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Germanium Transistor Gain?

Discussion in 'Electronic Basics' started by Bill Bowden, Aug 29, 2003.

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  1. Bill Bowden

    Bill Bowden Guest

    I'm trying to fix the oscillator/mixer section of an
    old AM transistor radio. All the voltages and parts
    look ok, but it won't oscillate. It amplifies pretty well
    since a generator signal can be seen across the oscillator
    coil and tuned to a sharp peak. If I play with the bias
    voltage, it will oscillate near the top of the band, but dies
    out when the frequency is lowered. I took out the germanium
    transistor and measured the gain at 600 on a DMM. Is this
    reasonable for a germanium transistor, or is the DMM being
    fooled by the low junction voltage?

  2. John Larkin

    John Larkin Guest

    It's more likely being fooled by the high leakage current
    characteristic of germaniums.

  3. Bill Bowden

    Bill Bowden Guest

    Yes, hadn't thought of that. Leakage measures about
    2 mA at 9 volts and base open. The leakage drops
    to about 1/2 mA with 10K resistor added from emitter to base.
    If I remove the 10K and use 250K from base to supply,
    the current increases to about 10 mA, so looks like the gain
    is about 8mA/36uA = 222

  4. Fred Abse

    Fred Abse Guest

    Measuring Ic/Ib at one point is neither

    If the transistor is
    Since the OP was writing about a self-oscillating mixer, small signal
    characteristics seemed appropriate.
  5. He also said the Iceo was 2 mA. That seems excessive, more like what
    you'd see on a Ge power transistor. I would think a small signal Ge
    would be at least ten times lower than that, max. Maybe try another
    lower leakage Ge? And of course check the tother things mentioned.

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  6. Hi, if the oscillation dies out at lower frequency's, I would check
    the capacitors. Especially old elko's (only used for lower frequency's
    of course) get bad. They probably aren't in your signal path, but
    could be the decoupling. What frequency's are you talking about?


    Pieter Hoeben
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