Resistors in an amplifier

Discussion in 'General Electronics Chat' started by Zanac-X, Apr 14, 2012.

  1. Zanac-X

    Thread Starter Member

    Dec 23, 2011
    51
    1
    hey to all

    can anyone explain to me in details about the use of resistors in an NPN transistor working as amplifier , R1,R2 in voltage divider in (B) ,RC ,RE

    if i have only a transistor used as amplifier with one resistor connected between Vcc and B , and RC resistor (Between Vcc and C) ,whats the difference between the first and second circuit

    any help is much appreciated

    thanks
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,360
    A picture is worth a thousand words. Post a circuit diagram.
     
  3. Zanac-X

    Thread Starter Member

    Dec 23, 2011
    51
    1
    maybe this will help explain what i want
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Case 1 has a voltage divider feeding the correct voltage to the base of the transistor. There is an emitter resistor so that any change in the current gain or base-emitter voltage is cancelled.

    Case 2 has only one resistor feeding base current. Since transistors (even with the same part numbers) have different current gains and different base-emitter voltages then it is cutoff or saturated and probably cannot amplify without severe distortion.
    I was taught to NEVER EVER bias a transistor like that unless the transistor is a simple switch.
     
    Zanac-X likes this.
  5. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,360
    Case 1 is the preferred way to bias a transistor. Re provides negative feedback and hence gives better temperature stability.

    As audio says, use Case 2 only when the transistor is used as a inverting logic switch .
     
  6. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    If you set up Case 2 with Rb/Rc >>β, it can be possible to get some linear operation, from the arrangement, but this is really not very satisfactory.

    The collector current will be quite unpredictable, being very dependant on β, which is normally a quite variable quantity.

    In fact, a crude indication of β can be made with a circuit of this kind. If Rc is replaced with a mA meter, and perhaps a small series resistor for safety, and Rb is chosen to give a suitable base current, provided the collector voltage drop is not too large, the collector current roughly indicates the gain.
     
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