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how to design voltage controlled amplifier using BJT.

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harikrishnam

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Using FET we able to design voltage controlled amplifier .simillarly is it posible design same circuit using BJT?if it so plz post circuit.
 

Basically each device with sufficient gain can be used to make a voltage controlled amplifier. Regarding BJT, many opamps are/were based on BJT. The trick is to use feedback to make the circuit properties virtually independent of active device properties. Only for the case where extremely low input (bias) current is required, a FET will be first choice.

For a BJT series feedback is used to raise the input impedance. For example, an emitter resistor converts a BJT into a voltage controlled current source. The input impedance increases and transconductance is determined by the resistor mostly.
 

thank u for u r reply.if u hav any circuit diagram plz post me.
 

I fear, the term voltage controlled amplifier isn't very clear. I presume, you are referring to a variable gain amplifier (VGA). It comes in different flavours with rather different specifications regarding bandwidth, linearity and dynamic range. The classical building block for VGAs is a gm multiplier respectively a Gilbert cell, comprised of BJT.

What's your VGA application?

simillarly is it posible design same circuit using BJT?
Which FET circuit are you exactly referring to?
 
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    WimRFP

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FvM: I also had doubts about the meaning. You are right it can be an amplifier with voltage controlled gain. Long time ago I used a Plessey RF VGA. It used the exponential diode characteristic of a BJT to get a nice dB/V relationship. I also made one with discrete components at about 50 MHz.
 
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    FvM

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