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Does bjt amplify dc signal? why or why not?

  1. Sep 6, 2011 #1
    does bjt amplify dc signal? why or why not?
     
  2. jcsd
  3. Sep 6, 2011 #2
    Re: bjt

    Of course, yes. A bjt is useful at signal amplification from dc all the way up to its transition frequency, "fT". At a frequency value of "fT", the current gain, "beta" or "hfe", is equal to one. Does this help?

    Claude
     
    Last edited: Sep 6, 2011
  4. Sep 6, 2011 #3
    Re: bjt

    Absolutely yes.
     
  5. Sep 7, 2011 #4
    Re: bjt

    thanks that helped
     
  6. Sep 7, 2011 #5
    Re: bjt

    but what is transition frequency?
     
  7. Sep 8, 2011 #6
    Re: bjt

    In order for a bjt to amplify, the b-e junction must be forward biased. This requires current & voltage. The base current needed is the desired collector current, Ic, divided by the ac beta or "hfe'.

    As frequency increases, more base current is needed to charge/discharge the base-emitter diffusion capacitance. Eventually, as frequency is increased, there will be a value of freq where the base current needed equals the collector current. In other words, at this freq, hfe = 1. This is called the transition frequency, & is denoted in data sheets as "fT". Hopefully this makes it clear.

    Claude
     
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