noise of pmos and nmos

[lhlbluesky]

pmos or nmos better for noise

for 1/f noise, large W L can decrease it; for the same size of pmos and nmos transistor , the 1/f noise of pmos transistor is smaller than nmos transistor, but why? besides, for the same size of pmos and nmos transistor , whose total noise(thermal noise, 1/f noise and shot noise etc)is smaller, pmos or nmos transistor? why? pls explain to me.thanks

 

[ieropsaltic]

noise nmos

I think that the most popular explanation for the lower flicker noise of the PMOS is because the PMOS i s implemented into separate wells which decreases its flicker noise.

Concerning which will have lower noise, PMOS or NMOS, there's no exact answer for that question. It depends on the circiut, transistor sizing and the frequency of interest. For example, If you're designing an amplifier, for lower frequency offsets, the noise of PMOS may be better than NMOS because of flicker noise. However, at higher frequency offsets, the input referred noise of the NMOS will be better because of the higher gm. If you're designing a current mirror, the output noise current will be better for a PMOS than an NMOS for the same sizing because of lower gm.

 

[endru]

why pmos is preferred over nmos

However, at higher frequency offsets, the input referred noise of the NMOS will be better because of the higher gm. If you're designing a current mirror, the output noise current will be better for a PMOS than an NMOS for the same sizing because of lower gm.

Hi ieropsaltic,

May I know why higher gm is preferred regarding the input-referred noise of amplifier, while lower gm is preferred for current mirror? Thanks!

 

[ieropsaltic]

flicker noise pmos nmos

Higher gm is preferred for gain devices, because signal (power) gain will be proprtional to gm^2, while the output noise power is proprtional to gm so the input referred noise power will be proprtional to 1/gm (i.e as gm increases input referred noise decreases).
For current mirrors, there's no signal, so the output noise will be proportional to gm.