*Question E7D04:
Which of the following describes a three-terminal voltage regulator?*  

*Correct Answer is  B: A series regulator*  

### AI Response: (may contain errors)  

A three-terminal voltage regulator is designed to maintain a constant output voltage regardless of changes in input voltage or load conditions. The correct answer is:

**B: A series regulator**

### Explanation of the Correct Answer:
- **Series Regulator**: A series voltage regulator uses a variable resistance in series with the load to control the output voltage. It adjusts the resistance based on feedback from the output voltage to keep it stable. This is the fundamental operation of a three-terminal voltage regulator, which typically has an input, output, and ground terminal.

### Why the Other Choices are Incorrect:
- **A: A series current source**: This is incorrect because a current source provides a constant current rather than a constant voltage. A voltage regulator specifically aims to maintain a stable voltage output.
  
- **C: A shunt regulator**: While shunt regulators can also regulate voltage, they do so by diverting excess current away from the load, which is not the same as the operation of a three-terminal series voltage regulator. Shunt regulators are typically used in different applications and configurations.

- **D: A shunt current source**: This option is incorrect for the same reason as A. A shunt current source would not maintain a constant voltage; instead, it would provide a constant current, which is not the function of a voltage regulator.

### Useful Formulas:
For a series voltage regulator, the output voltage can be expressed as:

$$
V_{out} = V_{ref} \times \left(1 + \frac{R_2}{R_1}\right)
$$

where:
- $V_{out}$ = output voltage
- $V_{ref}$ = reference voltage (typically a fixed value)
- $R_1$ and $R_2$ are resistors used in the feedback network to set the output voltage.

This formula illustrates how the output voltage can be adjusted based on the resistor values in the feedback loop, a common feature in series voltage regulators.