Yes, I agree that breaking the loop modifies the circuit. In this case, if Cin of the opamp is comparable to the load(5pF), it'll surely modify it. Also, in this particular case, the unity gain frequency is about 10MHz.
In any case, the original question was about the low frequency behavior of loop gain(the phase not being 180 degrees from the inverting input to the ouptut). With the LC circuit used to break the loop, about the only impedance seen at low frequencies (~ 1Hz) is the inductor. the 100F capacitor is practically a short, other capacitors are open circuits, and the internal output resistance of the cascode opamp is also too high to matter. So what is left is just a gm loaded by an inductor, giving 90 degree phase lead.
Ken Kundert wrote on Nov 24th, 2013, 12:18am:nrk1,
You've got it wrong. The issue is not the inductor is not large enough to break the loop. The issue is that it modifies the loop. As such, the loop gain you measure is not the actual loop gain of your circuit. The difference between the measured and actual loop gain is small at log frequencies, and so generally looks reasonable, but is often quite substantial at high frequencies. The error is quite substantial at the frequencies where you would measure phase and gain margin.
-Ken
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