[Amps] Some thoughts on a transistorized legal limit amplifier...

[Radio WC6W]

  In order to meet the price, weight and broadband
requirements discussed herein the past few days, I
offer the following approach:

  Start with the 1500 watt peak output amplifier
described in the old APT application note, APT9701,
which posits 25¢/watt for a Class C amplifier section
with 70% efficiency at 27 MHz (ISM band, not CB!). Add
some ferrite to the transformers for wideband
operation.

  Connect a double pi-net (3C's & 2L's) to the output
of the above amplifier for both impedance matching and
harmonic suppression -- No filter bank required. 
Switching the pi-net into 7 or 8 band segments (the
L's and one of the C's) plus padders for 160 should be
about right. These components, including the
switch(es), will have to handle some high currents
but, only the center C will need to be rated for more
than 1KV so, reasonably sized air variables will
suffice for the two variable C's.  

  Employ a power factor correcting (PFC) front end and
a single ended pulse width modulated (PWM) regulator
for the power supply.  This will be a direct off line
unit with AC isolation provided by the RF input &
output transformers.  A line input of 12 amps at
120VAC should be sufficient for 1500W PEP (750W
average) out.  

  The heaviest part of the unit will be the heatsink &
fans which will be required to remove about 600 watts
of heat, figuring an overall wall socket to antenna
efficiency circa 55%.  

  Now, recall this started with a class C amplifier,
so a bit of inexpensive Digital Signal Processing
(DSP) will necessarily be inserted between the mike
and the exciter to provide for envelope restoration
(linearization) of the amplifier via the PWM
modulator.  The exciter will only be required to
provide around 30 watts drive for this scheme to
function.  Note that the exciter must be operated
without compression, limiting, or ALC in this system. 
Those functions may be included in the DSP section.

  The above numbers are figured for SSB operation.  AM
could be implemented by setting a CW carrier at 375
watts output (which will coincidently be the SSB tune
point) and directly modulating the final with the PWM
regulator.   For CW/FM/FSK, the capacity of the
heatsinking and the PFC section would have to be
doubled.   

73 & Good morning,
  Marv WC6W

http://wc6w.50webs.com/  





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