Differential pair, current mirror: high frequency analysis

[eliotsbowe]

Hello, I have a doubt about the high frequency response of the MOSFET (or BJT as well) differential couple in the case it's polarized by a current mirror.

Here's a picture of my issue (it shows the semi-differential circuit of a MOSFET differential couple with a "small" common mode ac input; the current mirror was replaced by its high frequency small signal model)
http://s3.postimage.org/x84o1dsxv/asdasd.jpg

The picture says (sorry for posting a non-english text) that Cgdm is to be considered a bypass capacitor and the MOSFET should be replaced by its low frequency (!) small signal model.
The sense of this is that Cgdm "exists" at much lower frequencies than the internal capacitances of the MOSFETs composing the differential couple.

I found the same statement about the BJT differential couple (with a BJT-based current mirror connected to the emitter): the internal capacitance of the mirror dominates the low frequency response of the whole circuit and it acts like a bypass capacitor.

So, my question is: why is there such a difference between the "life" (on the frequency axis) of the capacitance of the mirror and the one of the internal capacitances of the two main transistors?

Thanks in advance for your help.

 

[eliotsbowe]

I found it. The Sedra-Smith's book says that the interaction between 2r0 and Cgdm creates a low frequency zero. Every other internal capacitance is to be considered as an open circuit, then.