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A few questions about transistor

Discussion in 'General Electronics Discussion' started by Benhe, Aug 6, 2013.

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  1. Benhe


    Aug 6, 2013
  2. duke37


    Jan 9, 2011
    The collector voltage should be at least 0.5V above the emitter, not equal to it.

    Neglecting base current, the emitter current equals the collector current so, with 4.8V across the collector resistor those voltages will be obtained when R3 is four times R4.
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    I assume you're talking about the "OK" diagram, with R3 present. This diagram shows typical operating voltages for a common emitter amplifier.

    You say that when a transistor is biased, its collector-emitter resistance should be minimum. That's only true when the transistor is saturated. In this application, the transistor is operated in the linear region of its base-current-vs.-emitter-current graph. It conducts enough current to cause 4.8V to appear across R3.

    At this bias current, the collector voltage can change by several volts in each direction, relative to its quiescent (idle) voltage of 4.2V. If it could not change smoothly over some range, in response to the input signal, then the circuit would not be able to amplify the input signal in a linear way.

    The 4.2V comes from the positive supply rail, through R3. The transistor is biased so that when there is no signal at the base, the base current is enough to cause the desired collector current. The desired quiescent collector current is the amount of current that causes the right amount of voltage drop across R3 to allow the collector voltage to sit somewhere around half way between the supply rails.

    If you have any more questions, please ask.
  4. Benhe


    Aug 6, 2013
    thank you very much, I get it now

    thank you very much, I get it now
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