[AMPS] PARALLEL CAPACITOR BY-PASSING

[T. A. Russell]

On Wed, 10 Sep 1997 08:45:27 +0000 w8jitom@postoffice.worldnet.att.net
writes:
.
>> * RF-ground the cold end of the GTR with a triple-bypass using 
>roughly 
>> 100pF, 1000pF and 0.02uF.  Use minimal lead lengths.  This method 
>assures 
>> acceptable bypassing at 160m to10m AND the lower half of the VHF 
>range.  
>
>73, Tom W8JI
...................................

One cautionary note:

When paralleling multiple by-pass capacitors, it is a good idea to
sweep the frequency response of the combination before applying
power.

I once tried this approach for a broadband J-FET follower circuit and
found that the large capacitor became inductive in the band of interest.
There was a high frequency PARALLEL  RESONANCE formed by the
inductive reactance of the large capacitor and the capacitance of the
smaller capacitor.  (These were for radial leaded low voltage ceramic
caps.)

In other words, two unequal value capacitors can actually form a
parallel resonant circuit at a frequency above the series resonance
of the larger capacitor.  BE  CAREFUL when paralleling unequal
capacitor values.

de  Tom  N4KG

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