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Finding bias current in atransistor circuit

Discussion in 'Hobby Electronics' started by Kristine Hyvang, Jan 22, 2004.

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  1. I'm about to build a small power amplifier using bipolar transistors. I
    think it is called a class AB amplifier with a push-pull concept, using the
    transistors 2N1613.

    My problem is: What should decide the bias current (the current running
    through both transistors when input is zero), and how does that circuit look
    like ?

    Maybe there is another newsgroup for this discussion (I found a great one,
    but it was in Dutch ?! lucky them !! )
  2. 1. The transistors are 800mW types, so you won't want much current -- 2mA
    might be about right.

    2. This is not a high fi amplifier, so don't let people worry you with talk
    of the optimum current. Especially if it is battery powered, you won't want
    more current - in fact, you might want zero current if you don't mind more
    distortion at low volume.

    3. You can find how the current is set and how the transistors are
    connected by searching for "Amplifier Schematic". You will soon see that
    most amplifiers have similar circuits.

    5. It would be much easier to use an LM386 amplifier integrated circuit.
    The LM386 is so easy to wire up and it will forgive you if you short the
    speaker leads. If you download a datasheet you can even see what is inside
    the IC - it has two output transistors also.

    Google for "LM386 datasheet" will get you plenty of hits..

  3. Hello Kristine,
    have a look here,
    The article above talks about making R2 a trimpot, therefore
    you can play around and set bias current where ever you
    like. Just for fun set the trimpot for minimum bias current
    and note what happens when you view the output waveform
    on your oscilloscope. Increase the bias current gradually
    and check your output waveform. Make your ouput
    waveform very small and then larger at various bias
    currents. Enjoy yourself, have a good play.

    There is a lot of good reading on that site Kristine, it will take
    days to go through it all. :)
    check out the projects also.
    Have a little read here.

    John Crighton
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