Friday, 3 February 2017

transistors - Why is BJT called as current controlled current source and FET as voltage controlled?




If BJT amplifies only current then why CB configuration has voltage gain but no current gain?



Answer



Output of BJT can be voltage as well as current


That is not correct, a BJT is basically a current amplifying device.


If a voltage comes out of a BJT based amplifier that voltage is either the BJT's collector current which is turned into a voltage by a resistor connected to the BJT's collector or the BJT is used as a voltage buffer (common collector or emitter follower) where it does not provide any voltage amplification but is does provide current amplification.


A FET has a high impedance control input (the gate) and as such it cannot accept current making it a voltage controlled device if you use it in a common source configuration. If you would use it in a grounded gate configuration (or cascode) you could get current buffering (no amplification) but when combined with one or more resistors you could get voltage amplification.


Similarly with the help of a few resistors you can also make a voltage amplifying circuit by using a BJT even though the BJT is intrinsically a current amplifier.


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