biasing circuit in op amp

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anhnha

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Hi,

I see that the biasing circuit below is used a lot in op amp design. However, I couldn't find any document explaining how it works. Could anyone tell me some document explain the operating principle of the biasing circuit?
Or could you explain it to me?
Thanks.

 

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No, this is a CMOS PTAT (delta-VT) current loop, similar
to what you'll find at the core of a BJT based bandgap.
Your operating principle is that there's a single current
point where Vgs_m=4(I)+I*40K=Vgs_m=1(I), and the
mirror loop will lock to it (assuming it starts at all).

Beware the starup issues, since there is no explicit
startup circuit evident, and a relatively poor HF PSRR
as a result of weak unbuffered P and N current ref
rails.
 
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    anhnha

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Thank you very much for the explanation!
I think the top 4 mosfets is the improved Wilson current mirror that ensures the current at two branches are equal.
Is that right?
And could you suggest a better biasing circuit that is good as power supply voltage varies a lot and maybe, temperature also changes?
 

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