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# frequency response of BJT amplifier

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#### funjoke

##### Member level 3
A cascode amplifier is shown . Given that VCC = 12V, RS = 50Ω, R1= 43kΩ, R2 = 15kΩ, R3 = 12kΩ, RC = 2.7kΩ, RE = 1.2kΩ, RL = 4.7kΩ, CB1 =10μF ,CB2 =CC2=4.7 and CE = 100 . Assume junction capacitance Cbe= 9pF, Cbc=3pF, thermal voltage VT =25.8mV, βac=150 and VBE=0.7V for both transistors

(i) Determine all high cutoff frequencies due to Cbe and Cbc.

can anyone got the formula of how to find VB1 .i use the (R2/R1+R2)VCC.but the answer not correct.

Please post schematics as pictures (PNG tends to be the best choice).

There is many reason for this:
* Much smaller file size
* It doesn't take a lot of time to open the office application.
* User might not have an office compatible application installed.
* Other users can see the picture directly.
* Better chance to get help becase some users might not see your schematics

So I was nice and did the conversion job

anyway do you know what is the formula for this question and how to find VB1 ?

No, I'm sorry I doesn't. If I had, I would try to help. This amplifier of you is unknown. Is there any reasons for using two transistors in cascade?

First thought I had about this was to forget about Q2, and just calculate it the same way as for single transistor amplifier. But I'm not sure if this is the way.

BTW: You can also apply labels on the schematic. I recomend Faststone Image Viewer if bitmap is all you have.

funjoke said:
can anyone got the formula of how to find VB1 .i use the (R2/R1+R2)VCC.but the answer not correct.
I think you forgot R3.

I think this is a little hard to solve. Hard part is how to start. T.ex you have to take in account for that Ie on Q2 (Ic on Q1) is not the same as current from Rl because you have to add Ib on Q2 also.

this question need to sketch bode magnitude and bode phase plot for high frequency.(indicate all high cutoff frequencies,roll-off rate and phase shift)

anyone who know how to draw this ?

funjoke said:
anyone who know how to draw this ?
Have you considering drawing this by hand? It's the least time-consuming task, because otherwise you have to find software, install software and learn to use software to do the drawing.

ya i will use hand draw but i dunno how to find Av and phase shift

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