how can we find the voltage gain of a certain transistor if we are using it as a amplifier.What I mean is to design an amplifier circuit in an accordance with the gain.can you please help........

 

A transistor does not have a voltage gain, it has a current gain, and that gain is highly variable depending on the current thru it, the temperature, how this exact transistor was made, the phase of the moon, ect.

To deal with all that circuits are designed to work with a wide variation in gain, usually above some minimum.

So the first question is: in what circuit does this transistor reside?

 

In short, the gain of a transistor amplifier depends upon the circuit design.

 

Nobody designs a transistor amplifier to run at the gain of the transistor because it will have lots of distortion. You use resistors to force a certain amount of gain for your circuit and expect the transistor to behave nicely because it is not being pushed to its limits.

 

\(Gain=\frac{Output}{Input}
\)

Lets see some numbers.