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Differential gain of a diff pair

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calculus_cuthbert

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Hi,

I had a very basic question:

For a MOS diff pair: two input NMOS transistors with equal gm and with load resistor Rd what is the differential gain:

so is (vout1-vout2)/(vin1-vin2) = 2(gmRd) ?

this leads to my next question:
For a fully differential amplifier is the gain twice the single ended gain?
Also should the gain of a fully differential amplifier be equal to that of a differential to single ended amplifier assuming the gms and load resistors/impedances are equal?

e.g a diff pair with current mirror load converts a differential input to a single ended output so should the gain of this amplifier be equal to the differential gain of a diff pair with resistive load?

Please help.
 

gain is gmRd! 2Rd/((1/gm1)+(1/gm2))=gmRd

you can get information about this problem in "Design of Analog CMOS Integrated Circuits" Behzad Razavi

2- yes
3-if the both of them is equal, you can be devided and equal that half of them
 

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