Multistage BJT amps for a mic

Discussion in 'Homework Help' started by gusmas, Aug 11, 2009.

  1. gusmas

    Thread Starter Active Member

    Sep 27, 2008
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    ok now i need to build a circuit that will amplify the signal of a mic, the signal that the mic is picking up i dont need to use it again let say for sound to come out a speaker i only need to amplify it then take the output into a comparator. Now my problem is i have no idea where to start. I think a mic has 2 wires inside it , one is ground and the other one carry the voltage signal once sound is picked up'd correct?
     
  2. ELECTRONERD

    Senior Member

    May 26, 2009
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    It sounds like you need a preamplifier. Yes, a microphone as two wires, the positive ternimal will go to the input of the preamplifier and the negative ternimal will simply go to ground.
     
  3. gusmas

    Thread Starter Active Member

    Sep 27, 2008
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    ok now i need to choose my cap values bcz the freq of a person voice goes from3 00 to 3400hz correct? am i correct in the way i am planning to aproach this
     
  4. ELECTRONERD

    Senior Member

    May 26, 2009
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    Indeed, capacitors will filter unwanted noise throughout the preamplifier, and you are correct, the voice can range from approximately 300Hz to 3.4kHz. I would strongly suggest that you read more about preamplifiers, and look at the typical block-diagrams in preamplifier IC datasheets. Analog Devices have such IC's.
     
  5. gusmas

    Thread Starter Active Member

    Sep 27, 2008
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    Any suggestions on websites where i can read?
     
  6. bertus

    Administrator

    Apr 5, 2008
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  7. Audioguru

    New Member

    Dec 20, 2007
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    The frequencies of a tinny and muffled sounding old telephone goes from 300Hz to 3.4kHz. What did you say?? Telephones were designed a long time ago when they couldn't do any better. Any sounds were better than no sounds.

    The frequencies of speech are from about 80Hz to about 15kHz. The high frequency consonants of speech are very important.
     
  8. gusmas

    Thread Starter Active Member

    Sep 27, 2008
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    So my freq response for the Transistor must be between 80Hz and 1.5kHz
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    No.
    You need voice frequencies up to 15kHz.

    First you must select the type of mic. An electret mic is inexpensive, available everywhere and works very well. It has a medium output impedance so the preamp for it must have a fairly high input impedance. Its frequency response is flat from 20hz to 20kHz.

    A dynamic mic has a low impedance coil and a magnet like a little speaker. Its preamp can have a fairly low input impedance. It usually has a peak in the higher frequencies.
     
  10. gusmas

    Thread Starter Active Member

    Sep 27, 2008
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    ok cool thx will check what type of mic i have.
    OK now the formules i have i am not completely sure they are correct so here is what i have plz feel free to comment on them.
    ----------------------------------------------------
    Vcc(Assume)
    Vce = 0.5*Vcc
    VRe(Ve) = 0.1*Vcc
    R2<= Rin(base)*0.1
    Rin(Base)= RE * (Beta + 1)
    R1 = [(R2*Vcc)/Vb] - R2
    Ic is given from Datasheet
    -----------------------------------------------------------------------------------------------------------------
    one last thing , how should i aproach this, first i need the impedence of the mic. I am planning on using a 2 stage CE amp each amp with a AC gain of round about 33. Should i first design the second stage and then get the freq response of that second stage amp correct ?
     
    Last edited: Aug 15, 2009
  11. Audioguru

    New Member

    Dec 20, 2007
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    Two stages each with a gain of 33 times give a total gain of 1089 which is 5 times to 10 times too high.
     
  12. gusmas

    Thread Starter Active Member

    Sep 27, 2008
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    Well i need a gain of a th0usand?
     
  13. Audioguru

    New Member

    Dec 20, 2007
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    Why do you need a gain of 1000?
    Are you trying to hear somebody who is on the other side of your country?

    A mic with an output of 10mV needs a preamp with a gain of only 100 for a 1V output.

    The preamp can feed a power amp that drives a speaker. Power amps have a gain from 20 times to 100 times.
     
  14. gusmas

    Thread Starter Active Member

    Sep 27, 2008
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    well i still need to test what output my microphone gives , and i dont need a speaker my project dont require me to have a speaker attach. So i just need to get my pre amp to work
     
  15. Audioguru

    New Member

    Dec 20, 2007
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    What connects to the output of your preamp?
     
  16. gusmas

    Thread Starter Active Member

    Sep 27, 2008
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    a comparator circuit,but i have decided againts a transistor amp circuit it takes to long to design ni my time frame, i am planning on using a LM324 with negative feedback to controle my gain. But now my problem is the datasheet for the lm324 does not supply the max gain of the lm324 before its driven into saturation. It supplies the bandwidth though
     
  17. Audioguru

    New Member

    Dec 20, 2007
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    With a gain of 1000 (60dB), an LM324 has a bandwidth to only about 400hz if its supply voltage is from 10V to 15V. Audio will sound horrible.

    The max gain before output saturation depends on the input signal amplitude and on the supply voltage of the opamp.

    With a gain of 1000, an input of 10mV and a 10V supply the output is saturated all the time. With a 10V supply, the max output of an LM324 is 8.8V p-p which is only 3.1V RMS.
     
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