Opamp Input pair: PMOS or NMOS?

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lladnar23

Joined: 20 Mar 2006
Posts: 221
Helped: 32

15 May 2007 4:56 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)

elbadry

Joined: 06 May 2005
Posts: 363
Helped: 62
Location: Egypt

15 May 2007 7:50 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 )

lladnar23

Joined: 20 Mar 2006
Posts: 221
Helped: 32

15 May 2007 21:35 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

chenjia

Joined: 16 Apr 2005
Posts: 68
Helped: 5

16 May 2007 4:25 Opamp Input pair: PMOS or NMOS?

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

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ravitest

Joined: 01 Feb 2007
Posts: 359
Helped: 12
Location: india

25 May 2007 11:14 Re: Opamp Input pair: PMOS or NMOS?

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

elbadry

Joined: 06 May 2005
Posts: 363
Helped: 62
Location: Egypt

27 May 2007 1:34 Re: Opamp Input pair: PMOS or NMOS?

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

zhaoshibin

Joined: 11 Oct 2006
Posts: 44
Helped: 2

27 May 2007 7:28 Opamp Input pair: PMOS or NMOS?

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.