[Amps] A simple route to a 28V mobile HF amplifier?

[nx7u at nx7u.net]

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 at contesting.com] on behalf of nx7u at nx7u.net
> [nx7u at nx7u.net]
> Sent: Sunday, June 08, 2014 12:41 PM
> To: amps at 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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