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Finding the bias current in transistor circuits.

Discussion in 'Electronic Basics' 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. Bob Masta

    Bob Masta Guest

    It's been a while for me, but the basic idea is to keep the
    bias current as low as possible to just eliminate crossover
    distortion. You can adjust this with a scope, or even by
    ear if you are careful. (Use a pure sine at about 400 Hz
    so any distortion products will be in the most-audible
    part of your hearing range.)

    If you get it too high, you will go into thermal runaway
    in very short order, so use caution.

    The standard bias regulator that I recall was a simple
    one-transistor regulator between the bases of the
    positive and negative drivers. This regulator had a
    small resistor in the emitter, and the base went to
    the wiper of a pot between the collector and the
    far end of the emitter resistor. It just forces a constant
    voltage between the top and bottom.

    The transistor must be in thermal contact with an
    output device.

    Simpler approaches that I've seen in cheap amps
    are to just put in a couple of diodes, but you have
    to err on the low side to stay away from thermal
    runaway. I've also see ordinary resistor strings used

    In the "bad old days" some of these cheesier methods
    were tolerable because the overall amp had huge
    amounts of overall negative feedback to reduce the
    distortion. We now know that tends to produce
    Transient Intermodulation Distortion, so global
    feedback is more restrained.

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis

  3. There is a short discussion of this in "Art of Electronics", chapter 2. The
    idea is to choose a quiescent current that is a 'compromise between low
    distortion and excessive quiescent dissipation'. They go on to mention that
    using feedback can be used to reduce distortion.

    The circuit is a push-pull output stage with small resistors between the
    emitters and the output. The bases are kept at 3 or so diode drops apart.
    The quiescent current is determined by a diode drop across those small
    resistors. So, if they are, say, 1ohm each, then the quiescent current is
    700mV/2 = 350mA. Thus, using different size resistors, you can pick the
    quiescent current.

    Bob Monsen
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