Varactor diode applications – Where is voltage-dependent capacitance actually used? A varactor (varicap) diode provides a controllable junction capacitance versus reverse bias. In which of the following applications is a varactor commonly employed?

Introduction:
Varactor diodes act as voltage-controlled capacitors. They are key to frequency-tunable resonators and frequency multipliers because the junction capacitance changes predictably with reverse bias. Understanding where this effect is practically exploited helps distinguish common RF/microwave circuitry from low-frequency active filter design.

Given Data / Assumptions:

• Applications listed: (1) TV tuning, (2) Active filter, (3) Microwave frequency multiplication.
• Reverse-biased junction supplies a non-linear, voltage-dependent capacitance Cj(V).
• Focus on mainstream, established uses.

Concept / Approach:

In television and other tuners, varactors are embedded in LC resonators; changing the bias changes C, thereby tuning the local oscillator and front-end filters across channels. In microwave multipliers, varactors exploit C(V) nonlinearity to generate harmonics efficiently, with appropriate matching networks selecting the desired multiple of the input frequency. By contrast, “active filters” in analog baseband commonly use op-amps with resistors/capacitors; while tunable RF filters can use varactors, the textbook association of “active filters” refers to op-amp topologies that do not require varactors.

Step-by-Step Solution:

Verification / Alternative check:

Consumer tuner schematics and microwave multiplier modules routinely show varactors; baseband Sallen-Key/biquad filters do not.

Why Other Options Are Wrong:

Options A, B, D, E include “active filter” alone or exclude TV tuning; these do not reflect the standard, widely taught use cases.

Common Pitfalls:

Assuming any tunable filter is an “active filter”; confusing parametric amplifiers/multipliers with op-amp filters.

Final Answer:

1 and 3 only (TV tuning; Microwave frequency multiplication).