Suppose we are given a fully differential (low voltage single supply) op amp with common mode feedback. It is common knowledge to break the loop in order to measure open loop ac performance for loop stability. In the first figure, there is a common setup to measure differential gain from input to output and measure the gain and phase response at the output (vop-von, differentially). The idea is to break the loop (using large inductor short at DC) for dc feedback only, but open loop to ac. The input on the other hand has a large capacitor for open to DC and short to AC. One can run a bode plot of the AC Gain and Phase, in order to look at Phase Margin of the differential gain.
Can someone show a working testbench setup for LTSPICE that will measure the CMFB (Common Mode Feedback) loop stability using AC analysis? A google search will return plenty of examples for stb analysis using a program like Spectre, but next to no examples for a more simple spice simulator like LTSPICE.
More specifically, given an OTA architecture (Folded Cascode Input and Dual Differential Pair common mode feedback) as in Fig 2., can you show a test setup (like Fig 1) with specific source stimulus and response measure location(s) to perform CMFB loop stability using AC analysis with LTSPICE only?
Fig. 1 LTSPICE test bench to measure Differential AC gain and Stability.
Fig. 2 Fully Differential Folded Cascode Operational Amplifier with DDP CMFB