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Opamp Input pair: PMOS or NMOS?

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lladnar23

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opamp gbw gm

I have a question regarding the choice of using a PMOS or NMOS devices for the input stage of an opamp.

Which is best to use, in terms of bandwidth of the resulting amp, and in terms of noise? (Obviously, the choice will set the input CM range)
 

pmos input pair vs nmos input pair

NMOS will be better in terms of gm and hence thermal noise and bandwidth ( bandwidth will be larger at the cost of a worse phase margin ). PMOS will be better in terms of flicker noise ( however this might not be true depending on ur technology )
 

why pmos input op amp

Elbadry,

Thank you for your input. Your comments on gm and thermal noise make sense, but could you elaborate a bit on the BW / Phase margin tradeoff of using NMOS versus PMOS? What is the reason?

Thanks
 

I think the PMOS input make the pole2 further away from 0 in foldcascode topology.
 

pmos pair is better than nmos pair , this is given following book with detailed explanation

analog integrated circuit design by david a jhons and ken martin
 

Considering the BW/PM issue:

GBW = gm/C . So higher gm means higher gain bandwidth product for the same C

PM depends on the distance between the non-dominant pole and the unity gain frequency. So higher GBW will mean a smaller PM ( of course this is a very crude approx. The actual thinfg ill depend on ur circuit )

Hope this helps
 

AND THE DFM consideration must been included,as the B and S terminal of NMOS can't been shorted in today's N-SUB processing,which can increase the factor of "base-gate"modulation.
 

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