Differential amplifier design

[serma]

Hi i have a doubt in designing differential amplifier, generally we will be designing the W/L ratio of diff amplifier in large, what is the reason behind this.... what happens if we increase the length in pmos diff amplifier?

 

[ducvilla]

Hi serma,

As far as i know, W and L of differential input pair will be consider by some following specifications:
1. W/L is large to increasing gm, so it means that gain also increases.
2. L need to be small, and W to be large to minimize device offset: Vos is ~ 1/(W*L)

Normally, depend on the gain and offset requirement, you will consider the range of W/L and W*L to get the answer: W=? and L=?

Hope this help!

 

[Baas Rietrot]

I am assuming you are using a PMOS current mirror for the differential and NMOS for the amplification together with a current source.

Remember: your PMOS should be larger in Width than your NMOS.

n=Kn/Kp

Wp=nWn

 

[dgnani]

Hi i have a doubt in designing differential amplifier, generally we will be designing the W/L ratio of diff amplifier in large, what is the reason behind this.... what happens if we increase the length in pmos diff amplifier?

Operating at large W/L pushes the diff pair towards weaker inversion which is the OP region of highest transconductance efficiency (for a given current). Increasing L will in general reduce the intrinsic bandwidth of your devices but if you do not have a stringent speed requirement then there are advantages in choosing longer devices (e.g. increased gds). As already noted above large W/L (or large L for that matter) also tends to increase area which has a positive impact on both matching and noise. Matching determines your random voltage offset but also your supply rejection...

 

[serma]

thank you frnds, hope this helpful