Home    Bloggers    Blogs    Article Archives    Messages    About Us
Tw  |  Fb  |  In  |  Rss
Michael Steffes

# Extracting Loop Gain and Phase Information from Simulation

50%
50%
Page 1 / 2 Next >
TucsonMike
8/21/2018 4:23:50 PM
User Rank
Blogger
Re: Alternative technique
Thanks Man21, I vaguely recall this technique and it is probably quite valid as well. There are usually multiple ways to tackle these issues. I could not find a description of what he was doing there, but I suspect he is just solving backwards for the LG inside the full transfer function. So if you have gain phase info closed loop you should be able to dump that data out into some code that will solve for the loop gain. That would be more steps, while the solution we actually use shown in the article is pretty quick and should give the same results. I may buy that book though, seems like a good addition to my library - thanks.

50%
50%
Man21
8/17/2018 7:31:54 AM
User Rank
Newbie
Alternative technique
Paul Tuinenga in his book SPICE: A guide to Circuit Simulation and Analysis Using PSpice (section 6.9, p59, Plotting Loop Gain, in my 1998 edition) develops a technique for determining loop gain in a closed-loop circuit without having to break or alter the circuit. It would be interesting to see if that approach produced similar results to those in the present  article. It may be noted that my version of Tuinenga's analysis results in a small difference in his final equationon for T on p63 with -2 rather than +2 in the denominator.

50%
50%
More Blogs from Michael Steffes
This part 3 will progress through the different VFA approaches with Part 4 moving on to CFA and FDA DC precision considerations
Modern Rail-to-rail (R-R) output stage op amps, or fully differential amplifiers (FDAs), can sometimes get into a low phase margin condition for relatively simple circuits. Using the phase margin simulation test of Reference 1, a few simple external phase margin improvement methods will be shown. These tests and fixes are best done in simulation prior to board layout. Where the device model for a R-R output device is perhaps not including the higher frequency reactance, places for these tuning elements might be put into the board layout as a hedge against future stability problems.
The differential outputs provided by fully differential amplifiers (FDA) can be used in a positive feedback scheme to generate a partially active output impedance
A recent >30MHz active filter design will be described here and updated to more accurately fit the desired response shape.
flash poll