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Please see attached image. I struggling to understand why Rs is negelected in the input impedance; should'nt the total input impedance be the parrallel combination of the capacitor impedance with the Rpi plus in series with Rs?

Cascode Input and Output Impedance

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Yes !

In the referenced equation, Rs is not referenced. The capacitance is. Clearly the signal at the base of Q1 does depend on the value of Rs -- it is in series with the input signal, and there is (at least) a capacitance to GND.

A little inspection will show that the true impedance is Rs+...

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