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OP AMP why input pmos transistor
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olzanin



Joined: 01 Sep 2003
Posts: 36
Post19 Aug 2009 7:51   

pmos transistor
Hello,

why most of the opamps I have seen have pmos differential input stage and not nmos. Is it regarding
noise or other effects too?

Thanks.

olzanin
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Post19 Aug 2009 7:51   

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mikersia



Joined: 19 Feb 2009
Posts: 41
Helped: 2
Post19 Aug 2009 8:30   

op amp,transistor
due to noise reasons and ground referenced signal design style mostly
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tlihu



Joined: 02 Jan 2002
Posts: 669
Helped: 5
Post19 Aug 2009 10:01   

pmos opamp
One reason is that PMOSFETs have lower flicker noise.
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rfsystem



Joined: 25 Feb 2002
Posts: 858
Helped: 96
Post19 Aug 2009 10:11   

pmos inpute opamp
Next reason is that the body voltage of the MOS differential pair, which is the NWELL for PMOS, could follow the input common mode voltage. Therefore the CMRR is much higher because the body effect and the mismatch of the body effect of the two differential pair MOS is avoided.

If the process have an isolated PWELL you can also use NMOS.
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