help- transistor bias

Discussion in 'The Projects Forum' started by deepak007, Jul 16, 2008.

  1. deepak007

    Thread Starter Active Member

    Sep 30, 2007
    59
    0
    hello

    i want to substitute the last two transistors, in the circuit given below , with 2N3866, and want to use a 9v supply. could anybody tell me the biasing resistor changes?

    thanks in advance
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The 2N3866 is a very old RF power transistor. It has a very wide range of current gain from 10 to 200 so each one would need its bias resistor values selected for it.

    A few other parts would need to be changed to get more output power.
     
  3. deepak007

    Thread Starter Active Member

    Sep 30, 2007
    59
    0
    thanku mr.audio guru. but, can you please explain in detail. i would be grateful to you if you give me the values directly.
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The circuit is designed for BC547 transistors with a current gain of from 100 to about 700, or 2N2222 transistors with a current gain of from 75 to abbout 275.

    The old 2N3866 has a current gain of from 10 to 200.
    So the 47k base resistor for the second transistor must be 4.7k if the transistor has a gain of only 10, then the first transistor won't be able to drive it. If the gain of the second transistor is 200 then the 47k resistor is fine.

    If the gain of the third transistor is only 10 then its 150k base resistor must be 15k.
     
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