biasing

Discussion in 'General Electronics Chat' started by bhuvanesh, Aug 29, 2013.

  1. bhuvanesh

    Thread Starter Member

    Aug 10, 2013
    268
    2
    what is the use of fixed bias circuit
     
  2. #12

    Expert

    Nov 30, 2010
    16,257
    6,763
    To keep the bias voltage stable.
     
  3. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    IIRC - Fixed bias dates back to the days of valves (tubes) a battery or stabilised supply from the mains transformer provided a fixed negative voltage that the grid leak resistors were tied to.

    Later on, designers realised that the cathode current could develop a small voltage across a resistor - as the cathode is allowed to rise above chassis, the grid leak resistor pulls the grid negative wrt the cathode, so fixed bias was no longer required.

    Transistors are a little different as the base doesn't have to be pulled negative to set the operating point - it needs a small forward current to set the collector operating point.

    Bearing in mind fixed bias was applied to germanium transistors with their huge temperature coefficient - it was soon found to be a *REALLY* bad idea!

    Even with modern silicon transistors, fixed bias in the form of a single resistor to base from Vcc or even a potential divider, thermal runaway would still be a problem - and gain spread means each resistor would be select on test for the production line.

    The first incarnation of automatic bias was to feed the base resistor from the collector - as collector current rises, its voltage falls and reduces the base current. That's basically negative feedback, but its AC nfb as well as DC nfb - which clobbers the stage gain a bit.

    The more usual automatic bias these days is to feed the base from a voltage divider and include an emitter resistor - as emitter current increases; so does the voltage across the emitter resistor - which cancels out some of the voltage applied to the base.
     
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