In a colour TV receiver, a varactor diode is used for tuning. The varactor diode, also known as a voltage variable capacitor, is a type of diode that exhibits a capacitance that varies with the applied voltage. This property makes it ideal for use in tuning circuits in TV receivers.

Tuning in a TV receiver refers to the process of selecting a specific frequency or channel to receive the desired TV signal. This is typically done by adjusting the capacitance in the tuning circuit. The varactor diode is used in the tuning circuit to vary the capacitance and tune to the desired frequency.

Here is a detailed explanation of why the varactor diode is used for tuning in a colour TV receiver:

1. Varactor Diode Operation:
- A varactor diode is a p-n junction diode with a reverse-biased junction.
- When a reverse-bias voltage is applied to the diode, the width of the depletion region increases, reducing the capacitance.
- As the reverse-bias voltage increases, the capacitance decreases, and vice versa.

2. Tuning in a TV Receiver:
- A TV receiver has a tuner circuit that selects the desired frequency or channel.
- The tuner circuit typically consists of an inductor and a capacitor in parallel, forming a resonant circuit.
- The resonant frequency of this circuit determines the frequency to which the receiver is tuned.
- The capacitance in the tuning circuit needs to be adjustable to tune to different frequencies.

3. Varactor Diode in Tuning Circuit:
- The varactor diode is connected in parallel with the tuning capacitor in the TV receiver's tuner circuit.
- By varying the reverse-bias voltage applied to the varactor diode, the capacitance can be adjusted.
- As the capacitance changes, the resonant frequency of the tuner circuit also changes, allowing the receiver to tune to different frequencies.

4. Advantages of Varactor Diode for Tuning:
- The varactor diode provides a compact and convenient method of varying capacitance in a tuning circuit.
- It allows for precise and accurate tuning of the receiver to different frequencies.
- The tuning can be controlled electronically, making it easier for the user to select channels or frequencies.

In conclusion, the varactor diode is used for tuning in a colour TV receiver because of its voltage variable capacitance property. It allows for precise and accurate tuning of the receiver to different frequencies, providing a compact and convenient method for channel selection.