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Cascoded diode connection for biasing single mirror transsitor

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Junus2012

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Dear friends,

I understand for the wide swing mirror we need cascode diode biasing voltage like the one seen below, no doubt about it

cascodemirror.PNG

However, for the following fully differential circuit shown below, the common mode feedback circuit as you see is using cascoding diode to bias single transistor (M3&M31 gate) of the main amplifier,

this scheme I don't understand why ? is it that necessary ?

biasing.PNG

thank you
 

The feedback loop would help adjust cmfb1 if the cascodes in the cmfb weren't there, but I guess from a DC operating point of view, it helps mimicking the high output impedance current sources in both ends and thus adding the cascodes and shifting up cmfb1 by some mVs.
 
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