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). 1.jpg
    I have found the collector currents through each resistor except for transistors Q7 and Q8. 2.jpg
    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). 3.jpg 4.jpg
    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
  3. hiphop

    Thread Starter New Member

    Oct 10, 2016
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    Thank you for replying.Also is the formula for Ad1(no load) and Ad2 (no load)correct.
     
  4. The Electrician

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    Your handwriting (the calculations, not the text) is too hard for me to read, so I can't offer an opinion about the correctness of Ad1 and Ad2.
     
  5. hiphop

    Thread Starter New Member

    Oct 10, 2016
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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 .
     
  6. The Electrician

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  7. hiphop

    Thread Starter New Member

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