[SOLVED] Amplifier Input Pair in Subthreshold

[nitishn5]

Is there any problem if the input pair of an error amplifier for an LDO goes into sub-threshold region?

In my case the error amplifier is an NMOS input differential pair with PMOS current mirror load.
In order to reduce flicker noise I have had to increase the sizes which in turn puts the transistors into sub-threshold.

Is there any thing I should watch out for here.

 

[erikl]

No, that's just fine - lower noise, lower Vds,sat and less mismatch. No disadvantage - apart from area consumption.

 

[nitishn5]

How is it low noise?
Flicker Noise would reduce with the larger area but thermal noise would increase with the higher gm won't it?

In my simulations I have found that for an amplifier in feedback, the thermal noise in the the output (not input referred) is while being higher in open loop is reduced in closed loop due the the higher bandwidth due to higher gm.

 

[erikl]

I thought you wanted to reduce the flicker noise - in this case the subthreshold operation would help - the drain-induced thermal noise current will be larger, however. How few noise you can achieve depends very much on your circuit, your application and its bandwidth.

 

[dick_freebird]

There may be a "sweet spot" between flicker and thermal,
which would depend on the attributes of your particular
technology. High Zout also amplifies RTN / RTS current
noise if you have any of that. You have to feel out the
box that you're in.