AIIMS 2005 Physics Transistor Amplifiers Assertion Reason Question

To analyze the given question, we need to delve into the operation of a common emitter transistor amplifier and the statements provided in the assertion and reason.

Assertion:

In a common emitter transistor amplifier, the input current is much less than the output current.

This assertion is true. In a common emitter configuration, the transistor is operated in such a way that a small input current (base current, $I_B$) controls a much larger output current (collector current, $I_C$). The relationship between these currents is characterized by the transistor's current gain ($\beta$), defined as:

$$\beta = \frac{I_C}{I_B}$$

For typical transistors, $\beta$ can range from 20 to 1000, which means that the output current can be several times greater than the input current. Therefore, the assertion is correct.

Reason:

The common emitter transistor amplifier has very high input impedance.

This statement is false. While it is true that common emitter amplifiers have a relatively low input impedance compared to other configurations (like common base and common collector), they are not characterized by "very high" input impedance. In fact, the input impedance is determined primarily by the base-emitter junction and is typically in the range of a few kilo-ohms (kΩ). High input impedance is more characteristic of a common collector configuration (emitter follower).

Conclusion:

Given this analysis:

1. The assertion is true because the input current is significantly less than the output current in a common emitter configuration.
2. The reason is false because the common emitter amplifier does not have very high input impedance; it has moderate impedance.

Thus, the correct answer is C) Assertion is true but reason is false.

Why Other Options Are Incorrect:

• Option A: Both assertion and reason are true and reason is the correct explanation of assertion. This is incorrect because the reason is false.
• Option B: Both assertion and reason are true but reason is not the correct explanation of assertion. This is incorrect as the reason itself is false.
• Option D: Both assertion and reason are false. This is incorrect because the assertion is true.

Key Concepts:

• Common Emitter Amplifier: A type of transistor amplifier where the input is applied at the base and the output is taken from the collector.
• Input and Output Currents: The relationship of input current ($I_B$) and output current ($I_C$) is governed by the current gain $\beta$.

To summarize:

• Assertion: True (input current is less than output current).
• Reason: False (common emitter amplifiers do not have very high input impedance).