# Transistor as an Amplifer

A transistor can be used as an amplifier to increase the signal's strength.

In a common-emitter configuration, the input signal is applied to the base of the transistor, and the output signal is taken from the collector. The transistor is biased so that a small change in the input signal causes a large change in the output signal.

When a small input voltage is applied to the base, the transistor amplifies the voltage and produces a larger output voltage at the collector. The amplification factor, or gain, of the transistor is determined by the ratio of the output voltage to the input voltage. The gain can be controlled by adjusting the bias voltage applied to the base of the transistor.

In a common-emitter configuration, the transistor operates in its active region, where small changes in the base current cause large changes in the collector current. This allows the transistor to amplify a weak input signal to a much higher output signal.

## Problems from IIT JEE

Problem (JEE Mains 2022): A transistor is used in common-emitter mode in an amplifier circuit. When a signal of 10 mV is added to the base-emitter voltage, the base current changes by 10 $\mu$A and the collector current changes by 1.5 mA. The load resistance is 5 k$\Omega$. The voltage gain of the transistor will be________

Problem (JEE Mains 2021): For a transistor in CE mode to be used as an amplifier, it must be operated in

1. cut-off region only
2. saturation region only
3. both cut-off and saturation
4. the active region only