Electronic – Should I be concerned when loop gain rises back above 0dB on simulation


Interim update

Okay, I just realized I screwed up on the simulation. When I added \$C_{bp}\$ I upset the "break" in the feedback loop necessary to get the loop gain. The left-side lead has to go to the FB node leaving the INV node the only thing connected to the inverting input. I just fixed it and now the plot looks like one would expect (and my phase margin is all gone :). I'll work it through and then may take the question down since I'm not sure it happens when one doesn't screw up like I did 🙂

I'm finally getting a handle on methodical frequency response compensation for op amp circuits. I got LTspice set up to show loop gain, and have compensated this circuit in a fairly conventional way. I'm not sure what the name of this compensation strategy is, if it has one, but as I understand it, after "isolating" the gate capacitance with \$R_{iso}\$ it adds a zero to the feedback circuit at the frequency determined by \$\frac{1}{2\pi C_{bp} R_f}\$, about 1MHz in this case.

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The feedback circuit frequency response looks like this:

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The phase margin is a healthy 64 degrees, which I'm happy about because I'd prefer this circuit to be overly stable rather than get every last bit of speed out of it.

My question is: Should I be concerned that the gain rises back above 0dB at about 1.5MHz?

Also I'm a little bit concerned that the phase drops so precipitously in that neighborhood, wondering if I should be looking to add another pole somewhere or perhaps relocate one I already have.

Best Answer

The rise of the gain above app. 1.5 MHz is caused by the capacitor Cbp, of course. As can be seen, a part of the test signal is coupled to the opamp output directly via Cbp: For rising frequencies the opamp output voltage continuously decreases and the output impedance increases so that the effect of the signal through Cbp also increases with frequency. (The same effect can be observed for Sallen-Key lowpass filters).

By the way - what is the purpose of Cbp? "Upset" the break of the feedback loop? This is not necessary for simulating the loop gain response. Note that this capacitor deteriorates the loop gain function.