DC opertaing point and the conduction angle issue

Discussion in 'General Electronics Chat' started by acelectr, Apr 20, 2011.

  1. acelectr

    Thread Starter Member

    Aug 28, 2010
    73
    0
    Hi I was workıng on this amplifier design project and I think I've misunderstood some important basic topics. What I want to ask is what determines the dc operating point of the for instance a bjt amplifier?
    Is it the collector voltage Vc or collector emitter voltage Vce?
    Secondly is the dc operating point only fact that would affect the conduction angle (i.e wether getting a full swing at the output or a distorted clipped one):confused:
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    The DC operating point of an amplifier is defined in terms of voltages and currents. Specifying Vc or Vce alone (for a bjt) is not sufficient to define the working point.

    The circuit configuration also requires to be considered. In some common-emitter stages the emitter is returned directly to ground, but in other cases there is an also an emitter network. Other circuits are common-collector, (where Vc is fixed), or even common base. The significance of Vc or Vce varies according to the situation.

    Perhaps it would be easier to discuss your question with reference to an actual example. Can you post the a schematic of the circuit which you have trouble with?
     
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    As to your second point: the conduction angle results from an interaction of the bias conditions and the drive signal. The amount of clipping (if any) is also determined this way, but note that some clipping may result from saturation, rather than from the device being driven out of conduction.

    Amplifiers are often intended to operate in class A, with 360 degree conduction. However, acceptably linear operation will only be possible up to some maximum level, beyond which excessive distortion or clipping will occur, due to current cut-off, saturation, or both, according to the situation. Sometimes excessive distortion is seen merely due to current or voltage variations varying the gain, without reaching cut-off or saturation.

    The DC conditions of the amplifier must therefore be defined to provide sufficient margin for current and voltage variations about the working point, so that the largest required signal can be delivered to the load without clipping.

    Once this is achieved, negative feedback may be employed if necessary to reduce distortion to an acceptable level. Note however that once clipping is encountered, negative feedback can tend to make it more harsh
     
  4. acelectr

    Thread Starter Member

    Aug 28, 2010
    73
    0
    How can you determine this working point. Consider a bjt having both an equivalent resistance at the collector and also at the emitter and also at the base. Is it Vce that one should worry about mostly? What would be the things that one should take in to account, for instance if the working point (dc operating point) is fine than one should not get any nonlinear amplifier, crappy, distorted signal... Would this be a right statement? When can you decide whether to apply a negative feedback for instance at a common emitter amplifying circuit?:confused:
     
  5. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    I think that there is no simply answer to your question.
    Is all up to designer skills. Designer considering the pros and cons, and on this basis he start his deign.

    In simply BJT amplifiers we usual set Vce = Vout_max +Vce(sat) + safety margin( 0.1...0.5 *Vout_max)
    Re = (0.1...0.4)Vcc/Ic
    Ic > Vout_max/ (Rload * safety margin (0.8)
    But even this recipe don't hold in every real life situation.
    http://forum.allaboutcircuits.com/showthread.php?t=36500
     
Loading...