# What is the phase margin of this inverting amplifier?

I'm having trouble finding the phase margin the telescopic cascode amplifier I'm currently building.

This source says that the phase margin for an inverting amplifier is the distance from -360 degrees at unity gain frequency and in general is:

$$PM = \text{180}-|\angle_{\text{change from normal operating point}}|$$

Is this the formula we should use or am I misinterpreting what they're saying? Below is the bode plot for my amplifier. According to the source my phase margin should be roughly 60 degrees but if that's not the case then my amplifier is likely unstable.

• PM looks to be about 60 degrees to me. Apr 20 at 15:48
• There seems to be no loop gain at DC. Does that mean that there is no feedback at DC? It'd be interesting to see your testbench and where exactly you have cut the loop for the AC or stb analysis
– sai
Apr 20 at 16:42
• One more point, I would expect the phase at low frequencies to be close to 180deg which indicates that there is negative feedback at DC. I don't see that in your plot. So, I am confused.
– sai
Apr 20 at 17:03
• @sai your confusion might stem from your inexperience in AC coupled circuits. I would estimate that the circuit used is a DC to high MHz amplifier with an input stage that blocks DC. Apr 20 at 19:21