# Input, output impedance of an BJT amplifier

Discussion in 'Homework Help' started by Javad Maleki, Dec 24, 2015.

Dec 24, 2015
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Hello
I should calculate the input and output impedance of the circuit shown below .
The thing I want to know is the basic information for this need .
As an example I don't know the DC source must be zero or the ac source and Exactly where is the input voltage and output voltage and input current and output current .
Thank you

2. ### Dr V S V Mani New Member

Dec 24, 2015
2
2
The circuit was simulated by me using 2N2222A. The output resistance at collector of Q1 is about 3.9k and Rin is about 18k. Measurements was done at 1kHz.
the power supply is considered to be having very low source resistance (less than 1ohm). the input of an ac source is applied at base of Q3 through a capacitor of 10uF. It is an RC coupled amplifier. the current output of Q2 is fed to emitter of Q1 and Q1 operates as a common base circuit with its base bypassed at ac by 10uF. the input impedance can be measured by placing a 1k in series with the ac source, and identifying its effect at base of Q3(not preferred) or collector of Q1 (preferred). Q3 and Q2 for a darlington and hence will have inherently high Rin but this is swamped by r1 and R6 in parallel as far as ac input is concerned. Note that 27||56 is about 18k.
Do let me what you need to know more.

Dec 24, 2015
3
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Thanks for your attention . But the issue is the algorithm of solving the problem . The thing I want is a little explanation about beginning the problem solving .
Thanks for simulation and those result are valuable for me .
But the basic knowledge of this problem is a thing that I haven't it .

4. ### Bordodynov Active Member

May 20, 2015
643
188
Rin=27k||56k||((Betaq3+1)(Betaq2+1)*2*req2)
req2=25.8mV/1mA=25.8Ohm
Rin=27000||56000||((100+1)(100+1)*2*25.8)=1/(1/27000+1/18000+1/(101*101*2*25.8))=17607.5 ==>
Rin=17.6kOhm

5. ### sailorjoe Member

Jun 4, 2013
361
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Do you have a textbook to follow?

Dec 24, 2015
3
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Not a suitable one !
The books have their own route and I can't find my question in them .
If you can please introduce a textbook or ebook !

7. ### sailorjoe Member

Jun 4, 2013
361
63
Electronic Amplifier Circuits: Theory and Design, by Joseph Mayo Pettit and Malcolm Myers McWorlter
Find it on Amazon.com.