Question
What can be done to prevent unwanted oscillations in an RF power amplifier?
Answer Options
- 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: C
Explanation
RF power amplifiers can suffer from two types of unwanted oscillations: parasitic oscillations (usually at VHF/UHF) caused by internal component inductances, and fundamental oscillations caused by unintended feedback from the output to the input at the operating frequency.
To prevent these issues, two corresponding measures are used: Install parasitic suppressors (such as ferrite beads on the leads or small RC networks) to dampen high-frequency oscillations. For fundamental oscillations, the stage must be neutralized, which involves feeding a small amount of the output signal back to the input, 180 degrees out of phase, to cancel the inherent, unwanted feedback, ensuring stability.
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