My amplifier doesn't work very well [2N3904][collector feedback bias][capacitor mic]

Discussion in 'General Electronics Chat' started by booboo, Jul 9, 2015.

  1. booboo

    Thread Starter Member

    Apr 25, 2015
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    Hi everybody
    I'm working on this circuit:

    [​IMG]
    I have wired it up but the output is oscillating between 20mv-3mv. the supply is 9v. I don't have anything about my microphone. I have picked it up from the headset of my PC. should I cascade the output to two or maybe three stage? I'm newbie to using BJT as amplifire. How can I calculate the gain of this amplifire?
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    First select R2 value so that the collector voltage is around 4.5V
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    R2 will probably need to be a much higher value (e.g. ~ 1MegOhm).
     
  4. booboo

    Thread Starter Member

    Apr 25, 2015
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    I changed R2 to 1MegOhm but nothing happened. a picture of my circuit:

    [​IMG]

    The pots in the picture is R1, R2 and R3.
     
  5. Bordodynov

    Active Member

    May 20, 2015
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    Your amplifier has a very low input impedance. This is due to the negative feedback (resistor R2 = 100k Rin ~ 100k / (A +1) || Ztrans ~ 0.5kOhm. Can optimize the amount of R2, and it is possible to add RC -filter.
    Mic.png
     
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  6. DickCappels

    Moderator

    Aug 21, 2008
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    Is the 20mv -3mv oscillation the problem, or are you just worried about the gain?

    This is a transimpedance amplifier (with low loop gain), as such the output P-P voltage will try to be the P-P current from the JFET in the mic times the value of R2.
     
  7. booboo

    Thread Starter Member

    Apr 25, 2015
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    I'm just worried about the gain.
     
  8. booboo

    Thread Starter Member

    Apr 25, 2015
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    I have measured the output of headset jack of my PC and it was 3.3v.
    Thanks but 70mv isn't enough. I want to get the output between 0v-3v3. any seggestion?
     
  9. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    At such low base and collector currents, your transistor probably will not have a gain of 200. But even if it did, with 5 mV coming from the mic that's only 1 V output voltage, and it would have distortion caused by the almost complete lack of feedback. For a circuit gain of 660 (57 dB) you will need two stages in series, each with a gain of 26, to get the output you want with reasonable distortion.

    Open question - This one-transistor preamp circuit has been around for decades, but I've never seen it done with a Darlington transistor. Anyone know why?

    ak
     
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  10. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Maybe because the voltage gain for a Darlington CE amplifier will be two times smaller than a single transistor CE amplifier.
     
    Last edited: Jul 10, 2015
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  11. DickCappels

    Moderator

    Aug 21, 2008
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    Jony130 has a point.
    Its been done with Darlingtons for many decades, in audio output stages, for example.

    By the way, (back to the single transistor gain stage) because of the dependence of the emitter resistance on current and because the current through the emitter is approximately equal to the current through the collector load, the maximum gain of a stage like will be less than the voltage across the collector load x 38. Therefore you will need multiple gain stages as AnalogKid indicated.

    You won't get a 3.3 volt peak-to-peak output voltage with a 3.3 volt power supply and that topology; how much do you really need?
     
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  12. booboo

    Thread Starter Member

    Apr 25, 2015
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    Thanks guys for reply

    If you were me, how would you do it? What type of biasing? which one exactly?
    Let's assume my supply is 9v. a 3.3v peak-to-peak output is what I need. no matter how many transistor or what type of biasing. I have also these transistors in stock:

    2N3906,2N3904,BC548,BC557,BC338,BC327,BC337,BF199

    my goal is just learning.;)
     
  13. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Try this circuit
    aa.png

    You can change gain by changing R5 value. Gain ≈ R4/R5
     
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  14. ScottWang

    Moderator

    Aug 23, 2012
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    You can shorted two input pins of microphone, normally it will be 0V at the output.
     
  15. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    In the diagram that I posted (13) a NPN transistor (T1 on the diagram) is wrongly labeled.
    T1 is a NPN so 2N3904,BC548,BC338,BC337 can be used. Also notice that 2N3904 ( and PNP version 2N3906) has a different pin out then the European transistors BCxxx.
    As T2 (PNP) you can use thus 2N3906,BC557,BC327. Don't forgot about
    different pin out.
     
  16. RichardO

    Well-Known Member

    May 4, 2013
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    To get rid of oscillation and power line induced hum you need to shorten the wires in your circuit. Also, don't forget to add power supply bypassing.
     
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  17. #12

    Expert

    Nov 30, 2010
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    The top thread in Completed Projects right now contains the circuit in post #1.
    You might examine how that person did this circuit.
     
  18. booboo

    Thread Starter Member

    Apr 25, 2015
    165
    2
    Ok and Thanks
    I will try this tomorrow. I was busy today.
    Two question:
    What type of biasing is used for T1 and T2? I think it's common-collector for T2. right? What about T1?
    What are the usage of R4 and R7 and R8?
    What's the point? I cannot get it. could you more explain, please?
    shorten? you mean I should use breadboard? I have to solder them? Is this what you are saying?
    This:
    http://forum.allaboutcircuits.com/threads/audio-preamplifier-project.113313/
    He/she hasn't said anything about its Mic.
     
  19. RichardO

    Well-Known Member

    May 4, 2013
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    The picture of your circuit shows many long insulated wires. These wires act as antennas and pick up stray signals from power line wiring creating hum. In addition, the long wires can couple to each other causing feedback and oscillation.

    I would recommend using short wires that are flat against the solderless bread board. Also, the resistors and capacitors should have the leads cut short so that they are against the SBB when installed. I always use the leads cut from the resistors and capacitors for my jumpers. Using un-insulated wires encourages using short wires.
     
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  20. ScottWang

    Moderator

    Aug 23, 2012
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    Connected two pins of microphone together and measuring the output pin to see does it has any noise.
     
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