Question E7B07:
Which of the following is the likely result of using a Class C amplifier to amplify a single-sideband phone signal?  

Choices:  
A: Reduced intermodulation products  
B: Increased overall intelligibility  
C: Reduced third-order intermodulation  
D: Signal distortion and excessive bandwidth    

Correct Answer is  D: Signal distortion and excessive bandwidth  

To amplify SSB, you need a linear amplifier. A Class C amplifier is intentionally nonlinear (conduction angle < 180°), so it distorts any signal whose amplitude varies, like SSB.

Why D is correct (Signal distortion and excessive bandwidth):
- Class C is efficient because it conducts for less than half the cycle and relies on a tuned circuit to restore a sinewave at one frequency.
- SSB has an amplitude-varying envelope. Nonlinearity in Class C clips/warps the envelope, creating harmonics and intermodulation products that “splatter” into adjacent frequencies.
- Result: audible distortion and increased transmitted bandwidth.

Why the other choices are incorrect:
- A: Reduced intermodulation products — Nonlinearity increases intermodulation, it does not reduce it. IMD gets worse with Class C.
- B: Increased overall intelligibility — Distortion reduces intelligibility; speech sounds harsh/compressed, and adjacent-channel splatter increases.
- C: Reduced third-order intermodulation — Third-order IMD increases with nonlinearity and is the main cause of adjacent-channel splatter in SSB.

Useful rules and formulas:
- Conduction angle by class:
  - Class A: $360^\circ$ (most linear, lower efficiency)
  - Class AB: $180^\circ$–$360^\circ$ (good linearity for SSB)
  - Class B: $180^\circ$ (can be linear in push‑pull)
  - Class C: $<180^\circ$ (nonlinear; OK for CW/FM, not SSB/AM)
- Nonlinear model (shows how IMD arises):
  $$v_o = a_1 v_i + a_2 v_i^2 + a_3 v_i^3 + \cdots$$
  Third-order terms ($a_3$) create IMD products at $2f_1 - f_2$ and $2f_2 - f_1$, which fall near the signal and widen bandwidth.
- Rule of thumb: In a two-tone SSB test, third-order IMD products rise about 3 dB for each 1 dB increase in drive near compression. Linear gain would rise 1 dB per dB. This is why nonlinearity rapidly worsens splatter.

Takeaway: Use Class A, AB, or properly biased B for SSB. Avoid Class C for any amplitude-varying modes.