crossover distortion in Op Amp

Discussion in 'General Electronics Chat' started by froten_140, May 13, 2008.

  1. froten_140

    Thread Starter Active Member

    Jun 4, 2006
    30
    4
    Hello all,

    I would like to understand an issue with the Op Amps. The inverting and non-inverting inputs of many of the Op Amp circuits like the summing circuit of the 741 Op Amp are not biased. Is there a crossover disturtion in the out put of the circuit? How can it be corrected if any? If not, how the crossoverdisturtion was eleminated ? when I design a signal amplifier using Op Amp, do I have to consider the biasing and crossover disturtion?

    thank you very much for any help.
     
  2. Caveman

    Active Member

    Apr 15, 2008
    471
    0
    They are generally biased using feedback. One of the effects of feedback is also to reduce non-linear distortion of the amplifier. This includes any crossover distortion.
     
  3. froten_140

    Thread Starter Active Member

    Jun 4, 2006
    30
    4
    Thanx for your replay.

    If I'm using a voltage comparartor then there will not be any feedback circuits. Do I have to consider biasing the comparator? for example LM319, Q1 & Q2 are not biased.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Bias on a comparator input is generally placed on an input to set the sensing point. When the voltage on the other input exceeds the set level, then the comparator output switched to the other state.

    Feedback in a comparator circuit is to place some hysteresis in the operating point to make a small dead spot and reduce the possibility of oscillation on a slow-changing input.

    A voltage comparator will not reproduce or amplify a waveform.
     
  5. Caveman

    Active Member

    Apr 15, 2008
    471
    0
    To add to beenthere's point, biasing is a linear circuit operation concept. A comparator is not a device that is used in the linear mode.
     
  6. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Most opamps have their output transistors biased into class-AB for very low crossover distortion.

    Except the low power LM358 dual and LM324 quad lousy old opamps which have their output transistors with not enough bias. They are biased into class-B for low power. Their crossover distortion is reduced to "only" 3% when they have 100% negative feedback.

    The MC3317x line of low power opamps use less power but the output transistors are biased properly into class-AB for no crossover distortion. They also operate with a single supply down to 3V and have a much improved bandwidth to 35kHz.

    The manufacturers of the LM358 and LM324 recommend an output load resistor to one supply voltage so that one output transistor operates in class-A for no crossover distortion.
     
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