To use a transistor as an amplifier

Options

A. both junctions are forward biased
B. both junctions are reverse biased
C. the emitter base junction is forward biased and the collector base junction is reverse biased
D. no biasing voltages are required

Answer

To use a transistor as an amplifier, we need to understand the biasing of its junctions. A transistor consists of three regions: the emitter, base, and collector. The way these regions are biased determines the operation mode of the transistor.

### Step-by-step Solution:

1. **Identify the Transistor Configuration**:
- A transistor can be in different configurations: common emitter, common base, or common collector. For amplification purposes, the common emitter configuration is most commonly used.

2. **Understand the Biasing Requirements**:
- To use a transistor as an amplifier, we need to bias its junctions correctly. There are two junctions in a bipolar junction transistor (BJT): the emitter-base junction and the collector-base junction.

3. **Forward Biasing the Emitter-Base Junction**:
- The emitter-base junction must be forward biased. This means that the emitter (which is typically n-type in an NPN transistor) is connected to a positive voltage relative to the base (which is p-type). This allows current to flow easily from the emitter to the base.

4. **Reverse Biasing the Collector-Base Junction**:
- The collector-base junction must be reverse biased. This means that the collector (which is n-type in an NPN transistor) is connected to a higher positive voltage than the base. This creates a barrier that allows the majority charge carriers to be pulled from the base into the collector, enhancing the current flow through the transistor.

5. **Conclusion**:
- Therefore, to use a transistor as an amplifier, the correct biasing condition is that the emitter-base junction is forward biased, and the collector-base junction is reverse biased. This corresponds to option 3 from the given choices.

### Final Answer:
**Option 3: The emitter-base junction is forward biased and the collector-base junction is reverse biased.**