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[Moved]: transconductance formula for transistor

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deepsetan

deepsetan

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Hi guys,

Can anyone explain to me what is the difference between this two transconductance formula?

gm =UnCox + (Vdd/2 -Vtn)

gm =2pi x gain-bandwidth x Cc
 

Actually, first one is a MOS transistor transconductance formula. And I think it is incorrect. it should be 2*Id/(Vgs-Vth) or sqrt (2*Un*Cox*Id). Second one is not gm of single transistor. This is probably effective gm of an amplifier and if Cc is compansation capacitor, it is probably a two stage miller opamp. In every situation, gm of a mos transistor is not equal to gm of whole amplifier. For example, if you have source degenerated stage, your effective gm is going to be gm/(1+gm*Rs), and gm that you mentioned in your post will be equal to this.

Shortly, first one is gm of a mos (nmos because of Un), second one is effective gm of an amplifier. They are not same in every situation.
 

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