[问]关于CASCODE运放

cisse

看allen的书，295页讲到一个单级cascode不太理解，说impedence level接近1/Gm,不是因为cascode结构而电阻变得很大吗？还有后面说到可以用shunt capacitance（并联电容？）来self-compensation，这个原理也没弄懂。
请大家指点一二！

langtaosha

你说得太不清晰

lianggu

1/gm (actually, 2/gm) is input impedence of common gate amplifier which is inside cascode, not output impedence of cascode amplifier.

As for differential cascode op-amp (one stage), the dominant pole is at the output node. Thus, load capacitor is also used for compensation. It is so-called "self-compensation". Most one-stage op-amps have this feature.

microuser

楼上的解释很棒，高手！！

cisse

the previous sentence：If we examine other internal node of Fig.6.5.1.So i think that the impedance levels means other node's impedance.For example,the source of the MC2,MC1,MC3,MC4.Am I right?What about impedance of the gate of them?

cisse

you say the input impedance is 2/Gm.I think when the Rdsmc2=R(the impedance looking into the source of the MC4),it's OK?Am I right?
And the sentance:the output capactiance is increased,the dominant pole decreases,maintaining a constant phase margin.I think its because the second pole is so large that it dosen't change.Am I right?

lianggu

原帖由 cisse 于 2007-6-18 09:03 发表
the previous sentence：If we examine other internal node of Fig.6.5.1.So i think that the impedance levels means other node's impedance.For example,the source of the MC2,MC1,MC3,MC4.Am I right?Wh ...

Yes. But why do you want to know the impedence of common gate? I do not think it makes sense.

cisse

i think there is a mirror pole

lianggu

原帖由 cisse 于 2007-6-18 09:33 发表
you say the input impedance is 2/Gm.I think when the Rdsmc2=R(the impedance looking into the source of the MC4),it's OK?Am I right?
And the sentance:the output capactiance is increased,the domin ...

I do not understand the first question. "2/Gm" is a general result for cascode structure. It is derived from Kirchoff law. However, if you try to derive it from the point view of physics, you will find it is "1/Gm". The difference is small but very critical since no similiar case exsits in other circuits.

As for the 2nd question, the output impedence is pretty larger than any other nodes. Thus, dominant pole is at the ouput while the other poles are high frequency.

cisse

for the first question i have some touble.The principle what you said is at the page of 203.I can read it,but i dont know how to differentiate the 1/Gm and 2/Gm.In other words,when should i conside the resistance looking into the source of MOS is 1/Gm or 2/Gm.
the second i understand.thank you very much.I have posted message to you.