Hello,
I have designed a fully differential folded cascode OTA with SC-CMFB.
I have tested the circuit using a SC-CMFB network using Verilog-A macromodel switches. And the OTA seems to work very well.
When I replace the switches with transistors the clock injection is so bad that the CMFB voltage (which should be around 900mV) never is mantained at a fixed value and varies in a much wider range . The common mode open loop gain seems to be as large as the differential gain (~70dB) as well is the Unit gain Bandwidth (>125MHz).
I attached the images of the schematic and the simulations.
When using the MOS switches the CMFB voltage varies in a much wider range. As you can see in the simulation screenshots.
Schematic ScreenshotTransient simulation using Ideal SwitchesLeft: Diferential output, Rigth: VCMFB, Differential input: 300uV
Transient simulation using MOS SwitchesLeft: Diferential output, Rigth: VCMFB, Differential input: 300uV