question about transistor design

Discussion in 'General Electronics Chat' started by samy555, Oct 20, 2012.

  1. samy555

    Thread Starter Active Member

    May 24, 2010
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    I often see many electronic circuits using a simple fixed biased common emmiter transistor like Q2 in the following [​IMG] and transistor T3 here: [​IMG] Why those designers didn't use a more strong configurations like voltage divider or collector feedback? Thank you very much
     
  2. nsaspook

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    Sometimes you just need a gain stage where the exact bias is not critical because the signal is low-level. With collector feedback you also get a increase in input impedance that's needed for a condenser mic input stage.
    T3 is a RF amp stage that looks to be class AB so the input bias on it is a function of the drive signal from T2 and the R7 quiescent bias is to have it operate in a more linear part of the collector current curve during small audio signal modulation to reduce harmonic distortion.
     
  3. samy555

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    May 24, 2010
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    I think that class AB uses two transistors please explain thank you
     
  4. Audioguru

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    The audio amplifier transistor Q2 is poorly biased so that many transistors WILL NOT WORK!
     
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  5. Audioguru

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    Sorry but you are wrong. The feedback reduces the input impedance of the transistor.
    The mic is NOT a condenser type, it is an electret type.

    No.
    The RF transistor has class A bias. It might be almost cutoff or almost saturated so the tuned LC circuit at its collector keeps the waveform linear.
     
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  6. samy555

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    May 24, 2010
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    Thank you Audioguru
    That was a high-value answer
    But you did not answer my question which is:
    Why do designers use this weak configuration and it operate well in their designs and not using a strong voltage divider one?

    thank you very much
     
  7. Ron H

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    I think Q2 is intended to be a limiter, i.e., the transistor is driven from saturation to cutoff. It doesn't need typical "linear" amplifier stage biasing.
     
  8. nsaspook

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    In a RF amp you don't need both because the collector has LC (tank) circuit and provides a flywheel effect producing a mainly sine wave output.

    http://users.tpg.com.au/users/ldbutler/OutputCoupling.pdf

     
    Last edited: Oct 20, 2012
  9. nsaspook

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    The circuit says COND. so I just used that for the mic type.

    http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/feedn.html

    It's a inverting amp so the collector-base resistor provides negative feedback.
     
    Last edited: Oct 20, 2012
  10. Audioguru

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    I showed you that some BC547 transistors will work, some will be cutoff and will not work and others will be saturated and also will not work.

    Why do you say the lousy design works well?
     
  11. Ron H

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  12. nsaspook

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    The first circuit looks to be a small speaker or headphone driver (something with a coil) so I hope it's not a limiter at the mic's normal input levels. :)
     
  13. nsaspook

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    You can't use the static 1M resistor to calculate that. The input impedance is determined by the transistor Zin.
    http://en.wikipedia.org/wiki/Negative_feedback_amplifier#Input_and_output_resistances

     
  14. Jony130

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    For sure Ron H knows this stuff very well. And yes we can use this equation to find Rin thanks to Miller theorem.
    Rin = (1M/(Av+1))||((hfe+1)*re)≈ 1M/Av
     
  15. nsaspook

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    Rin vs Zin, there is a difference.

    Static resistance vs dynamic signal impedance. The mic signal sees Zin, the transistor bias sees Rin.
     
  16. Jony130

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    In 99% of the audio amplifiers circuits we assume Zin = Rin. Because error is negligible.
    But for the purists I should write
    Zin ≈ (1M/(Av+1))||((hfe+1)*re) ≈ 1M/Av for small signal and "low" frequency.
     
  17. Audioguru

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    The base of the transistor becomes a virtual ground like an inverting opamp. Then the input impedance is low and is almost the value of the series input resistor.

    The opposite, a "bootstrap" circuit increases the input impedance.

    I found the horrible circuit. It is a hearing aid or a "spy microphone" that is EXTREMELY sensitive. It can pickup the voice of somebody talking in the next country.
    I simulated it. Its input is only 8 micro-volts and its output is clipping!!
     
    Last edited: Oct 20, 2012
  18. Audioguru

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    I will try to post some images:

    Nope, I click on Manage Attachments and it winks normally but it does not do anything else.

    Edit: Now I can attach images.
     
    Last edited: Oct 20, 2012
  19. nsaspook

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    Granted you might assume that but my old professor would want a proof.
     
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  20. nsaspook

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    Series negative feedback is used in a bootstrap circuit and parallel negative feedback is used in the OP circuit. So it can be used either way to increase or decrease impedance. My bad for getting them reversed. I stand corrected.
     
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