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

Choices:  
A: It bypasses rectifier output ripple around D1  
B: It is a brute force filter for the output  
C: To prevent self-oscillation  
D: To provide fixed DC bias for Q1    

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

To calculate the effect of C2:  
C2 provides a low-impedance path for AC ripple while blocking DC (once charged), so the DC component continues through the rectifier path, but the ripple is shunted away.

Why the correct answer is correct (A):
- D1 is a rectifier diode producing pulsating DC. C2 is placed so that AC ripple components are diverted “around” D1 to ground (or to the low-impedance side), reducing ripple on the DC taken from the rectifier.
- A capacitor passes AC but not steady DC; thus it is ideal for bypassing ripple without disturbing the DC level.

Why the other choices are incorrect:
- B: “Brute force filter for the output” — A brute-force output filter would be a large capacitor placed at the regulator/output node to ground. C2 here is a small bypass specifically around the rectifier, not a main output reservoir.
- C: “To prevent self-oscillation” — Oscillation control uses small compensation capacitors in an amplifier’s feedback or across active devices. C2 is tied to the rectifier path, not part of an amplifier feedback network.
- D: “To provide fixed DC bias for Q1” — Capacitors do not set fixed DC bias; resistors, diodes, or references do. A capacitor can couple or bypass AC, but it cannot establish a steady bias voltage by itself.

Useful formulas and rules of thumb:
- Ripple frequency from line rectification:
  - Half-wave: $f_\text{ripple} = f_\text{line}$
  - Full-wave: $f_\text{ripple} = 2 f_\text{line}$
- Approximate ripple with simple RC filtering:
  $$V_\text{ripple} \approx \frac{I_\text{load}}{C \cdot f_\text{ripple}}$$
  Solve for the needed bypass capacitance:
  $$C \approx \frac{I_\text{load}}{V_\text{ripple} \cdot f_\text{ripple}}$$
- Small “bypass” capacitors are placed to shunt unwanted AC (ripple/noise) while leaving DC operating points undisturbed.