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Cross couple differential pair

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IvanTheTerrible

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In the text, "If a high impedance current source is provided to the source follower and gm5/cgs4 is much higher than signal freq, VB is considered to be constant."

question 1: why a high impendance current source is needed at the source of M5.

question 2: How do you come with the constranit gm5/cgs4 is much higher than signal freq?

<Text taken from "Linear RF CMOS design" , a thesis by YongwangDing. >
 

1. As the input voltage changes, the voltage at source node of m5 starts to change. If the ouput impedance of current source is low, then current of m5 will change and therefore vb will change.
Also, here is preferred to connect the bulk of m5 to it's source to get rid of nonlinearity of vth as z function of vin. For this you isolated p-well for m5.

2. Here it is assumed that voltage across the gate-source of m5 is constant. The dominant pole of the source-follower stage is gm/cgs which means that at frequencies higher than this you can assume that vb is constant and changes as frequency changes.
 
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