Question

For a transistor in CE mode to be used as an amplifier, it must be operated in :

• A. Cut-off region only
• B. Saturation region only
• C. Both cut-off and Saturation
• D. The active region only

Hint:

Associate the transistor regions with their applications: - \textbf{Active Region} \(\rightarrow\) \textbf{Amplifier} (Linear operation) - \textbf{Cut-off & Saturation Regions} \(\rightarrow\) \textbf{Switch} (Digital/logic operation)

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks about the required operating region for a transistor when it is used as an amplifier in the Common Emitter (CE) configuration.
Step 2: Key Formula or Approach:
This is a conceptual question based on the operating principles of a Bipolar Junction Transistor (BJT). We need to understand the characteristics of the different operating regions.
Step 3: Detailed Explanation:
A transistor has three main operating regions, defined by the biasing of its two junctions (Emitter-Base and Collector-Base):
1. Active Region: The emitter-base (EB) junction is forward-biased, and the collector-base (CB) junction is reverse-biased. In this region, the collector current (\(I_C\)) is approximately proportional to the base current (\(I_B\)), with a large amplification factor beta (\(\beta\)), i.e., \(I_C = \beta I_B\). This linear relationship allows a small input signal at the base to be amplified into a larger output signal at the collector. This is the region required for amplification.
2. Cut-off Region: Both the EB and CB junctions are reverse-biased. The transistor acts like an open switch, and ideally, no current flows (\(I_C \approx 0\)).
3. Saturation Region: Both the EB and CB junctions are forward-biased. The transistor acts like a closed switch. The collector current reaches its maximum possible value, determined by the external circuit, and is no longer controlled by the base current.
For amplification, we need the output current to be a magnified but faithful reproduction of the input current. This linear control is only possible in the active region. The cut-off and saturation regions are used for digital logic and switching applications, where the transistor is either fully OFF or fully ON.
Step 4: Final Answer:
For a transistor to function as an amplifier, it must be operated in the active region.