Question E7B08:
Why are switching amplifiers more efficient than linear amplifiers?  

Choices:  
A: Switching amplifiers operate at higher voltages  
B: The switching device is at saturation or cutoff most of the time  
C: Linear amplifiers have high gain resulting in higher harmonic content  
D: Switching amplifiers use push-pull circuits    

Correct Answer is  B: The switching device is at saturation or cutoff most of the time  

Correct answer: B — The switching device is at saturation or cutoff most of the time

Why B is correct:
- In a switching (Class D/E/F) amplifier, the device is either fully on (very low voltage across it) or fully off (very little current through it). Since device dissipation is the average product of voltage and current:
  $$P_{\text{diss}} = \langle v(t)\,i(t) \rangle$$
  keeping either $v \approx 0$ or $i \approx 0$ most of the time makes $P_{\text{diss}}$ very small, so efficiency approaches 100% in the ideal case.

Why the other choices are incorrect:
- A: Switching amplifiers operate at higher voltages  
  Not inherently. Efficiency is not determined by supply voltage level; it’s determined by minimizing overlap of voltage and current in the device. High voltage does not guarantee high efficiency.

- C: Linear amplifiers have high gain resulting in higher harmonic content  
  Linear amplifiers aim for linearity (low distortion), not higher harmonic content. In fact, switching amplifiers produce more harmonics due to fast edges and require output filtering. Harmonics aren’t the reason for efficiency differences.

- D: Switching amplifiers use push-pull circuits  
  Push-pull is a topology that both linear and switching amplifiers can use. It can help with symmetry and even-harmonic cancellation, but it is not what makes switching amplifiers efficient. The key is the on/off operation minimizing $v \cdot i$ overlap.

Useful formulas and rules of thumb:
- Efficiency:
  $$\eta = \frac{P_{\text{out}}}{P_{\text{in}}} = \frac{P_{\text{out}}}{P_{\text{out}} + P_{\text{diss}}}$$
- In switching devices:
  - Conduction loss (when “on”): $$P_{\text{cond}} \approx I_{\text{rms}}^2 R_{\text{on}}$$
  - Switching loss (during transitions): $$P_{\text{sw}} \approx f_s \cdot \tfrac{1}{2} V I (t_r + t_f)$$
- Ideal/theoretical efficiencies:
  - Class A: up to 25% (resistive load) or 50% (transformer-coupled)
  - Class B: 78.5% (theoretical)
  - Class AB: between Class A and B
  - Class C: often 75–85% with a tuned load
  - Switching (Class D/E/F): can exceed 90% in practice, 100% ideal

Key takeaway: Switching amplifiers are efficient because device dissipation is minimized by operating the active device either fully on or fully off, keeping $v \cdot i$ near zero most of the time.