# Simplify AC analysis of bjt amplifier circuit with two-ports

Discussion in 'Analog & Mixed-Signal Design' started by Hangster, Sep 23, 2016.

1. ### Hangster Thread Starter New Member

Sep 23, 2016
2
0
I recently got interested in signal processing and amplifiers. In the following file "Two_port_network_bjt" on the first page i could simplify a simple common emitter amplifier with two ports. Afterwards i think its easier to tell the input impedance Zi, ouput impedance Zo and the voltage gain Vu...

Am i right if:
(without R load and R collector)

Zi = (B+1)*RE+rBE
Zo = RE+rCE

Vu = VCE/VBE = (RE-BrCE) / ((RE+rCE) * ((B+1)*RE+rBE))

On the second page i am stuck at this collector feedback circuit with emitter resistor RE and feedback resistor RF. Could please anybody help me out or give an advice and correct me if i made a mistake? Its not clear how the two port would look like if there is this RF resistor.

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2. ### debdut123 New Member

Aug 30, 2016
10
1
Hello Hangster, the circuit at page 2 is correct.
I think you need to name the voltage at positive terminal of Re as, say, Vx, and solve it in terms of Vin, using KCL and KVL.

3. ### Hangster Thread Starter New Member

Sep 23, 2016
2
0
Hello debut123, firstly thank you for your response.

I am quite sure what you mean, but if i get this equations with KCL and KVL and after merging them, there is one equation left with Ic or B*IB, which i can't get rid of by using two port rules...

Maybe i do not see the trick hiding behind this circuit, nethertheless i have already contacted a teacher on my university and hopefully i will get an answer