PMOS diff pair as 1st stage and nmos common source 2nd stage

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kavitha_rapolu

Joined: 04 Jan 2008
Posts: 6

06 Jan 2008 4:03 why pmos has low flicker noise than nmos

Does any one know the advantages of having the combination of
pmos diff pair as 1st stage and nmos common source stage as 2nd stage
for designing opamp ?
Is it related to phase margin and bandwidth ? If yes , why ?

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milkdragon

Joined: 03 May 2007
Posts: 64
Helped: 6

06 Jan 2008 6:44 Re: PMOS diff pair as 1st stage and nmos common source 2nd s

With PMOS as first stage input, slew rate can be maximized because typically first stage have large Veff. Also, with NMOS second stage can maximize transconductance for high frequency operation

watersky

Joined: 20 Aug 2004
Posts: 78
Helped: 2

06 Jan 2008 7:12 PMOS diff pair as 1st stage and nmos common source 2nd stage

you can read book of Allen, on which there are some different aspects about nmos and pmos as input.

wael_wael

Joined: 03 Dec 2005
Posts: 225
Helped: 2

06 Jan 2008 8:53 Re: PMOS diff pair as 1st stage and nmos common source 2nd s

ok, i can till u from my experience that using the PMOS transistor has many disadvantages because is reducing the the gain and the phase margin(phase margin is related to selw rate) of the OP AMP, advantage of using PMOS is low fliker ngardsoise whish can be solved by using many techniques, my advice to u if u r looking to design high performance OP AMP use NMOS, pls if i am not correct, correct me
regards

safwatonline

Joined: 19 Nov 2005
Posts: 1418
Helped: 172
Location: EGYPT

06 Jan 2008 9:02 PMOS diff pair as 1st stage and nmos common source 2nd stage

well, i am not sure but i dont get the PM degredation part from my point of view u need to have the second stage's gm larege for pole splitting which means that using a second NMOS stage after the first PMOS stage is good for PM, i think the main advantage of using a first PMOS stage is the noise and PSRR

A.Anand Srinivasan

Joined: 15 Oct 2005
Posts: 1792
Helped: 216
Location: India

06 Jan 2008 10:51 PMOS diff pair as 1st stage and nmos common source 2nd stage

PMOS has lower flicker noise,,,... it is one of the important reasons behind using PMOS input stage for many circuits....

santi001

Joined: 24 Sep 2007
Posts: 17
Helped: 5

06 Jan 2008 11:07 Re: PMOS diff pair as 1st stage and nmos common source 2nd s

1) With pmos input pair the noise numbers will be better because of inherent physical property.

2) If you have pmos input stage and the stage is diff in and diff out, the second
has to be nmos to ensure devices in saturation region. Specially if you are trying
for high bandwidth amp

3) With PMOS in the 1st stage and nmos in the second stage. You will have symmetry in the signal path. In this case it is single ended input and single ended output.

vivek_raj_verma

Joined: 28 Aug 2005
Posts: 23
Helped: 2

15 Mar 2008 11:01 PMOS diff pair as 1st stage and nmos common source 2nd stage

I think its better to use Nmos op amp. but depending on situation like whats the input common mode ...we need to decide which will be best in terms of OVD and SAT margins. Second stage must be a gain stage and NMOS always has adv of smaller size (so smaller parasitics) over Pmos counter part.

Sadegh.j

Joined: 29 Nov 2006
Posts: 803
Helped: 33

15 Mar 2008 14:25 Re: PMOS diff pair as 1st stage and nmos common source 2nd s

PMOS transistor are low noise, at least lower than NMOS, but their gain is also lower. The noise of the first stage affects the output noise figure considerably. Thus the first stage is PMOS and the second stage is NMOS.

jnuhope

Joined: 10 Apr 2007
Posts: 53
Helped: 4

15 Mar 2008 18:52 PMOS diff pair as 1st stage and nmos common source 2nd stage

Textbook says by using pmos as 1st stage followed by nmos as 2nd stage, you get a higher slew rate and lower flicker noise. John Martin's book even says "...hence most knowledgeable designer use pmos input."

But I havn't seen many pmos as 1st stage input in recent jssc papers. Most authors use nmos-nmos combinations. Can anyone comment on this? Tks in advance!