Question on class B operation using push-pull pair

Discussion in 'Homework Help' started by circuitashes, May 16, 2008.

  1. circuitashes

    Thread Starter Active Member

    May 13, 2008
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    The attached file contains a diagram of a transistor push-pull pair I found on this site (the only modification to the diagram being the inclusion of the AC input signal). From what I know about class B operation and push-pull pairs, the transistors must operate in active mode alternately for only half a cycle. From my analysis - I stand to be corrected - of the diagram, both Q1 and Q2 are active in the 1st half- cycle since both B1/E1 and B2/E2 junctions are forward biased (assuming the AC signal is +ve with respect to B1 for 1st half-cycle). In the 2nd half-cycle both junctions are reverse-biased and therefore are in cut-off mode. Where am I going wrong in my interpretation of the diagram?
     
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Two things.

    Firstly
    your alternating input is incorrect. Remove it from its present position and rejoin the bases of Q1 and Q2.

    Now place the input between the common connection of the two bases and earth (ground). The phase splitting is then automatic as one transistor is cut off and one conducting in each half cycle.



    Secondly
    Neither transistor will actually conduct for the full half cycle. Each will only start to conduct when the signal rises above (below) the base emitter voltage plus the emitter resistor voltage.
    It is normal to overcome this by including some biasing circuitry in the link between the two transistor bases.
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    The biasing would put the mode of operation onto class AB. Class B has that dead spot around the 0 volt level.
     
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Yes you are right adding the bias brings the amplifier operation into a class known as AB.
     
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