# BJT op amp equivalent circuit AC and DC analysis.

Discussion in 'Homework Help' started by hiphop, Oct 10, 2016.

1. ### hiphop Thread Starter New Member

Oct 10, 2016
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We have to find the overall voltage gain of this circuit i.e Ad=V0/(V1-V3).
I have found the collector currents through each resistor except for transistors Q7 and Q8.
Also I have found the voltage gain for the first stage(consisting of Q1,Q2,Q3 and Q4) and the second stage (Q5 and Q6).
Now I know that to find the total voltage gain , I have to multiply the voltage gain of each stage taking in account the input resistance of the next stage .
But how do I find the voltage gain and input resistance of the third stage (Q7 and Q8).

2. ### The Electrician AAC Fanatic!

Oct 9, 2007
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Since there is no circuitry to bias both Q7 and Q8 into their linear operating region simultaneously, only one of Q7/Q8 will be on when the output is more than .7 volt above or below ground. For a complementary class B emitter follower like this, it would be a reasonable approximation to say that the Q7/Q8 voltage gain is unity, and the input resistance is just β times the output load resistance. The Q7/Q8 output circuit will produce substantial crossover distortion.

Last edited: Oct 10, 2016

Oct 10, 2016
5
0

Oct 9, 2007
2,281
326

5. ### hiphop Thread Starter New Member

Oct 10, 2016
5
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Sorry for that , anyway I did the calculations again and it is correct.
Also can you recommend a book which deals with differential amplifiers , op amps and their quantitative analysis .

Oct 9, 2007
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7. ### hiphop Thread Starter New Member

Oct 10, 2016
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Ok. Thank you for your help.