Vfd For Discrete Diodes VS Transistor Diodes

Discussion in 'General Electronics Chat' started by Glenn Holland, Jan 17, 2015.

  1. Glenn Holland

    Thread Starter Member

    Dec 26, 2014
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    I'm wondering if a diode can be connected in parallel with the base/emitter of a transistor so Vfd will maintain the Q Point at exactly the cut off point of the transistor.

    However is the forward voltage (Vfd or barrier voltage) across a discrete diode different than the diodes that form transistors? For a discrete silicon diode, Vfd is 0.7 and for germanium, it's 0.3 and the Q Point could be set at cut off simply by using diodes.

    However, the doping for transistor diodes is different than for the discrete type and Vfd may be different. Consequently, using diodes to maintain an exact setting may not work.
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    It can be done and is done. However, "Exactly" can mean different things to different people. The voltage drop also depends on the amount of current flowing through a given PN junction (whether diode or transistor).
     
  3. Glenn Holland

    Thread Starter Member

    Dec 26, 2014
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    The main use for "exact" cut off bias would be for the complementary pair in a Class B amplifier.

    To avoid crossover distortion, the Q Point needs to be kept at a flea's hair next to cut off.
     
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Yes, that is one way and it is commonly done with two diode drops one for the PNP and one for the NPN. More usually, a class AB is used as you will always want some small conduction at zero input signal to eliminate cross-over distortion.

    Another option is to use a potentiometer, a transistor and a capacitor. The pot lets you dial in the exact bias you want. Mounting the transistor on the same heat-sink as the power transistors.

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