phase margin
Raymond_di said:
I want the effect of feed back. Do I have to cut A? Could I cut B? If only to break the loop, then what's the rule to choose the point to break it? Thanks
If the circuit is like this:........, is it different from the former circuit?
Best Regards!
Hi raymond,
It´s true what was said about the Middlebrook method - however in many cases it is not necessary to apply this somewhat complicated method.
Here are some basics:
1.) You have to break the loop, inject a signal and measure the output at the other "open" point.
2.) This breaking should not change the loading at this point; therefore, the cut should be at a point where a low output impedance sees a high input impedance
(opamp output, for example).
3.) Problem: Due to opamp offset you will most probably loose the operating point which is established by feedback. Therefore, some tricky circuits like the Middlebrook method.
4.) But for relatively simple circuits (like opamp with low output impedance) there is a simple method which is normally sufficient:
Break the loop at the opamp output and inject an AC signal BETWEEN both open ends, that means: not with reference to ground.
5.) Then, the loop gain is the voltage RATIO between those two points.
(The load resistor of your second circuit is not part of the loop but it should be connected if it is present in reality due to its loading effect).
6) This method, normally works good - even until a frequency region where the loop gain approaches unity (for phase margin calculation). For higher frequencies the calculation of the loop gain becomes more and more false.