In colour TV receiver, varactor diode is used for

Varactor diode is commonly used for Electronic tuning.

Varactor diode:

• It is represented by a symbol of diode terminated in the variable capacitor as shown below:

• Varactor diode refers to the variable Capacitor diode, which means the capacitance of the diode varies linearly with the applied voltage when it is reversed biased.
• The junction capacitance across a reverse bias PN junction is given by

​           \(C=\frac{A\epsilon}{W}\)

• As the reverse bias voltage increases, the depletion region width increases resulting in a decrease in the junction capacitance.

•  Varactor diodes are used in electronic tuning systems to eliminate the need for moving parts
• Varactor [also called varicap, variable reactance diode, or tuning diode] diodes are the semiconductor, voltage-dependent, variable capacitors
• Varactors are used as voltage-controlled capacitors and it operated in a reverse-biased state. Hence option (3) is the correct answer.