Question
What devices are commonly used as VHF and UHF parasitic suppressors at the input and output terminals of a transistor HF amplifier?
Answer Options
- A) Electrolytic capacitors
- B) Butterworth filters
- C) Ferrite beads
- D) Steel-core toroids
Correct Answer: C
Explanation
Transistors designed for HF amplification often have high gain at unintended VHF and UHF frequencies. This excess high-frequency gain can cause the amplifier to oscillate uncontrollably at a parasitic frequency, leading to instability, spurious emissions, and transistor failure. These parasitic oscillations must be suppressed at the amplifier’s terminals.
Ferrite beads are commonly used as simple yet effective parasitic suppressors. A small coil (or simply a wire) is passed through the ferrite bead, which presents a very high impedance (acting as an RF choke) at the parasitic VHF/UHF frequencies, dampening the oscillation without significantly affecting the desired HF signal (where the ferrite is transparent).
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