关于Folded-cascode中，input-cascod管子作用的直观分析！

199619919

为什么能纠结这么久。。我觉得楼主是对的，小信号也分低频高频，有很多人认为小信号就一定高频电容就一定短路了，其实是不对的

feynmancgz

回复 94# JoyShockley


   I think I have already explained it clear enough.
   To be more clear, if the frequency is lower than the dominant pole, of course, the resistance is ro.
   When the frequency is higher than the dominant pole, the resistance starts to decrease.
   Some maths: just approm
   the dominant pole, 1/(gm*ro*ro*CL), the pole at the folded node: 1/(ro*Cfolded)
   the resistance at the folded node is (ro+gm*ro*ro)/(1+gm*ro)
   so the speed of the increasing of this pole and the decreasing of the resistance is the same.
   So when this pole can have effect on the circuit, its resistance is already 1/gm.

   Of course, you are right. This happens at high frequency. But the CL does not have to be large.
   At low frequency, the pole at the folded node is of course low. But it has no visible effect on the circuit.
   That's why we regard cascode single stage, instead of two stage.

feynmancgz

回复 96# pigintree


   why do you think the frequency of the pole is fixed for all operation frequencies ?

pigintree

回复 103# feynmancgz


    原来如此，明白了，多谢版主！！

gaojun927

回复 103# feynmancgz

你这么说的话，会让很多人更困惑的。你这里所说的pole已经不是教科书里定义的pole，教科书中的pole位置是不会变的。你所说的pole更接近于”如果真有一个节点代表一个pole这个事，那么这个pole差不多可以用这个节点的RC时间常数来反映，不过这么简单的事情是不会经常在电路里发生的，所以你引入了一个可变的RC时间常数来反映这个事实。在不同频率段RC时间常数是不同的，但是只有这个pole起作用的频率段下的RC时间常数才是你需要的“。

pigintree

回复  feynmancgz

你这么说的话，会让很多人更困惑的。你这里所说的pole已经不是教科书里定义的pole，教 ...
gaojun927 发表于 2013-10-12 08:57

                               
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    恩，我觉得版主就是这个意思。不过我从未在书本里见过这种说法

feynmancgz

回复 105# gaojun927


   Yes, you are right. This may makes people confused.
   The frequencies of poles and zeros are fixed for a specific circuit.
   What I mean actually is the way to analyze the poles and zeros at low frequency, then assume the poles and zeros are just there, is wrong. Ok, still confused.
   Actually, the fact is that the "pole" and "zero" we get at low frequency is not "real" poles and zeros, seems like poles and zeros, but they are not. The "real" poles and zeros are those who can have effect on the circuits. If they can't show effects, they are not poles or zeros.
   Thank you a lot to mention that !

feynmancgz

回复 106# pigintree


   Actually that's not what I mean, my saying makes people confused.
   I posted something, but it didn't show

feynmancgz

回复 105# gaojun927


   Yes you are right, what I said makes people confused

feynmancgz

回复 105# gaojun927


   What I mean is actually, the "poles" and "zeros" we get at low frequencies are actually not real poles and zeros.
   Poles and zeros are fixed for a specific circuit.
   Poles and zeros are those can show effects on circuits, if they didn't show because of feedback reasons, they are not real poles or zeros.

   Thank you a lot for mentioning that !