How determine OPamp phase margin

Thread Starter

RMyska

Joined Oct 23, 2016
3
Hi all,

I tried determine phase margin in LTspice of the circuit in the picture but I am not sure if I choosed correctly nodes for calculating(Vout/Vin). Can anybody give his point of view? Thanks

upload_2016-10-23_11-7-17.png
 

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crutschow

Joined Mar 14, 2008
34,285
You just have to do an AC analysis and click on Vout.
The resulting plot shows the ratio of Vout to the AC value of V6.
Make sure the circuit is biased in the linear region when you do the AC test.
Note that C25 creates a lag in the loop which may cause a high frequency peak (and possible transient ringing or oscillation) in the response.
 

Thread Starter

RMyska

Joined Oct 23, 2016
3
Hi Crutschow,

Thanks for your reply, in my simulation I was use AC simulation but I observed ratio Vout/Vin and from resulting plot I determined phase margin. My question was if I should use for calculating anothor nodes for example Vfb/Vin, but according your answer I should use only Vout, so my question now is why in LTspice is enough use only Vout for determine phase margin, thanks ;)

upload_2016-10-23_20-43-1.png
 

crutschow

Joined Mar 14, 2008
34,285
Because plotting Vout automatically uses the voltage across the AC source in the loop for the ratio denominator and that will give the loop response.

Plotting Vout/Vin gives you the response of the op amp, not the response of the loop.
 
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