Hi teachers,

I am new in Analog IC Design. I want to simulate frequency response of a fully differential OTA using 'A Test Bench for Differential Circuit by Kenneth S. Kundert. (The paper can be download from www.designers-guide.com).

When implementing a test bench, I follow exactly what have been written in the paper. I set Vdm=1V ac, Vcm=0V, Rid=Ric=Rod=Roc=0 ohm. Run ac analysis and probe differential output at node d.

The result is a bit strange for me. The phase response curve looks fine but the gain response curve is somehow funny. It looks like a response for high pass filter.( Gain increase linearly until it reaches certain level at certain frequency the gain become constant)

Anyone can tell me why is this happening? Is it something wrong with my setup? How about the setup for settling time and other related quantities?

thanks

My personal guess would be that the operating point of the circuit is not correct. Maybe a different value should be chosen for Vcm. The highpass characteristic might be due to capacitive coupling from the gates of the input transistors which are not in their active region.