Dear All,
First of all I wish to thank all those who sent in material for the
LF book and also for those who have given permission for me to use
material from their websites. It is interesting that when was looking
for material for the earlier edition, finding suitable transmitter
circuits was a problem. Now I have as much on this subject as I can handle!
I will try to include as many of these circuits as I can.
Which brings me to a technical aspect of PA design on which I am a bit hazy.
Most LF PA designs employ FETs in class D push-pull, with the output
taken via a transformer. A Pi section filter is then used to remove
the harmonics.
The documentation on the Decca transmitter modules shows a different
method, with the the output FETs driven in a bridge configuration.
Furthermore the output is fed via a large (airspaced coil) high C
tank circuit
With the old valve PA class C circuits a tank circuit was necessary
to convert pulses of current into a sine wave. However this tank
circuit could be a pi-section arrangement.
I discussed most of the above in a telephone conversation yesterday
with Andy G4JNT. Andy followed this up with an attached file to an
e-mail if a Harris Semiconductor device called a HIP4080 80v/2.5A
peak, high frequency full bridge FET driver. The HIP4080 can switch
at frequencies up to 1MHz and appears to be normally used for driving
voice coil motors, switching amplifiers in class D high frequency
switching audio amplifiers and power supplies.
Is there any advantage to the bridge circuit? and what is the
difference between a tank circuit and a filter in a Pi configuration?
--
Regards, Peter, G3LDO
<[email protected]>
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