac source at the input and do ac analysis and then plot the amplitude frequency response curve and the phase frequency response curve to see the phase margin.
But for measurement, you must do the close loop analysis because in reality the DC operating point are not stable. you must stable the operating point of the opamp through close loop connection. And then do the same as the simulation.
sixth

 

[holddreams]

so what's the output when loop closed?

 

[v_c]

Actually, even in simulation, you probably cannot open the loop because you will have a hard time establishing the operating point if you do. The way to measure the loop gains can be done by signal injection into the loop. A paper by R. D. Middlebrook written in the 70s (?) showed this.

Here are some recent discussions on feedback, loop gain and stability
**broken link removed**