[AMPS] Solid State Amp Problem

[William L. Fuqua III]

  This reminds me of a problem I was having with a solid state amplifier
output in a system we use for research here at U. of Ky .
It was being modulated at a low frequency and I tracked it down to
the LC power supply filter. It did not have a regulated power supply
but a full-wave rectifier and choke input filter to provide about
28 volts DC.
  This frequency was not a harmonic with the  line frequency and
varied with the power output of the amplifier.
It then occurred to me that many solid state amplifiers appear to
have a negative resistance from the view point of the power supply.
   This one had negative feedback from collectors to the bases to
provide a fixed gain. If I increased the power supply voltage and kept
the drive constant the amplifier would put out the same power as
before but the collector current dropped. This makes sense if the 
efficiency either remains constant or increases with an increase
in power supply voltage.
   If there is a drop in current with an increase of voltage from 
the power supply, the power supply is seeing a negative resistance.
And the filter circuit in the power supply may oscillate, this power supply
had no active components.
   It is possible that the same thing could happen with RF chokes
and bypass capacitors between the amplifier and the power supply.
However I have only seen this problem once and I eliminated it by
reducing the Q of the LC filter.
  I would look first for feedback into the transceiver or power supply.
Just try another power supply first. Some are better shielded and bypassed
for RF than others.
  I wanted to mention this experience because it took me a day or two to
figure it out and others may have similar problem.
73
Bill wa4lav 

  William L. Fuqua III  P.E.  E-mail WLFUQU00@POP.UKY.EDU  Phone (606) 257-4155
  Department of Physics and Astronomy      CP-177 Chem. Phys. Bldg.  
  University of Kentucky ,                 Lexington, Ky 40506-0055
 
  


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