17. In a bipolar junction transistor (BJT), the current gain `beta` is defined as

Options

A. The ratio of the change in collector current to the change in emitter current for a constant collector voltage in the common base configuration
B. The ratio of the vhange in emitter current to the change in base current for constant emitter voltage in the common emitter configuration
C. The ratio of the change in collector current to the change in base current for constant collector voltage in CE configuration
D. The ratio of the change in base current to the change in collector current for constant collector voltage in CE configuration

Explanation

### Step-by-Step Solution:

1. **Understanding the BJT Configuration**:
A bipolar junction transistor (BJT) has three regions: the emitter, the base, and the collector. It can be configured in different ways, primarily as a common emitter or common base.

**Hint**: Recall the structure of a BJT and the roles of the emitter, base, and collector.

2. **Defining Current Gain (β)**:
The current gain, denoted as β (beta), is a key parameter of a BJT. It is defined as the ratio of the change in collector current (ΔIC) to the change in base current (ΔIB) under specific conditions.

**Hint**: Remember that β is a measure of how effectively a BJT can amplify current.

3. **Conditions for Measurement**:
For the definition of β, it is important that the collector-emitter voltage (VCE) remains constant. This ensures that the changes in currents are due to the input signal and not influenced by variations in voltage.

**Hint**: Think about why keeping VCE constant is crucial for accurate measurements in transistor operation.

4. **Formulating the Expression**:
The mathematical expression for current gain (β) in a common emitter configuration is given by:
\[ \beta = \frac{\Delta I_C}{\Delta I_B} \quad \text{(at constant VCE)} \]
This means that for a small change in base current, there is a corresponding change in collector current.

**Hint**: Focus on the relationship between the changes in collector and base currents.

5. **Identifying the Correct Answer**:
From the definitions and conditions discussed, the correct answer to the question is:
- The ratio of change in collector current to change in base current for a constant collector voltage in common emitter configuration.

**Hint**: Look for the option that matches the derived expression for β.

### Final Answer:
The current gain β in a bipolar junction transistor (BJT) is defined as the ratio of change in collector current to change in base current for a constant collector voltage in common emitter configuration.