Does bjt amplify dc signal? why or why not?

  • Thread starter asad1111
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  • #1
asad1111
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does bjt amplify dc signal? why or why not?
 

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  • #2
cabraham
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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:
  • #3
yungman
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Absolutely yes.
 
  • #4
asad1111
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thanks that helped
 
  • #5
asad1111
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but what is transition frequency?
 
  • #6
cabraham
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but what is transition frequency?

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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