Bill,
An interesting point about SOAR.
Looking further, I notice some inconsistencies.
Both Motorola and Philips/NXP publish SOAR curves for their RF FETs.
Motorola SOAR curves follow Id,max all the way down to Vds=0.
However, Philips shows a "current may be limited by Rds,on" region to
the upper left (low Vds, high Id). In all the examples I've seen, the
current limiting begins at Vds=4V (no matter the rated Vds of the
device). To me, this implies that you should not drive the FET hard
enough so that Vds drops below 4V, which has a direct effect on how much
power you can extract from a device.
I suppose the simple route is to play it safe and call Vds=4V the
minimum (and therefore for a nominal 28V supply, the peak swing is now
24V), but it's curious why there is a difference between the two
vendors.
An example pair of parts to examine would be the MRF173 and the BLF246.
Both are 28V and 80W parts.
On 2014-06-08 12:31, Fuqua, Bill L wrote:
A tech that used to work for me called them "Blowup Junction
Transistors" after trying to operate one
biased in linear range rather than using them for a switching
application. Take a good look at the safe operating
region of current and voltage published in the spec. sheet.
73
Bill wa4lav
________________________________________
From: Amps [amps-bounces@contesting.com] on behalf of nx7u@nx7u.net
[nx7u@nx7u.net]
Sent: Sunday, June 08, 2014 12:41 PM
To: amps@contesting.com
Subject: [Amps] A simple route to a 28V mobile HF amplifier?
Lately I have been looking at transitioning my mobile HF amplifer from
12V to ...more.
Currently it seems that 50V is all the rage; but that's not so simple
to
implement. 28V would be a lot simpler.
I have a number of DU2880U HF MOSFETs. They are 28V devices, 2-150
MHz.
I'm guessing they must have been among the first 28V HF MOSFETs,
because they are rated for only 80W output. What's intriguing about
them is that they are physically identical to the common MRF454/458 and
2SC2290/2SC2879 BJTs.
It occurred to me that maybe the fastest and easiest way for me to get
to 28V is to take an existing HF solid-state amplifier and substitute
in
the DU2880U devices. There are a few things that would have to be
changed in the donor amplifier that I've thought of:
1. The existing control circuitry is all 12V based, but that could
easily be managed in a battery-powered situation by wiring the DC feed
as 0/+12/+24V.
2. Impedance levels are different, so the input and output transformers
for each amplifier module would need to be changed. One plus is that
the impedance ratios are more favorable at 28V.
3. RF circuit changes (for example, the input gain equalization), but
those should be pretty simple.
4. Bias circuit would need to be modified for a higher voltage. But
the
current capability of the existing circuit would be way more than
adequate.
Other than that, everything else should be reusable as-is...Low-pass
filters, T/R relays, monitoring, input splitters and output combiners,
cooling...and most importantly, all the chassis work and packaging is
already done for me. As an added bonus, I'd have a set of BJTs to sell
to offset incurred costs.
I do have a "parts" Ten-Tec Hercules II chassis that is 95% complete.
The nice thing about the Herc II is that the construction is very well
modularized, and I think it would lend itself well to such a
modification.
Am I missing anything? Or just crazy? I would very much welcome any
comments or pointers.
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