Hybrid Pi

[Micheal]

Hi All,

Please help to find an expression for Beta cutoff frequency f(beta), which is simply a high frequency current-gain cutoff point of the transistor with collector and emitter terminals shorted as shown in the circuit attached.

Thanks..
Micheal

 

[The Electrician]

Are you sure that's the right model? You have RΠ (that's Rpi) in there twice.

 

[Micheal]

Hi Electrician,

Yup. The question exactly like this, this is the right model. Please help.

Thanks
Micheal

 

[The Electrician]

Have you tried to write the expression for the voltage appearing at the B' node with respect to an applied signal at B? It's just a simple voltage divider. Since the output is shorted, there is no further frequency dependence past B'.

 

[indulis]

There is the cap in parallel with Rπ that would come into play...

 

[Micheal]

Hi Electrician,

What should I do once I get the voltage at B' ?

Hi indulis,

What do you mean on that? What should I looking for at first? current , voltage or ...... ?

Thanks

Micheal

 

[The Electrician]

You see the dependent current source, labeled gm*Vb'e; that provides the output current, which feeds into a short.

The transconductance, gm, is not frequency dependent, so the rolloff with frequency only occurs due to the shunting effect of the two capacitors.

Calculate the voltage at B' as a function of frequency, and you've got the cutoff frequency.

 

[Micheal]

Thanks Electrician, I will try to solve it and anything qurries will come back again.