Tuesday, June 22, 2010

While repairing a rig I installed a replacement transistor ...

Q Paul Taylor, WB2GIN, asks, “While repairing a rig I installed a replacement transistor, but now the circuit oscillates on a frequency near TV channel 2! I have read that tiny ferrite beads can be employed to correct this problem. How are they used?

A We occasionally see this on replacement transistors—they can have a higher cutoff frequency than the original so they may oscillate. This happens because parasitic resonances in the circuit that would not have mattered with the original device now become a problem because the replacement transistor is able to amplify at the higher frequency.

Most of the time, a ferrite bead and/or a 10-Ω resistor in the base circuit usually tames things down. If you have a few beads, you may need to experiment, perhaps with two beads on the base lead. Try different material (for VHF, I would use #43 or equivalent material) and perhaps even adding a bead to the emitter and/or collector if the base bead didn’t do it. Unfortunately, the exact
circuit configuration often determines where suppression is needed. If the resonance is in the base circuit, base beading will be most effective. If it is in the collector circuit, a bead on the collector would be most useful.

From QST March 1999