Cathy,
When we get to 3000V, we can have drop-in replacements for our tube
amplifier finals! :-)
That would be fun... :-)
But not really practical. The high voltage needed for tubes is a
nuisance rather than an advantage. And the low voltage needed by most
transistors is also a nuisance. The golden spot lies in between.
Paul,
> Yesterday, it was +50V, today it's 65V,
tomorrow 75V, then 100V.? Where will it end?
Impossible to predict.
Where is the point of diminishing returns on operating voltage?
This is an interesting question.
My idea on that subject is that amplifiers should work at such a voltage
that the circuit impedances are very roughly in the neighborhood of 50
ohm, or a bit higher, but not much higher nor much lower. Because at
very low impedances, such as are common with most transistors, stray
inductance is a big problem, while at very high impedances, such as
those used by tubes, stray capacitances are an equally big problem. You
can't make a broadband high power VHF amplifier, neither with tubes nor
transistors, because of this. You always need tuned networks to absorb
and "tune out" the stray reactances.
With transistors at least you can make broadband HF high power
amplifiers, although with some performance penalties relative to tuned
designs. With tubes you can't even do that! Tubes are really suitable
for broadband operation only on pretty low frequencies - MF, LF or VLF
depending on the tube.
1500 watt on 50 ohm is 274 volt RMS. A supply voltage of roughly 400V is
optimal to run this power in a nice, simple, single-ended amplifier.
Half that voltage would be OK for push-pull amps, but that requires more
complex circuitry. Something higher than 400V is OK too, say 500 or even
600V. Higher than that is too much.
Such amplifiers would most likely need to be tuned, just like tube
amplifiers. For two reasons: Transistor capacitances will probably be
too high to allow broadband operation at those voltages, and broadband
transformers for high voltage get hard to make, as they require a
proportionally larger product of turns number and ferrite cross-section,
and this creates phasing problems due to conductor length.
Three basic concepts for tuned solid-state amps come to mind:
1) Bandswitched tuned networkd instead of bandswitched low pass filters;
2) Manually tuned tanks like in most tube amps;
3) Automatically tuned tanks like in some tube amps.
At present there are some high voltage RF power MOSFETs available, but
not many, and they are all VDMOSFETs rather than LDMOSFETs. That means
higher capacitances.
Some developments around silicon carbide MOSFETs and gallium nitride
MOSFETs look interesting.
Manfred
========================
Visit my hobby homepage!
http://ludens.cl
========================
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|