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Transistors as V clamps?

Discussion in 'Electronic Design' started by Steve Sousa, Jan 26, 2004.

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  1. Steve Sousa

    Steve Sousa Guest


    I've seen the following circuits used to protect the IC's on multimeters.
    I think that they use the reverse breakdown voltage of the transistor to
    achieve this effect, is that correct?
    Why not use a zener?
    Isn't it bad to reverse bias transistors beyond their RBV?
    How does circuit B work?


    In o---------o---------o Out
    | |\
    Circuit A | |

    In o---------o---------o Out
    | |\
    | |
    Circuit B | |
    | |/
    created by Andy´s ASCII-Circuit v1.24.140803 Beta
  2. Fred Bloggs

    Fred Bloggs Guest

    Circuit B is the same thing except for bipolar signals- note the
    symmetry for signal +/- with respect to GND. Circuit A clamps negative
    signals at -Vbe. The transistor clamps differs from the zener in that
    Vce falls with conduction, a zener clamps at a constant voltage. This
    Vce fall is due to the increase of transistor reverse beta with emitter
    current-and is a form of positive feedback action.
  3. henryf

    henryf Guest

    I've seen similar things used to protect the inputs of op amps.
    Pretty much. For circuit A, it depends on the polarity of the
    Based on a quick google search and my limited sampling of
    data sheets, it seems that real zener diodes have greater
    parasitic capacitance when used this way. (The zener
    datasheet I checked showed 50pF to 100pF, while the transistor
    datasheet indicated 5pF or less.)
    Only if you ever plan to use them again as transistors. It
    messes with the gain.
    Like two back-to-back zener diodes. It clamps the input
    voltages (either polarity) at Vz + Vbe.
  4. Steve Sousa

    Steve Sousa Guest

    So all i need to do is pick a transistor with a RBV that suits my app,
    nothing else to consider?

    Thanks for the explanations.
  5. Steve Sousa

    Steve Sousa Guest

    So the clamping voltage will vary with the input voltage->current?

  6. henryf

    henryf Guest

    No, it's not that simple.

    First of all, circuits A and B use a transistor's emitter-base
    breakdown voltage, which is typically near 7 volts -- it's
    unlikely you'll find any outside the range of 5V to 10V.
    Unless your app can live with that, you'll need to use a
    different voltage-clamping circuit.

    Second, there are other things to consider, including:
    - power and thermal considerations
    - packaging
    - price
    - availability
  7. Fred Bloggs

    Fred Bloggs Guest

    Yes- I was assuming there is a resistor in series with the signal source
    not shown. The only advantage this circuit may have over a zener is a
    better leakage characteristic, but I am not 100% sure. All the breakdown
    devices will have more or less reverse current through them as the input
    approaches the breakdown rating. For the standard 500mW zener, this can
    be as much as 10's of ua at 80% Vzener, just as an example. If you say
    this protects a meter, then that level of leakage may introduce an
    unacceptable inaccuracy at signal levels they do want to measure, and
    increasing the zener breakdown rating to reduce leakage is not an
    option. Unlike the zener with a relatively soft knee, when the
    transistor starts reverse breakdown, the voltage required to sustain
    larger currents is reduced and there is an abrupt increase in shunt
    current- this should be a sharper breakdown characteristic.
  8. Steve Sousa

    Steve Sousa Guest

    I'll have to give it a try on real parts and see what comes up.
    Thank you and Fred for your help.
  9. GPG

    GPG Guest

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