# Does bjt amplify dc signal? why or why not?

1. Sep 6, 2011

does bjt amplify dc signal? why or why not?

2. Sep 6, 2011

### cabraham

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
3. Sep 6, 2011

### yungman

Re: bjt

Absolutely yes.

4. Sep 7, 2011

Re: bjt

thanks that helped

5. Sep 7, 2011

Re: bjt

but what is transition frequency?

6. Sep 8, 2011

### cabraham

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