Question E7D01:
How does a linear electronic voltage regulator work?  

Choices:  
A: It has a ramp voltage as its output  
B: It eliminates the need for a pass transistor  
C: The control element duty cycle is proportional to the line or load conditions  
D: The conduction of a control element is varied to maintain a constant output voltage    

Correct Answer is  D: The conduction of a control element is varied to maintain a constant output voltage  

To regulate a DC output, a linear regulator uses feedback to continuously adjust how much a control element (usually a transistor) conducts so the output stays constant despite changes in input voltage or load.

Why D is correct:
- In a linear regulator, a pass device (series or shunt transistor) operates in its active region. Its conduction is varied by an error amplifier comparing the output to a stable reference.  
- By increasing or decreasing the pass element’s conduction, the regulator drops just enough voltage to hold the output at the set value.

Why the other choices are incorrect:
- A: “It has a ramp voltage as its output”  
  - A linear regulator produces a DC output, not a ramp. A ramp/sawtooth is typical of PWM oscillators in switching supplies.
- B: “It eliminates the need for a pass transistor”  
  - Linear regulators depend on a pass element (series or shunt transistor) to control voltage; they do not eliminate it.
- C: “The control element duty cycle is proportional to the line or load conditions”  
  - Duty cycle control is characteristic of switching regulators (PWM). Linear regulators use continuous (analog) conduction, not duty-cycle modulation.

Useful formulas and rules of thumb:
- Adjustable linear regulator setpoint (e.g., LM317-type):  
  $$V_{out} \approx V_{ref}\left(1 + \frac{R_1}{R_2}\right)$$
- Dropout requirement:  
  $$V_{in} - V_{out} \ge V_{drop}$$
- Pass element power dissipation (series regulator):  
  $$P_{pass} = (V_{in} - V_{out}) \, I_{out}$$
- Efficiency (ideal estimate):  
  $$\eta \approx \frac{V_{out}}{V_{in}}$$

Key idea: Linear regulators maintain a constant output by analogly varying the conduction of a pass device, not by switching.