Question E7D05:
Which of the following types of linear voltage regulator operates by loading the unregulated voltage source?  

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

Correct Answer is  D: A shunt regulator  

Correct answer: D — A shunt regulator

Why D is correct:
- A shunt regulator holds the output voltage by drawing current to ground (shunting) as needed. This intentionally “loads” the unregulated source so that the voltage across the load stays constant.
- Typical implementation: a series resistor from the unregulated source feeds the regulated node; a shunt element (zener or transistor/op-amp) to ground varies its current to keep the output at the set voltage.

Key relations for a shunt regulator with series resistor $R_S$:
- Source current: $$I_S = \frac{V_{in} - V_{out}}{R_S}$$
- Kirchhoff’s current law at the output node: $$I_S = I_L + I_{sh}$$
- Regulation condition (zener-like): $$V_{out} \approx V_Z$$
- Worst-case shunt current (light load, high input): $$I_{sh,\max} \approx \frac{V_{in,\max} - V_{out}}{R_S} - I_{L,\min}$$
- Power in shunt element: $$P_{sh} = V_{out}\, I_{sh}$$
Rule of thumb: Shunt regulators are simple and fast but waste power, especially at light loads, because they always “load” the source.

Why the other choices are incorrect:
- A: A constant current source — Regulates current, not voltage. Not a type of linear voltage regulator for voltage regulation.
- B: A series regulator — Uses a pass element in series with the load; it doesn’t regulate by loading the source. It supplies (approximately) only the load current plus small control current.
  - Pass-device power: $$P_{pass} = (V_{in} - V_{out})\, I_L$$
- C: A shunt current source — Not a standard voltage-regulator topology; again implies current regulation, not voltage regulation.

Summary rule:
- Series regulator: controls series drop; efficient at light load; does not intentionally load the source.
- Shunt regulator: controls shunt current; operates by loading the source; best for low-current, simple applications.