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cascode BJT output impedance

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tshankar501

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When I do not connect any load resistance Rd, what is the output impedance of the cascode BJT in terms of gm,ro,r π (pi)?
 

gout=VE/IE

VE: Early Voltage 100-500V for some discrete bipolars, 5-40V for integrated bipolars
IE: Emitter Current
 

The impedance looking into the collector of a cascoded BJT transistor is:

82_1163833204.JPG


Re is the impedance from emitter to ground, if there is another BJT there just replace it with the ro of that. Rb is a resistor tied to the base if any, otherwise it is 0.
 
Thanks for your formula....but my objective is to obtain the output impedance of the cascode configuration of three or bjts in order to obtain the high output impedance.....like a triple cascode or the quadraple cascode....In this formula, if i assume Rpi small, just a base emitter resitance, Rb=0,

The 3rd expression is just Re in parallel with Rpi => Rpi(small).

The 2nd expression after approximation is gm*ro*Rpi....this is the answer for 2 stage in this approximation.....

Even if i have triple cascode or more cascoded stages, then I get the same formula....gm*ro*Rpi.....

So I am confused now......
 

You are getting the same expression due to your approximation Rpi small, which I don't understand why you are taking that, Rpi is supposed to be quite large
Rpi=β/gm and β is in hundreds and gm may be in µS or mS, so Rpi is quite large!
 
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