Draw a circuit diagram of n-p-n transistor amplifier in CE configuration. Under what condition does the transistor act as an amplifier?

The transistor in CE configuration can be operated as an amplifier.

If we fix the value of V_BB corresponding to a point in the middle of the linear part of the transfer curve then the dc base current I B would be constant and corresponding collector current I C will also be constant. The dc voltage V_CE = V_CC - I _C R_C would also remain constant. The operating values of V_CE and I _B determine the operating point, of the amplifier.

Conclusion:

The transistor behave as an amplifier when the input circuit is forward biased having low voltage V_BB and the output circuit is reverse biased having high voltage V_CC.