*Question E7D07:
What is the purpose of C2 in the circuit shown in Figure E7-2?*  

*Correct Answer is  A: It bypasses rectifier output ripple around D1*  

### AI Response: (may contain errors)  

To understand the purpose of C2 in the circuit shown in Figure E7-2, let's break down the function of each component and why C2 is used:

### Correct Answer: A: It bypasses rectifier output ripple around D1

1. **Why the correct answer is correct:**
   - **C2 as a Bypass Capacitor:** In power supply circuits, capacitors like C2 are often used to smooth out or bypass the AC ripple that appears on the DC output after rectification. The rectifier (D1) converts AC to DC, but the output is not perfectly smooth. C2 helps to filter out this ripple, providing a more stable DC voltage.

2. **Why the other choices are incorrect:**
   - **B: It is a brute force filter for the output:** While C2 does filter the output, the term "brute force filter" typically refers to a more comprehensive filtering approach, often involving multiple components like inductors and additional capacitors. C2 alone is not sufficient to be considered a brute force filter.
   - **C: To prevent self-oscillation:** Self-oscillation is typically prevented by feedback network design or by using components like resistors and capacitors in specific configurations to stabilize the circuit. C2's primary role here is not to prevent oscillation but to smooth the DC output.
   - **D: To provide fixed DC bias for Q1:** A capacitor like C2 does not provide a fixed DC bias. Biasing is usually achieved with resistors or active components that set the operating point of a transistor like Q1. C2's role is related to filtering, not biasing.

### Useful Concepts:
- **Ripple Voltage:** The residual periodic variation of the DC voltage within a power supply after rectification. Capacitors are used to reduce this ripple.
- **Capacitive Filtering:** The formula for the ripple voltage ($V_r$) in a simple rectifier circuit with a filter capacitor is approximately:
  $$ V_r = \frac{I}{f \cdot C} $$
  where:
  - $I$ is the load current,
  - $f$ is the frequency of the AC supply (usually double the line frequency for full-wave rectification),
  - $C$ is the capacitance.

By understanding these concepts, you can see why C2 is crucial for reducing ripple and stabilizing the DC output in the circuit.