Subthreshold Amplifier

[bigheadfish]

subthreshold common source amplifier design

When mos transistor is in subthreshold region,why common source amplifier's gain is a constant value?

 

[suria3]

op amp subthreshold

I don't really understand your question. please write properly again so that people can give their input.

 

[bigheadfish]

I don't really understand your question. please write properly again so that people can give their input.

Voltage gain versus Id for a typical mos transistor,at very low current(i.e mos is in subthreshold region), the mos voltage gain approaches a constant value.Why? Thanks!

 

[bamboo]

I don't understand about your question clearly.
But in subthreshold region for a mos, we can get
Id = Io EXP[Vgs/(ζ*Vt)] k is a constant number
gm=δ(Id)/δ(Vgs) = Id/(ζ*Vt)
Av = gm*ro ≈Id/(ζ*Vt) * 1/(λ*Id*k) => Av is indepedent of Id
So if in your question, the condition is like this.
Then the Av is indepedent of Id.

 

[scheruku]

Hi,
In subthreshold region Id is exponetial function of Vgs, so gm will directly proportional to Id.
So gain (gm/Id) in subthreshold region is constant..., I hope this will clear your doubt.

Cheruku

 

[mickey_melomane]

Is Gain of the amplifier very large in subthreshold region... I get values of gm of around 5mS...

If that is so, why dont opamps use MOS in subthreshold region...

Sidharth

 

[yschuang]

Opamp is very slow in sub-threshold region due to low gm.