Question E6D09:
What devices are commonly used as VHF and UHF parasitic suppressors at the input and output terminals of a transistor HF amplifier?  

Choices:  
A: Electrolytic capacitors  
B: Butterworth filters  
C: Ferrite beads  
D: Steel-core toroids    

Correct Answer is  C: Ferrite beads  

Correct answer: Ferrite beads

Why C is correct:
- Ferrite beads act as lossy inductors whose impedance increases with frequency. At VHF/UHF they present a large, mostly resistive impedance that damps parasitic oscillations on transistor leads, while having minimal effect at HF.
- They are slipped over the base/gate and collector/drain leads at the amplifier’s input/output to suppress unintended high-frequency gain.

Key idea:
- A ferrite bead’s impedance can be modeled as $Z(f)=R(f)+jX(f)$ with $R(f)$ rising strongly at VHF/UHF, providing broadband loss (damping).
- For a simple inductor, $X_L=2\pi f L$; ferrite beads add loss so they don’t just detune—they dissipate energy and prevent oscillation.

Why the other choices are incorrect:
- A: Electrolytic capacitors
  - Poor at VHF/UHF due to high ESR/ESL; they are for low-frequency decoupling, not for suppressing VHF parasitics on device leads.
- B: Butterworth filters
  - These are intentional filter networks (e.g., low-pass) used at an amplifier output for harmonic suppression, not as lead-mounted parasitic suppressors.
- D: Steel-core toroids
  - Steel is not used for RF suppression on device leads; RF chokes/cores use ferrite or powdered iron. Steel cores have high eddy currents and saturate easily, unsuitable for VHF/UHF parasitic control.

Rules of thumb:
- Place beads as close as possible to the transistor terminals with shortest leads.
- One or more beads in series increases suppression; use suitable mixes (e.g., 43 for HF/VHF, 61 for higher frequencies).
- Goal: add high resistive impedance at unintended VHF/UHF while keeping HF loss small.