[AMPS] Capacitors & Resonance

measures measures@vcnet.com
Thu, 4 Nov 1999 05:00:09 -0700


>
>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.

A capacitor that is resonant at 50MHz looks like an inductance at 150MHz. 
 A better bypass for 150MHz might be 200 pF.  

cheers.  

-  Rich..., 805.386.3734, www.vcnet.com/measures.  


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