Question

Draw a simple circuit of a CE transistor amplifier. Explain its working. Show that the voltage gain AV, of the amplifier is given by AV = - (βacRL)/ri,where βac is the current gain, RL is the load resistance and ri is the input resistance of the transistor. What is the significance of the negative sign in the expression for the voltage gain?

Answer 1

Circuit diagram of CE transistor Amplifier.

When an ac input signal Vi (to be amplified) is superimposed on the bias VBB, the output, which is measured between collector and ground, increases. We first assume that Vi = 0. Then applying Kirchhoff'* law to the output loop.

Negative sign in the expression shows that output voltage and input voltage have phase difference of π.