Hi
Can someone tell me what i have done wrong in the following question?
The question and my solutions are below:
View attachment question10.pdf


The answer is supposed to be fH=2.97MHz

P.S

 

so can anyone help me on this question?

 

Well I don't have a time to do any calculations. But for my in this circuit there is no millers effect.

 

ok, but from my solutions i have Q1 => C-B and Q2 => C-E is this correct?

 

Yes Q2 works as a CE amplifier but the gain of this amplifier is close to one.
And Q1 work as CB amplifier and CB amplifier is free from miller effect

 

i have worked it out, forgot to use the gain for common emitter in order to work out the miller's effect

 

Interestingly, I've found a more careful analysis / simulation gives the upper -3dB point as ~3.8MHz [rather than 2.97MHz] which suggests the approximations applied aren't particularly robust. The model I used was the Hybrid-Pi. Rather than applying the Miller theorem I left Cμ [2pF] from the two BJT's collector-to-base terminals. Cbe was 5.96pF per the relationship fT=gm/(2∏(Cμ+Cbe). Also used gm=20mA/V and r∏=5k.