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

Choices:  
A: A series current source  
B: A series regulator  
C: A shunt regulator  
D: A shunt current source    

Correct Answer is  B: A series regulator  

A three-terminal voltage regulator (e.g., 78xx, LM317) uses a pass element in series with the load to maintain a constant output voltage. That’s the definition of a series regulator.

Why B (A series regulator) is correct:
- The device places a controllable pass transistor in series with the load and adjusts its conduction to hold $V_{out}$ constant despite changes in $V_{in}$ or load current.
- Typical pins: Input, Output, Ground/Adjust.

Why the others are incorrect:
- A (A series current source): Regulates current, not voltage. While you can configure some regulators as current sources with an external resistor, the basic three-terminal regulator is a voltage regulator.
- C (A shunt regulator): A shunt regulator (like a Zener or TL431 in shunt mode) varies current to ground in parallel with the load. Three-terminal regulators do not operate this way by default.
- D (A shunt current source): Not how three-terminal voltage regulators operate; also unusual terminology for standard regulator types.

Useful formulas and rules of thumb:
- Adjustable regulator (LM317) output:
  $$V_{out} = V_{ref}\left(1 + \frac{R_2}{R_1}\right) + I_{adj}R_2$$
  with $V_{ref} \approx 1.25\ \text{V}$ and $I_{adj}$ typically small.
- Dropout requirement:
  $$V_{in} \gtrsim V_{out} + V_{dropout}$$
  Rule of thumb: $V_{dropout} \approx 2\ \text{V}$ for classic 78xx; much lower for LDOs.
- Power dissipation (heat in the regulator):
  $$P_{diss} = (V_{in} - V_{out}) \cdot I_{out}$$
  Ensure adequate heatsinking if $P_{diss}$ is significant.