folded-cascode operational amplifier design

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Patrick Yang

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Hi guys，
I‘m a freshman in IC design. I try to design a folded-cascode opa, and i met a problem that when Vcm increases, the Av and GBW also increase and vice versa. simulation shows that the gm changes little inspite of increasing in Vcm. GBW = gm/Cc, this value shouldn't change. I really need help !Thank in advance
 

Av (and so GBW) changes over the CMR (common mode range), s. e.g. here:
 

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erikl said:
Av (and so GBW) changes over the CMR (common mode range), s. e.g. here:
Click to expand...

THx for u help, still i want to know about why Av changes much when Vcm varies. I read the pdf, personally, i think in the ICMR, the Av changes little in spite of Vcm changes, while out of ICMR, distortion appears. Is it right?
Thank U in advance
 

Patrick Yang said:
... still i want to know about why Av changes much when Vcm varies.
I read the pdf, personally, i think in the ICMR, the Av changes little in spite of Vcm changes
Click to expand...

This depends on the definition of the ICMR. Sometimes it's defined for 2% Av drop, sometimes for 5%, 10% or even 50% Av drop.
The Av drop is due to a degradation of the operating point of one or more transistors (shifting from saturation region --> triode region, less gm).

Patrick Yang said:
... while out of ICMR, distortion appears. Is it right?
Click to expand...
Yes. So farer from ICMR, so more distortion.
 

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