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Operating a NMOS in subthreshold

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mischivis

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I have an NMOS cascode current mirror which provides a reference voltage for a folded
cascode amplifier. Right now this transistor is going into sub-threshold region (Vgs<Vt).
What impact will this have?
 

The standard sub-threshold advantages and disadvantages:

+ low power consumption
+ best gm/ID
+ low VDS,sat
+ low thermal noise voltage

- high thermal noise current
- low bandwidth
- high layout area
- high Vt mismatch (if matching needed)
 
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    naras1

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Why is a low overdrive (Vdsat) a good thing? I thought it is good
to have a high overdrive for mismatch purposes.
 

There's mismatch, and there's mismatch.

Natural VT mismatch is going to be insignificant compared
to operating two FETS, one in linear and one in saturation,
for Id matching. Lower Vgs gives you a lesser headroom
requirement for Vds induced mismatching to recede as a
problem.
 

BTW, Vgs<Vt is not necessarily a condition for subthreshold. What is your technology? And how much is Vgs lower than Vt?
 

For the current mirror,In sub-thresold region you are not able to provide high impedance for tail current requirement as currents flowing is in nano amps.
 

sevak112 said:
For the current mirror,In sub-thresold region you are not able to provide high impedance for tail current requirement as currents flowing is in nano amps.
Click to expand...

On the contrary: with a relatively low W/L you can achieve a very high output impedance VA/ID ~ 10V/100nA = 100MΩ !
 

erikl said:
On the contrary: with a relatively low W/L you can achieve a very high output impedance VA/ID ~ 10V/100nA = 100MΩ !
Click to expand...
you are right.Don't need to keep all transistor in sub-thresold by that we can increase voltage headroom and also constant mirror current.
 

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