# BJT High frequency analysis

Discussion in 'Homework Help' started by Digit0001, Jun 2, 2012.

1. ### Digit0001 Thread Starter Member

Mar 28, 2010
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0
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

2. ### Digit0001 Thread Starter Member

Mar 28, 2010
89
0
so can anyone help me on this question?

3. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Well I don't have a time to do any calculations. But for my in this circuit there is no millers effect.

4. ### Digit0001 Thread Starter Member

Mar 28, 2010
89
0
ok, but from my solutions i have Q1 => C-B and Q2 => C-E is this correct?

5. ### Jony130 AAC Fanatic!

Feb 17, 2009
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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

6. ### Digit0001 Thread Starter Member

Mar 28, 2010
89
0
i have worked it out, forgot to use the gain for common emitter in order to work out the miller's effect

7. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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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.

Last edited: Jun 4, 2012