biasing a differential amplifier

[a1s7trea]

differential amplifier bias

18 Sep 2009 16:00 differential amplifier issue



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Hi,

i am trying to bias a simple differentila amplifier

there is a 1:1 current sink at the bottom, theoretically the o/p of the current mirror should give 500uA but as soon as I am connecting it to the sources of the nmos differential pair the current drops down

I hope the attachemnt will help

Please advice

Regards

George

 

[dick_freebird]

biasing differential amplifiers

You are way overdriving the devices for their size, so
they do not act as current sources, but resistors with
a loose voltage control. In the linear region you must
match both vds and vgs to match current. Up the width
by a factor of 10-100 and see how it changes the fidelity.
Or raise the feed resistor 10X and the mirror output 10X,
so you don't wast half the circuit current in overhead.

You want subthreshold and low current density despite
the relatively large current.

 

[a1s7trea]

What about increasing the input volatge at M2 and M3 will that help on order to give more headroom for M1

Regards

George

 

[dick_freebird]

It may, but from the look of things you would need
more headroom than your 1.2V supply voltage (Vgs
is already at 1.15V, you need well more than this on
the drain before it becomes anything like a current
source).

 

[a1s7trea]

Thanks

I will try this one first

Best Regards

George