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A unconventional use for a NPN BJT

Discussion in 'Electronic Basics' started by Gene, Sep 2, 2004.

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  1. Gene

    Gene Guest

    It is known that with a high value resistor, a capacitor and NPN transistor
    is possible to build a relaxation oscillator. The trick is to "turn upside
    down" the transistor, that is, the emitter is connected to the positive
    terminal of the capacitor (charged by the resistor) while the collector is
    connected to ground and the base is left floating. Above 12-15V the
    transistor displays a region of negative resistance which is the source of
    the oscillation.
    What is the explanation of this behavior from solid state physics viewpoint?
    Any ideas?
  2. Bob Masta

    Bob Masta Guest

    Boy, this takes me back a few decades! I recall making
    these relaxation oscillators in a lab class in the late 1960s!
    I think this is an avalanche effect, where the conduction electrons
    knock loose others that increase the conduction further, so the
    resistance goes negative. A lower voltages there isn't enough
    energy to knock others loose, so resistance is positive.
    At higher voltages the current is limited by bulk resistance
    or something so the resistance is again positive.

    Best regards,

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  3. One comment, there is also a mode (that worked with some 2n2369s) that
    uses the transistor in 'forward' mode (where the collector is positive
    and emitter grounded.) The circuit is like this:

    | ---------
    | |
    Collector |
    ---Base |
    Emitter Capacitor
    | |

    Note that the base is 'open'. The transistor is partially damaged
    when operated in this mode. The V+ would be fairly high (like 40volts).

  4. Active8

    Active8 Guest

    You're probably right. I remember learning to check trans with a VOM
    and was told that some - like horizontal outs - would zener from
    C-E, so I had to make sure I knew where the base lead was.
  5. John Larkin

    John Larkin Guest

    The Zetex avalanche transistors are cool in this mode. 300-volt,
    25-amp pulses, reliably, from a SOT-23 transistor.

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