*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 best described as a **series regulator**. Here's why:

1. **Why the correct answer is correct (B: A series regulator):**
   - A series regulator controls the output voltage by varying the resistance in series with the load. It maintains a constant output voltage despite variations in input voltage or load current.
   - The three terminals typically include an input, output, and ground, allowing the device to regulate the voltage by adjusting the current flow through the series pass element.

2. **Why the other choices are incorrect:**
   - **A: A series current source**: This is incorrect because a series current source is designed to provide a constant current, not a constant voltage. Voltage regulators focus on maintaining a stable voltage output.
   - **C: A shunt regulator**: This is incorrect because a shunt regulator works by diverting excess current away from the load to maintain a constant voltage. It is not a series device and typically has a different configuration.
   - **D: A shunt current source**: This is incorrect because a shunt current source is not related to voltage regulation. It is designed to provide a constant current by shunting excess current, not regulating voltage.

3. **Useful formulas or rules of thumb:**
   - The basic operation of a series voltage regulator can be understood by the formula for output voltage:
     $$ V_{\text{out}} = V_{\text{in}} - I \times R_{\text{series}} $$
   - The regulator adjusts $R_{\text{series}}$ to keep $V_{\text{out}}$ constant as $V_{\text{in}}$ or load current $I$ changes.

Understanding these concepts will help you identify the function and operation of a three-terminal voltage regulator in various circuit designs.