[Mobile-Portable] HF Mobil Amp

[K7fe@aol.com]

David,
I was a rf design engineer in my past life and have been designing ss amps 
for over 35 years I have worked for TRW Semiconductors, Motorola and Eimac(CTC). 
 The transistors can be obtained from distributors such as RF Devices.  Its 
not that hard.  Build one amp (pair) and do the same thing three more times, 
larger box, heat sink, combine them and add a fan. I am not sure which app note 
from Motorola you are referring to, just make sure that it is designed around 
12V not 28V devices.  The output impedance is much lower for the 12 V and will 
more closely represent what you want.  Keep in mind that the App Note circuit 
was designed for Motorola devices not Toshiba, so while it will be a good 
starting point the input and output impedance of the transistors is different and 
some changes in the value of your matching capacitors will be required.  (The 
Toshiba's cost less than Mot.)  Build the bias power supplies with voltage 
regulator IC's and output power transistors.  Do not use a silicon diode to 
establish your bias level -- it won't necessarily be "linear" and unless you mount 
the diode on the heat sink, the amplifier can go into thermal runaway.  Some 
application notes used the emitter - base junction of an identical rf power 
transistor in the bias circuit (which acts as a diode & CTC marketed a two 
leaded RF transistor for bias called a "Byistor," ) and mount it on the heat sink 
so as to prevent thermal runaway, but frankly I prefer a regulated bias supply. 
 
You can adjust the variable voltage bias for proper linearity using a scope & 
two tone test generator or other equipment that displays linearity.  Heath, 
Yaesu and others build Ham type monitor scopes.  Don't rely upon "on the air" 
reports, louder doesn't mean distortion free. 
Have fun with it.
73,
Terry, K7FE  


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