Question

According to a graph drawn between the input and output voltages of a transistor connected in common emitter configuration, the region in which transistor acts as a switch is

• A. Cutoff or saturation region
• B. Active region
• C. Active or saturation region
• D. Cutoff or active region

Hint:

A simple way to remember the applications of transistor regions: - Switching: Uses the two extremes - Cutoff (OFF) and Saturation (ON). - Amplification: Uses the middle ground - Active region.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
A transistor has three main regions of operation: cutoff, active, and saturation. Each region has distinct characteristics and applications. When a transistor is used as a switch, it is operated at the two extreme ends of its operation, corresponding to the "fully OFF" state and the "fully ON" state.
Step 2: Detailed Explanation:
The characteristics of the three regions in a common emitter configuration are as follows:
1. Cutoff Region: In this region, the input voltage (\(V_{in}\)) is very low, and the base current (\(I_B\)) is approximately zero. Consequently, the collector current (\(I_C\)) is also nearly zero. The transistor offers very high resistance between the collector and emitter. This corresponds to the OFF state of an open switch.
2. Saturation Region: In this region, the input voltage (\(V_{in}\)) is high enough to drive a large base current. This causes the collector current (\(I_C\)) to reach its maximum possible value, limited by the external load resistor. The voltage drop across the collector-emitter junction (\(V_{CE}\)) is very small (ideally zero). The transistor offers very low resistance. This corresponds to the ON state of a closed switch.
3. Active Region: This is the region between cutoff and saturation. Here, the collector current (\(I_C\)) is proportional to the base current (\(I_B\)), related by the current gain \(\beta\) (\(I_C = \beta I_B\)). The transistor functions as an amplifier in this region, not as a switch.
Therefore, for a transistor to act as a switch, it must be operated alternately between the cutoff region (OFF) and the saturation region (ON).
Step 3: Final Answer:
The region in which a transistor acts as a switch is the cutoff or saturation region. Option (A) is correct.