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Problems while making a simple class A amplifier

Discussion in 'Audio' started by Siampo, Mar 7, 2021.

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  1. Siampo

    Siampo

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    Mar 7, 2021
    Using a bc547b (hfe = 100/450) I wanted to build a class A amp for boosting up a microphone, but I have not been able to make it work. I don't have an oscilloscope so I can't analize the output, but when connecting my phone 3.5 jack to the input and headphones to the output nothing can be heard. I think everything was built ok and the components calculated correctly. I changed the transistor and soldered the components just in case (they where in a breadboard), but the results are the same. Can anyone notice some kind of mistake? Any help would be really apreciated.
    PS: Sorry if it is something really obvious, but I haven't done electronics in a while and the information that I find online seems to indicate my values should work. photo_2021-03-07_12-38-55.jpg
     
  2. bertus

    bertus Moderator

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    Nov 8, 2019
  3. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    The problem may be the impedance of your headphones. Usually headphones have an impedance from 8 Ω to 32 Ω, high impedance headphones up to 600 Ω. The output impedance of your amplifier is very high, so the load presented by the headphones may draw down the output level so much you can't hear anything.
    A quick simulation gives an output impedance of ~1.6 kΩ for your amplifier (I assumed 100 nF capacitors - what values do you use? You haven't given the values in your schematic).
     
    Last edited: Mar 7, 2021
  4. Audioguru

    Audioguru

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    Sep 24, 2016
    You did not say the type of microphone. Most mics today are electret type that need to be powered. A dynamic mic has a coil and a magnet like a tiny loudspeaker.
    Your circuit has the transistor biased to be almost saturated and not amplifying. The 20k resistor should be about 3.3k. The 50 ohm
    resistor should ha a 47uF capacitor parallel with it for much more voltage gain.
     
  5. Audioguru

    Audioguru

    3,133
    698
    Sep 24, 2016
    Without including the base current, I calculated simple voltages on yours and on mine.
    Then I simulated them:
     

    Attached Files:

    Last edited: Mar 8, 2021
    Harald Kapp likes this.
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