[AMPS] Capacitors & Resonance

[G0MJW]

The ideal capacitor for an amplifier ? IT depends what it is for. Quite
often, we don't want a capacitor at all. We want some form of DC blocking to
stop the EHT appearing at the antenna, or we want to ground the cathode
and/or screen to RF while maintining a DC bias voltage.

In these cases, does not really matter if the capacitor looks like an
inductor or a capacitor at the operating frequency as long it isolates the
voltage and the reactance can be tuned out. In the latter case, it may be
impossible to effectively ground the electrode with a real capacitor. If the
capacitor is chosen to have a resonance with its own inductance at the
frequency of interest, it will look like a short circuit and be a very
effective RF bypass.

It might seem that a 30pF 10kV doorknob capacitor resonant at 200 MHz would
be a good anode bypass capacitor at 150MHz, only 10 ohms reactance at a
point with several kohms impedence. It won't work though, the amp will be
unstable. But a 1000pF doorknob resonant at 50 MHz might.


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