Pat,
Are 32 V LDMOS amplifier transistors becoming scarce, or hard to
find, as spares, due to the newer 50V LDMOS taking over new designs?
I would say that definitely the trend is toward higher voltages.
13V devices are still with us, because there is a large demand for
mobile use in cars. 28V (or 32V, as you say) devices are aimed basically
at the aeronautical market, which is FAR smaller, so these devices are
being discontinued faster than the 13V ones. The telcom industry has
been using 48V supplies for ages. For fixed use with power supplies,
this 48 or 50V has become a very strong de facto standard, but in truth
it's still too low for high power devices. MOSFETs tend to work best at
higher voltages, like 80-100-120V. At lower voltages it's increasingly
hard to achieve good IMD and efficiency. There is even a trend pushing
toward a supply voltage around 300V for high power MOSFETs.
Looks like maybe new Ham
designs should migrate to 50 volt devices. yes/no?
Well, definitely they shouldn't use 28/32V. In the 100W class, I think
it's good to stick to 13V, because devices are available, new ones are
ocassionally developed, and it's always good to be compatible with the
electrical system of a car, so one can use a single car battery for
emergency power. For mid power using conventional devices, indeed 50V is
THE standard. And for new designs using new parts, specially in power
levels of 1kW and higher, at least 100V should be considered.
Interestingly the trend in digital electronics is going the other way.
In the 1970's, logic supply voltages were either 5V, or much higher,
like 12 or even 15V. There were forecasts that future logic components
would use even higher voltages, for increased noise immunity. But then
the opposite happened: Newer logic families worked at ever lower
voltages. Today 12V logic is out, 5V is the highest one ever sees, with
3.3V being probably the most common, and several lower voltages such as
1.8 and 1.4V being the norm in high speed logic. Lower voltages allow
increasing the speed of logic devices without incurring in excessive
dissipation.
The bad about this is that when designing equipment, it's often hard or
impossible to power everything from the same voltage, as was commonly
done in the 70's. In my latest radio, I'm using 12V for the audio and IF
circuitry, 6V for the mixer, 5V for the CPU and display, 3.3V for the
periphery and 1.8V for the direct digital synthesizer. I couldn't find a
suitable synthesizer chip and a display sharing the same voltage!
Always keep in mind that the greatest thing about standards is that
there are so many to choose from!
Manfred
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