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CE BJT Amplifier - Collector Resistance

  1. Apr 30, 2013 #1
    We are studying single transistor amplifiers, and in the text where the collector resistance of the CE amplifier, Ric, is derived, the final equation is

    Ric = ro[1+gm(Re||r∏)] (approximation)

    This equation is highlighted and emphasized as something we should remember, then shown again in a large blue box on the next page as a "Design Note" (large emphasis in this text).

    However in the table at the end of the section that summarizes the CE/CS amplifier design summary, and the big table at the end of the chapter that lists the design summary for all of the BJT amplifiers, the equation is given as

    Ric = ro(1+gm*Re)

    I have been looking for hours and there is nowhere that I can find where this transition takes place. The text is Microelectronic Circuit Design, 4th edition by Jaeger and Blalock. I am at a loss.
  2. jcsd
  3. May 1, 2013 #2
    Well they simply assume the Re<<r∏ so Re||r∏ ≈ Re

    And sometime we use this version

    Rci ≈ ro * (re + Re)/re


    re = 1/gm = 26mV/Ic

    But Ric cannot be greater than Rci_max = β*ro for Common-base amplifier.
  4. May 1, 2013 #3
    Thank you Jony130 for your reply. I apologizr for not being clearer, this is a common emitter amplifier (CE), I don't know if this makes a difference.

    Also you have pointed out something that has been driving me crazy with this text, that is approximations are often made without explicitly stating so. This is frustrating when learning a new subject and not completely confident with the subject matter.
  5. May 1, 2013 #4
    Simply you should always remember that if you use this equation
    Ric = ro[1+gm(Re||r∏)]
    If result is greater than β*ro your approximation equations tricks you. Because Ric can never exceed β*ro.
    Last edited: May 1, 2013
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