how to analyse the bias circuit?

[wjxcom]

Hi all: please look at the OPA shown in the attachment. I do not know why the gate of M45 connectes to the drain of M44? at the same time, why the source of M5 correctes to the source of M2?

I do not know what is the advantage of this bias! may be somebody can introduce some papers to me!

Help me, please. Thanx!!

 

[niezimei]

for providing a bias voltage for the cascode transistors.

 

[johnsmith101]

Gate of M45 connects to drain of M44 to provide the second gain stage in the amplifier. M45 is a typical common source stage.

The source of M5 connects to the source of M2 in order to provide the proper gate voltage to M3 and M4 since the source of M2/M1/M5 varies with common-mode input voltage.

 

[wjxcom]

Hi johnsmith101, you said "Gate of M45 connects to drain of M44 to provide the second gain stage in the amplifier", but I think this circuit has no the second gain stage, because the OUTPUT is the drain of M44 and M4.

 

[liuyonggen_1]

in my opinion, there are a feedback(not only bias the cascode transistor)

 

[wjxcom]

Hi liuyonggen_1, but can you explain detailed that why there have a feedback? and how to bais the cascode transistor?

thanx!

 

[liuyonggen_1]

Hi liuyonggen_1, but can you explain detailed that why there have a feedback? and how to bais the cascode transistor?

thanx!

In my opinion, the transconductance of the amplifier could be varied with the different voltage of the input terminals.
when there is no differential voltage, the transcoductance become minimum.
maybe, you should chech the application of the product.

usually,we use a self-biasd cascode to bias another cascode.

in my opinion, it's not good to use m5 to bias m3 and m4, which will degrade the output resistance of the amplifier. however, i think it's the simplest way.