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Transconductance of input stage in folded cascoded opamps

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salmiakki

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Hi!

What things I have to take in consideration when I'm dimensioning the transconductance of the diff.pair in folded-cascoded opamp? Large gm -> more gain, unity-gain freq better on the other hand to get larger gm the area of input transistor's will increase.

Thanks!
 

Re: Transconductance of input stage in folded cascoded opamp

operate the transistors at low overdrive voltages.
 

Re: Transconductance of input stage in folded cascoded opamp

it`s depend of the specification tou are trying to cover, you see in some cases gain is the main variable but in other it could be the BW. try to review the specification and find the rigth one
 

Re: Transconductance of input stage in folded cascoded opamp

I think the input stage in any ops is very important. It related to offset voltage, nosie behavior, gain and etc. So it depends on your spec.

Yibin.
 

Re: Transconductance of input stage in folded cascoded opamp

I think large input gm have many benefit, such larger gain, less input noise, lower offset. but a disadvantage is that linearity become poor, beacuse of larger overdrive voltage .
 

Re: Transconductance of input stage in folded cascoded opamp

parasitic gate resistance shoule be considered for very large w input transistor, using multi-slice layout and connecting both end may reduce it.

And for very large w/l and low drain current, transistor may work at weak inversion or sub-threshold region. gm stays constant with respect to drain current in this region.
 

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    salmiakki

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Re: Transconductance of input stage in folded cascoded opamp

firstly, u must know what performances you care, there are many tradeoffs in designs

kind regards,
 

Re: Transconductance of input stage in folded cascoded opamp

Take care, gm increase with sqrt(W) while the cap increseas with W, so increasing the W is not a good bargain always. It might hurt the performance in a switched cap circuit (is an optimum there, you should find it).
Other problem is that it lowers the second pole => decreased phase margin => overshoot/ ringing (bad performance) or worse, stability problems.
 

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    salmiakki

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