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question about the difference between the NMOS and PMOS load

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abcyin

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Hi, all

the pic below shows two amplifier, the left is with a PMOS active load while the right one with an NMOS active load, and we always use the PMOS as the load, what confused me is can I use the NMOS as the load, what's the difference between them, or is there any advantages with PMOS load?

Thx in advance!
 

Re: question about the difference between the NMOS and PMOS

hi abcyin,

there is large body effect with NMOS because of nwell process, so the output swing is restricted with NMOS load.
 

Re: question about the difference between the NMOS and PMOS

But I use p-sub, and there is no nwell in the process, what do you mean by "large body effect with NMOS because of nwell process", however, I got a message from your reply, the large body effect will increase the threashold voltage of the NMOS load, then compress the swing, am I right?

and any other reasons?
 

Re: question about the difference between the NMOS and PMOS

n-well is used in p-sub, so n-well process and p-sub process is the same. due to body effect the threshold voltage of nmos is large and sensitive to positive power supply. there is no this problem for pmos in nwell with nwell contact connected to source.
 

Re: question about the difference between the NMOS and PMOS

I do not think there is much differences. Diffremnce is that PMOS load use to load the current while NMOS use to supply the current. In the sense NMOS load work as a source and PMOS load use to get the output.
 

Re: question about the difference between the NMOS and PMOS

abcyin said:
But I use p-sub, and there is no nwell in the process, what do you mean by "large body effect with NMOS because of nwell process", however, I got a message from your reply, the large body effect will increase the threashold voltage of the NMOS load, then compress the swing, am I right?

and any other reasons?
Click to expand...

theoretically, they are both diode connected and identical.
but in fact, it's a really bad design having your source voltage varying with signal (if using NMOS load).
That means if your Vout goes up and down, body effect will change the NMOS threshold voltage, which changes gm also. That's a disaster for AC operation.

If it's used for DC operation only, I don't see any difference between them.
 

Re: question about the difference between the NMOS and PMOS

digito said:
abcyin said:
But I use p-sub, and there is no nwell in the process, what do you mean by "large body effect with NMOS because of nwell process", however, I got a message from your reply, the large body effect will increase the threashold voltage of the NMOS load, then compress the swing, am I right?

and any other reasons?
Click to expand...

theoretically, they are both diode connected and identical.
but in fact, it's a really bad design having your source voltage varying with signal (if using NMOS load).
That means if your Vout goes up and down, body effect will change the NMOS threshold voltage, which changes gm also. That's a disaster for AC operation.

If it's used for DC operation only, I don't see any difference between them.
Click to expand...

Thx very much!
 

Re: question about the difference between the NMOS and PMOS

abcyin said:
Hi, all

the pic below shows two amplifier, the left is with a PMOS active load while the right one with an NMOS active load, and we always use the PMOS as the load, what confused me is can I use the NMOS as the load, what's the difference between them, or is there any advantages with PMOS load?

Thx in advance!
Click to expand...

Why use this structure amplifier?
I think it has bad linearity and the current of it can't be controlled.
 

Re: question about the difference between the NMOS and PMOS

digito said:
abcyin said:
But I use p-sub, and there is no nwell in the process, what do you mean by "large body effect with NMOS because of nwell process", however, I got a message from your reply, the large body effect will increase the threashold voltage of the NMOS load, then compress the swing, am I right?

and any other reasons?
Click to expand...

theoretically, they are both diode connected and identical.
but in fact, it's a really bad design having your source voltage varying with signal (if using NMOS load).
That means if your Vout goes up and down, body effect will change the NMOS threshold voltage, which changes gm also. That's a disaster for AC operation.

If it's used for DC operation only, I don't see any difference between them.
Click to expand...

Not the complete story, you're neglecting the afore mentioned body effect. The PMOS will be in it's own well, connected to Vdd the same as the source. Whereas an NMOS device the source is not tied to the the same point as the bulk, thus the body effect will be at play.

There's also the layout area - much greater por a PMOS (if an NWELL is required).
 

let me add one,when you use nmos connect to Vdd,ESD is a problem. for safty should add a res
 

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