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Design >> Analog Design >> Different gain in Folded Cascode ????
https://designers-guide.org/forum/YaBB.pl?num=1098642727

Message started by Rave on Oct 24^th, 2004, 11:32am

Title: Different gain in Folded Cascode ????
Post by Rave on Oct 24^th, 2004, 11:32am

Hi All

I have a folded cascode amplifier. When i run an AC analysis with CMFB (its a transistor based CMFB which i wud replace later by SC-Cmfb) i get a gain of 60dB and UGF of around 55MHz.

But if i remove cmfb, and just connect load capacitance, i get gain of only 30dB and UGF of 35MHz.

Can someone suggest me why is that and which one should i consider correct.

Title: Re: Different gain in Folded Cascode ????
Post by Paul on Oct 27^th, 2004, 8:31am

Hi Rave,

did you check your operating point in the second simulation. I expect one of the devices to enter triode region resulting in loss of gain.

Also check an earlier discussion of this issue:
http://www.designers-guide.com/Forum/?board=analog_design;action=display;num=1079101945;start=0#0

Paul

Title: Re: Different gain in Folded Cascode ????
Post by Andre on Nov 15^th, 2004, 4:29pm

Generally speaking a differential circuit will not work without the CMFB. Your output will saturate at either ground or vdd. The simulation will not be correct.
Use a Verilog-A CMFB if you want to evaluate the circuit without its effects.