Question E7B05:
What can be done to prevent unwanted oscillations in an RF power amplifier?  

Choices:  
A: Tune the stage for minimum loading  
B: Tune both the input and output for maximum power  
C: Install parasitic suppressors and/or neutralize the stage  
D: Use a phase inverter in the output filter    

Correct Answer is  C: Install parasitic suppressors and/or neutralize the stage  

Correct answer: C — Install parasitic suppressors and/or neutralize the stage.

1) Why C is correct
- Unwanted oscillations usually come from unintended feedback and parasitic LC resonances at VHF/UHF, even in HF amplifiers.
- Parasitic suppressors (e.g., a small series L with a resistor, grid/collector “stoppers,” or ferrite beads) add loss at high frequencies, lowering Q and loop gain so the Barkhausen criterion is not met:
  - Avoid oscillation by ensuring $|A\beta| < 1$ at all frequencies.
  - Adding resistance reduces Q: $Q = \dfrac{|X|}{R}$; increasing $R$ lowers $Q$ and damps oscillations.
- Neutralization creates an out-of-phase feedback path to cancel the device’s internal feedback (e.g., triode/pentode $C_{gp}$), breaking the positive feedback loop that would otherwise meet the oscillation condition.

Useful ideas/formulas:
- Parasitic resonance often from stray $L$ and $C$: $f_0 = \dfrac{1}{2\pi\sqrt{LC}}$.
- Add loss at or above $f_0$ to kill the Q.
- Neutralization cancels the effective feedback capacitance so the net feedback approaches zero at the operating frequency.

2) Why the other choices are incorrect
- A: Tune the stage for minimum loading
  - “Light loading” raises the tank circuit Q and voltage gain, reducing damping and making oscillation more likely.
- B: Tune both the input and output for maximum power
  - Good matching improves desired power transfer but does not address unintended high-frequency feedback; can even increase gain at parasitic frequencies.
- D: Use a phase inverter in the output filter
  - An output network is passive and frequency-selective; adding a “phase inverter” is not a standard or effective technique to stop parasitics and does not cancel internal device feedback.

Rules of thumb for stability
- Use grid/gate/collector/emitter “stopper” resistors close to the device leads (e.g., 10–100 Ω).
- Add ferrite beads or RL parasitic suppressors in anode/collector leads.
- Neutralize triode tube stages; ensure solid RF grounding and short lead lengths.
- Bypass supply lines well; avoid light loading—slightly heavy loading often improves stability.