Question E7B02:
What is a Class D amplifier?  

Choices:  
A: An amplifier that uses switching technology to achieve high efficiency  
B: A low power amplifier that uses a differential amplifier for improved linearity  
C: An amplifier that uses drift-mode FETs for high efficiency  
D: An amplifier biased to be relatively free from distortion    

Correct Answer is  A: An amplifier that uses switching technology to achieve high efficiency  

To understand Class D:

1) Why the correct answer is correct
- Class D amplifiers use high-speed switching devices (often MOSFETs) that are either fully ON or fully OFF. The input is converted to a pulse stream (e.g., PWM), then a low-pass filter reconstructs the analog output.
- Because power dissipation is minimal when a device is fully ON ($V \approx 0$) or OFF ($I = 0$), efficiency is very high.

Key idea:
- Device dissipation: $P_{\text{diss}} = V \cdot I$
  - ON: $V \approx 0 \Rightarrow P \approx 0$
  - OFF: $I = 0 \Rightarrow P = 0$
  - Losses mainly occur during switching transitions and in the filter.

2) Why the other choices are incorrect
- B: “Low power differential amplifier for improved linearity” describes a differential pair (Class A small-signal) concept, not a power amplifier class. Class D is switching, not linear differential.
- C: “Drift-mode FETs” is not a recognized amplifier class. Amplifier classes (A, B, AB, C, D) refer to conduction/switching behavior, not a specific FET mode.
- D: “Biased to be relatively free from distortion” describes Class A operation (continuous conduction, high linearity, low efficiency), not Class D.

3) Useful formulas and rules of thumb
- Ideal efficiencies:
  - Class A (resistive load): $\eta \le 25\%$
  - Class A (transformer-coupled): $\eta \le 50\%$
  - Class B (ideal): $\eta = \pi/4 \approx 78.5\%$
  - Class AB: $\eta$ between Class A and B
  - Class C: $\eta$ can exceed $80\%$ (nonlinear, resonant load)
  - Class D: $\eta$ can exceed $90\%$, ideal $\approx 100\%$
- Conduction angles (for linear classes):
  - Class A: $360^\circ$
  - Class B: $180^\circ$
  - Class AB: $180^\circ$ to $360^\circ$
  - Class C: $<180^\circ$
  - Class D: switching class; conduction angle concept does not directly apply.

Exam tip: Class D = switching amplifier with very high efficiency; not inherently linear, so not used for linear RF modes without special techniques.