General question: stability of a feedback system -- transimpedance amplifier

Hi Wayne,

Although you are close in your understanding in some areas, you are off in others.  The stability of a closed loop system requires looking at the loop gain of the circuit which is not equal to the circuit closed loop Vo/Vin transfer function.  The Loop Gain is equal to the difference between the AOL curve and the 1/Beta (inverse feedback factor).  You will probably recognize this term as:  AOL*B.

The system stability is therefore determined based on the remaining phase margin when the AOL*B product is equal to 0dB.   Tim Green's paper's which you've reference go into great detail showing how to obtain the AOL, 1/B, and AOL*B curves which can then be used for stability analysis.  In short, you need to break the loop around the amplifier and look at the difference between the source input (injected at the point where the loop is broken), the feedback signal, and the output.  I suggest you continue your reseach on Tim Green's papers and I'm sure you will figure it out.

Regarding your circuit, there are some obvious problems with your lack of negative feedback on the composite amplifier second stage that requires some heavy re-work.  You may have just flipped the terminals on accident so please double check the circuit.

Hope this helps.

Best Regards,
Collin Wells
Precision Linear Applications

Thanks Colin,

I am still going through Tim Green's paper.  The first two parts of the paper seems to be most relevant in discussing how to obtain the open loop gain (A_OL) and 1/beta value of a circuit.  I followed figure 2.3 given in part 2 (p. 3) to change the circuit in order to conduct a loop gain test: 1) grounding the input, 2) place a high value inductor at the output (block low frequency signal), 3) put a voltage source with a high value capacitor at the start of the feedback loop.  The paper tests a first-order circuit, now my question is, how would I use the same technique for a n-order multistage amplifier circuit?  Do I test each amplification stage separately; if all stages are stable, then I can assume the entire circuit is stable?  Or, do I test the entire circuit together?

I attached the modified TINA circuit, hopefully you can tell me if I am doing this correctly.  In it, I test for the second stage of amplification alone, where I defined N2(s)/N1(s) as A_OL and N3(s)/N1(s) as 1/beta.  Now, if I were to test both stages together, like N3(s)/N0(s), I will get a floating point divided by zero error, most likely because I am not suppose to measure open gain test like this.

TINA circuit with open loop configuration:  8787.TunnelingCurrentPreamp-OpenLoop.tsc.zip

Just curious, I cannot find the rest of Tim Green's paper (part 11 to 15), was that ever made available online?

Thank you again and really appreciate your help!

Wayne