Problem 44 - Master Resource Book in JEE Main Physics

Question

For CE configuration of a transistor, (a) input resistance is very small while output resistance is very high (b) input resistance is very large while output resistance is very small (c) both input and output resistances are very small (d) both input and output resistances are very large

Step-by-Step Solution

Answer

The correct option is (a) input resistance is very small while output resistance is very high.

Step 1: Understanding Transistor Configurations

In electronic circuits, transistors can be configured in different ways: common emitter (CE), common base, and common collector. Each configuration has distinct input and output resistance characteristics. The common emitter (CE) configuration is known for its unique properties compared to the other configurations.

Step 2: Analyzing Input Resistance in CE Configuration

In the common emitter configuration, the input is given between the base and the emitter. Since the base-emitter junction is forward-biased like a diode, the input resistance is relatively small. This is because the base-emitter junction approximates a low resistance path when forward-biased.

Step 3: Analyzing Output Resistance in CE Configuration

The output in the CE configuration is taken across the collector and emitter. The collector-emitter junction is reverse-biased, which provides a high resistance path. Hence, the output resistance in a CE configuration is significantly high compared to the input resistance.

Step 4: Conclusion Based on Analyzed Characteristics

Considering the analyzed characteristics, the common emitter configuration has a small input resistance because of the forward-biased base-emitter junction, and a high output resistance due to the reverse-biased collector-emitter junction. Therefore, the correct option for a CE configuration is having a small input resistance and a high output resistance.

Key Concepts

Input Resistance

In a common emitter (CE) configuration, input resistance is an important concept that determines how easily the transistor can be driven by the incoming signal. The input is connected between the base and the emitter. Here, the base-emitter junction behaves like a forward-biased diode.
This forward-biasing creates a condition where electrons can easily cross from the base to the emitter, resulting in a low input resistance.

• The low resistance means that even a small input voltage can cause a significant base current, allowing the transistor to amplify incoming signals effectively.
• This property makes the CE configuration suitable for applications requiring signal amplification.

Output Resistance

Output resistance in a CE configuration is the opposition to current flow in the output circuit, which is connected between the collector and the emitter. In this setup, the collector-emitter junction is reverse-biased.
The reverse-biasing increases the resistance path for electrons, greatly enhancing the output resistance.

• High output resistance means less current is required to achieve a higher voltage at the output, which is beneficial for cascading stages of amplification.
• This property ensures that the load connected to the output is less loaded by the transistor stage itself.

Transistor Configurations

Transistor configurations define how the transistor's three terminals (base, collector, and emitter) interact with external circuits. The common emitter (CE) configuration is one of the most popular due to its excellent amplification properties.

• In CE, the emitter terminal is common to both the input and output, serving as a reference point.
• This configuration provides high voltage and current gain, making it ideal for amplifier circuits.

Understanding these configurations helps in selecting the right setup for specific electronic applications.

Base-Emitter Junction

The base-emitter junction in a CE configuration is crucial for understanding how the transistor can be manipulated to perform its amplifying role.
This junction is typically forward-biased, making it comparable to a diode in operation.

• As a diode, it conducts current easily from base to emitter when forward-biased, initiating the transistor's operation.
• This characteristic contributes to a low input resistance in the CE configuration, which is necessary for efficient signal amplification.

Collector-Emitter Junction

In a CE configuration, the collector-emitter junction plays a key role in controlling the output characteristics. This junction is reverse-biased, which distinguishes its behavior from the base-emitter's.
The reverse biasing ensures that electrons do not easily cross the junction, providing high output resistance.

• The high output resistance is essential for the transistor to efficiently transfer amplified signals to the load connected at the output.
• This behavior is particularly useful in circuits where maintaining signal strength and integrity is critical.