Question E7D08:
What type of circuit is shown in Figure E7-2?  

Choices:  
A: Switching voltage regulator  
B: Common emitter amplifier  
C: Linear voltage regulator  
D: Common base amplifier    

Correct Answer is  C: Linear voltage regulator  

To identify Figure E7-2, look for a series pass transistor controlled by an error amplifier and a reference (often a Zener or bandgap). The output is sampled with a resistor divider and fed back to the amplifier to hold the output constant. This is the hallmark of a linear voltage regulator.

Why C (Linear voltage regulator) is correct:
- A linear regulator uses a pass device (transistor) operated in its linear region to drop excess voltage and maintain a steady output.
- It includes:
  - A reference voltage source (e.g., Zener).
  - An error amplifier comparing the output (via a feedback divider) to the reference.
  - A series pass element supplying current to the load.
- No inductors or switching waveforms are present; regulation is continuous (analog), not pulsed.

Why the other choices are incorrect:
- A: Switching voltage regulator
  - Would show a switching device driven by PWM plus energy storage components (inductor and diode/catch diode, and often a capacitor in a defined buck/boost topology).
  - These elements are absent in the linear-regulator schematic.
- B: Common emitter amplifier
  - A CE amplifier is a single-transistor gain stage with the emitter at AC ground (through a resistor/bypass), input at the base, and output at the collector. It’s not configured with a precision reference and feedback to regulate DC output voltage.
- D: Common base amplifier
  - A CB amplifier has the base at AC ground (bias fixed), input at the emitter, and output at the collector. Again, it’s a small-signal RF/IF stage, not a DC regulation circuit with a reference and error amplifier.

Useful formulas and rules of thumb:
- Adjustable linear regulator with divider:
  $$V_{out} = V_{ref}\left(1 + \frac{R_2}{R_1}\right)$$
- Simple Zener-referenced emitter follower:
  $$V_{out} \approx V_Z - V_{BE}$$
- Power dissipation in the pass device:
  $$P_{diss} = (V_{in} - V_{out}) \, I_{out}$$
- Headroom requirement (dropout):
  $$V_{in} \gtrsim V_{out} + V_{drop}$$
  where $V_{drop}$ is the minimum voltage the pass element and circuitry need to regulate (often a few volts for classic designs).
- Heat management: if $P_{diss}$ is significant, a heatsink is required.