Does bjt amplify dc signal? why or why not?

[asad1111]

does bjt amplify dc signal? why or why not?

 

[cabraham]

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

Claude

 

[yungman]

Absolutely yes.

 

[asad1111]

thanks that helped

 

[asad1111]

but what is transition frequency?

 

[cabraham]

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, I_c, divided by the ac beta or "h_fe'.

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, h_fe = 1. This is called the transition frequency, & is denoted in data sheets as "f_T". Hopefully this makes it clear.

Claude