Input impedance BJT with input capacitance and bridge capacitance & pole

[AMSA84]

Hi guys,

I would like to know if someone can give me a help here to remember one detail of thevenin equivalent and BJT input impedance.

See the attached image.



I would like to compute the frequency pole at the point B1. From what I understood the solution is something like 1/[(rpi//Rs)(Cpi + 2Cµ)]

What I can't understand is how to get the (rpi // Rs). How's that possible? To get that result we should short-circuit the Vin source, but why, for what reason we should short the input (if that is the solution to get that parallel resistance).

Thank you in advance.

 

[FvM]

but why, for what reason we should short the input

?? A voltage source has zero output impedance by definition, it's always acting as a short for other signals.

 

[D.A.(Tony)Stewart]

Sources never have 0 impedance in practice, so Rs is added for realistic model.
The pole is computed by the net // impedance of source (output Rs) and transistor input Zin.

So the input is not as you have shown. It is after Rs.