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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