I need to do the stability analysis of my amplifier circuit. I implemented schematic in LTspice, where I did some of simulations, so I would like to keep the same tool also for the stability analysis. The amplifier schematic is the following:
I followed some video tutorial that advices the following steps for stability analysis:
Break feedback loop for the AC signals by inserting large - 1TH inductor between the output and feedback network (this way we keep the DC bias but block the AC),
Add large capacitance (1TF) between the feedback network and AC stimulus,
- Ground the input.
Applying the previous, I got the following schematic:
If I correctly understand, the stability check is based by measuring the phase margin of the closed loop gain (that should be V(out)) on the intersect point of 1/β (that should be 1/V(fb), fb is feedback point) and open loop gain plots (that should be V(out)/V(fb)). In general, we should ensure that the phase shift of the feedback signal is less than 180° when amplifier reaches the unity gain (avoid amplification when feedback signal subtraction become addition, because of phase shift). In practice, as a rule of thumb, the minimum phase margin should be 45°. After simulation I get the following plot:
From the plot, I can notice that the phase margin is about 113° (180° - absolute_value(phase at intersection point)), so I can conclude that the amplifier is stable (right?). This also confirms the transient simulation. However, when I remove the 100pF capacitor at the transistor Q4 (the second differential pair), I get phase margin of 145°, but the transient analysis reveals oscillation, like on plot:
I am definitely missing something, so I need help to make correct setup and simulations (probably and interpretation).