Circuitry; Tx Circuits; 200W Final Section - Kenwood TS-480 User Manual

Circuitry

TX circuits

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200W final section

Explained here is the circuitry for the 200W final section, the crown jewel of the TS-480 Series.
This circuit is responsible for developing 200W output with a DC 13.8V power supply. Of course,
various approaches are possible. The typical one would be to use a high voltage (28V or more)
with the FETs in a push-pull arrangement. However, we did not adopt this approach since a
DC-DC combination that raises the voltage to 28V exclusively for this purpose was considered
inappropriate for a compact rig. The final section of a regular transceiver delivers 100W from 13.8V,
so the normal approach would be to use this as the basis for a 200W design. Hence, we
considered the pros and cons of using 4 final devices, each with 50W output, to produce a total of
200W.
I will not go into details here, but following our calculations and tests we discovered that simply
hooking up the devices in parallel would not be a practical solution because of issues related to the
output transformer. The solution we finally adopted was to have a pair of 100W final sections with a
standard push-pull arrangement, combining these to obtain an output of 200W. Since this is the
most popular method, we should perhaps have adopted it from the start, but having no past
experience with a transceiver producing 200W from a 13.8V supply, we looked at the design
issues from various angles including performance, quality, size, cost, and manufacturability.
When one simply says "combine", there are in fact different ways to do this. For example, you can
take a pair of 100W final circuits and connect in series the secondary circuit of the output
transformer in phase to double the output, thus producing 200W. When we actually experimented
with this, we found that it worked okay. Frequency characteristics were good. However, using this
method means that one cannot provide isolation between the amplifiers. So what we finally
adopted was the old standby in situations like this – namely, a wideband hybrid combiner.
The circuit for this combiner is straightforward: if you reverse the input and output it will actually
work as a splitter. For the TS-480HX, we put together a 200W output final circuit by using this type
of combiner on the input/output of a pair of 100W final amplifiers. For the 50MHz band, we have
limited the output to 100W because of the heat that we knew would be generated from loss.
A hybrid combiner only works on the condition that the two signals are identical in amplitude and
phase. Since our final section was to operate in the HF~50MHz bands, it would qualify as
wideband in terms of frequency but there would be some concern for the balance frequency
characteristics. However, this sort of power combiner has been used before for general
applications, so in that sense it is an approach that can be adopted with some confidence.
When it came to the actual design (mounting), an ideal, symmetrical layout of the components was
not possible; however, care was taken to preserve the balance, for example by employing
isometric wiring for patterns in which there are many high-frequency currents flowing.
The device used was the 2SC2782 bipolar transistor. Since this has a collector loss of 220W, there
would a total loss of 880W in a 200W set equipped with four of them; this represents more than
enough leeway for operations.
Continuous transmission performance with such a compact design is explained in the section on
the TS-480 structure.
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