Input Signal of Common Drain Amplifier

The common drain amplifier is also known as a source follower or a voltage follower. It is a type of field-effect transistor (FET) amplifier configuration where the output is taken from the source terminal. The input signal is applied to the gate terminal.

Coupling Capacitor

In the common drain amplifier configuration, the input signal is coupled to the gate terminal using a coupling capacitor. The coupling capacitor allows the AC signal to pass through while blocking the DC component.

Function of the Coupling Capacitor

The coupling capacitor serves two main purposes in the common drain amplifier:

1. Blocks DC Voltage: The coupling capacitor blocks the DC voltage present at the input, allowing only the AC component to pass through. This is important because FETs are voltage-controlled devices and require a biasing arrangement to establish a suitable DC operating point. The coupling capacitor ensures that the DC bias voltage is not affected by the input signal.

2. Provides High Input Impedance: The coupling capacitor presents a high impedance to the input signal, allowing it to pass through without loading down the source terminal. This helps maintain the input signal integrity and minimizes any distortion caused by loading effects.

Alternative Options

- Input Resistor: An input resistor could be used to provide DC biasing to the gate terminal, but it would not allow the AC signal to pass through. It would block the AC component and only allow the DC bias to reach the gate terminal.

- Output Capacitor: The output capacitor is typically used in the common drain configuration to couple the output signal to the load. It is not involved in coupling the input signal to the gate terminal.

- Transformer: A transformer is not typically used in the coupling of the input signal to the gate terminal of a common drain amplifier. Transformers are more commonly used for impedance matching or coupling signals between different stages of an amplifier.

Therefore, the correct option is (B) Coupling Capacitor as it allows the AC component of the input signal to pass through while blocking the DC voltage and maintaining a high input impedance.