how can i differentiate a classB and a classAB amplifier in just looking in the ckt??

Discussion in 'Homework Help' started by juniorece, Mar 19, 2008.

  1. juniorece

    Thread Starter Member

    Mar 17, 2008
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    how can i differentiate a class B and a class AB pushull amplifier in just looking in the ckt??:confused:
     
  2. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Look at the bias in relation to the current flow (Ip, Ik or Ic, Ie) during the quiesent state.

    I'm sure the definitions of Class B and Class AB already told you this.
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    Class-AB circuits have two diodes in series between the bases of the output transistors.
    Class-B amplifiers have the bases shorted together.
     
  4. techroomt

    Senior Member

    May 19, 2004
    198
    1
    as mr audioguru mentioned, class b amps have a terminal of each (push & pull) transistor shorted together, could be emitters or bases depending on the configuration, with the inputs coming off a center tap transformer. while a class ab will have some components (diodes, resistors) providing bias to move the q point off zero, eliminating zero crossover distortion.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    Center-tapped driver transformers have not been used in audio amplifiers for 50 years!
    The output transistors are a complimentary pair. One NPN and the other PNP.
    They are both emitter-followers and have their emitters connected together or each has a low value resistor in series with its emitter.

    When their bases are shorted together then they don't conduct any current when at rest. So it is class-B with horrible crossover distortion in the "dead" center of the waveform.

    When the transistors have two diodes in series between their bases then they always conduct a little so the waveform flows smoothly from one transistor to the other in class-AB.
     
  6. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    While not much in the audio spectrum, push-pull amplfiers are still around, in the higher power applications ... even those using center tapped transformers.

    Althought not in commercial service, there are push-pull parallel circuits used in other applications to achieve higher power levels.

    Class A, AB1, and AB2, B, and C are still characterized by the current flow during quiesence or the current flow during the input cycle.
     
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