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In RF amplifiers, what is the purpose of neutralization?

Difficulty: Medium

Correct Answer: Stop oscillation

Explanation:


Introduction / Context:
Neutralization in RF amplifiers is a stability enhancement technique. It is used to prevent unwanted oscillations caused by internal feedback through interelectrode capacitances of active devices such as vacuum tubes or transistors.


Given Data / Assumptions:

  • High-frequency RF amplifier stage.
  • Parasitic capacitances between input and output cause positive feedback.
  • Neutralization network introduced to cancel this feedback.


Concept / Approach:
The amplifier can inadvertently become an oscillator if uncontrolled positive feedback occurs. Neutralization provides an out-of-phase feedback path that cancels the unwanted coupling, thereby stabilizing the amplifier without affecting desired gain significantly.


Step-by-Step Solution:

Unneutralized amplifier: input-output coupling through C_m.Feedback may satisfy Barkhausen criterion → oscillation.Neutralization introduces counter-signal of equal magnitude, opposite phase.Oscillation is suppressed, amplifier remains stable.


Verification / Alternative check:

Classic RF tube circuits used neutralizing capacitors for stability.


Why Other Options Are Wrong:

Neutralization does not primarily increase bandwidth or improve selectivity.It focuses on stability, not on reducing noise.


Common Pitfalls:

Assuming neutralization improves selectivity—this is handled by tuned circuits, not neutralization.


Final Answer:

Stop oscillation
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