# Voltage Amplifier with Feedback BJT

Discussion in 'Homework Help' started by Vicros, Jan 30, 2012.

1. ### Vicros Thread Starter New Member

Jan 22, 2012
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Hey guys,

Been studying a new type of question, part of the BJT family i suppose. This ones a voltage amplifier with feedback, I understand that there is 4 kinds of configurations? Such as current sampling and etc.

I havent tried any other configurations yet apart from this one. Are the process of finding the voltage gain and input/output resistance the same???

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2. ### The Electrician AAC Fanatic!

Oct 9, 2007
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Are you constrained in the methods you can use to solve this problem?

Most of your calculations that I compared to my own were close to the same as mine. There would be better agreement if you would carry more significant digits in your calculations; I carried 12 digits in mine.

One value which I think you have wrong is Rif. The various components of Ri are not all multiplied by the (1 + Av*B) factor. For example, the resistance Rs is not increased by the feedback.

Another interesting thing is that the ratio Av/Af is apparently not the same as Ri/Rf.

You might have to calculate a complete solution to the entire circuit when the feedback resistor is connected. I don't think that application of feedback factors to parameters such as Av and Ri will correctly give the corresponding Af and Rf.

3. ### Vicros Thread Starter New Member

Jan 22, 2012
13
0
thanks for your time for into looking at my solutions.

This is the way it was shown to me, and i was told this method is similar for other configurations, but i havent tried as i would like to get this correct first.

I can sorta understand what you mean about Rif, i will have to think about it a bit more though.

As for Av and Ri, i thought it included the feedback because of the R11 and R22 values that i did right at the start where the 6.8k was the feedback

4. ### Vicros Thread Starter New Member

Jan 22, 2012
13
0
i just had a look into my textbook (microelectronics circuits by sedra/smith) where it showed that the series-shunt configuration is in face Rif = Ri(1+AB)

maybe i have mixed up my topology, series-shunt = voltage amplifier?

5. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,255
311
If Av and Ri include the feedback, then what are Af and Rif?

Last edited: Jan 31, 2012
6. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,255
311
The problem with the standard formalism (the A and B blocks) is that the gain (A) and feedback (B) blocks are assumed to be unilateral. That is, there is no transmission of signal in the reverse direction. In the case of the feedback block, this can be seriously in error. The circuit you are analyzing here has signal transmission from the emitter of the first transistor to the output through the 6800 ohm feedback resistor. This causes the actual values of various parameters (Af, Rif, etc.) to be different from what you would get if the feedback resistor could only transmit signals in one direction.