Question

The value of depletion capacitance on decreasing the reverse bias on varactor diode (A) decreases (B) increases (C) becomes zero (D) does not change

Short answer

When the reverse bias on the varactor diode decreases, the depletion capacitance increases. Therefore, the correct answer is (B) increases.

Step-by-step solution

Understand the behavior of a varactor diode

A varactor diode consists of a p-n junction, which has a capacitance (called depletion capacitance) when reverse biased. Varactor diodes are commonly used in tuning circuits, such as radio receivers, where the capacitance needs to be adjusted electrically. The depletion capacitance is determined by the width of the depletion region, which in turn depends on the reverse-bias voltage applied across the diode.

Relate depletion capacitance to reverse bias voltage

For a varactor diode, the depletion capacitance (C) is inversely proportional to the square root of the reverse bias voltage (Vr) applied across the diode. This relationship can be mathematically represented as: $C={\displaystyle \frac{C_0}{\sqrt{1+\frac{V_r}{V_0}}}}$ Here, $C_0$ is the capacitance at zero bias, and $V_0$ is a junction characteristic constant.

Analyze the effect of decreasing the reverse bias voltage

Now, let's analyze the effect of reducing the reverse bias voltage on the depletion capacitance. According to the above equation, when we reduce the reverse bias voltage (Vr), the denominator in the equation becomes smaller while the numerator remains constant. As a result, the overall capacitance (C) becomes larger.

Choose the correct answer

Based on our analysis, we can conclude that when the reverse bias on the varactor diode decreases, the depletion capacitance increases. So, the correct answer is: (B) increases

Key concepts

Depletion Capacitance

The depletion capacitance is an essential concept in understanding varactor diodes. A varactor diode is a type of semiconductor device. It uses the voltage-dependent capacitance of a reverse-biased p-n junction to provide electronic tuning. This capacitance is known as depletion capacitance.

Depletion capacitance changes with the width of the depletion region in the diode. This behavior is important because it allows the diode to store small amounts of electrical charge. As the reverse bias voltage changes, so does the depletion region width, affecting capacitance.

This property makes the varactor diode very useful in tuning applications. The ability to change the amount of charge stored allows precise control over frequency in radios and other communication devices by adjusting the capacitance.

Reverse Bias Voltage

Reverse bias voltage plays a crucial role in the operation of a varactor diode. In reverse bias, the positive side of the voltage is connected to the n-type side, and the negative to the p-type side, increasing the width of the depletion region.

The reverse bias voltage not only affects the depletion capacitance, but it is also the reason varactor diodes can function as variable capacitors. As you increase the reverse bias voltage, the depletion region widens, reducing the capacitance. Conversely, decreasing the reverse bias makes the depletion region narrower and capacitance larger. This inverse relationship is key to the functionality of tuning circuits.

Materials and design parameters also play a part, but the voltage applied is the primary operator in adjusting capacitance.

Tuning Circuits

Tuning circuits are vital in many electronic devices, and varactor diodes often find a place in these circuits due to their unique characteristics. Tuning circuits allow devices to select different frequency channels by varying capacitance.

One can achieve precise frequency control in radio and television systems by using varactor diodes in tuning circuits. Here, changing the reverse bias voltage adjusts the capacitance, allowing the circuit to resonate at different frequencies.

In variable frequency oscillators and phase-locked loops, this ability of varactor diodes to provide variable capacitance makes them crucial components, providing flexibility and control in electronic design.