[Amps] Price per Watt Conversation

[Leigh Turner]

I concur with your more conservative design strategy here Jim.

The latest MRFX1K80H devices are certainly a step in the right direction for
high-power RF amplifier devices; but limitations remain particularly when
pressing them into service for Class AB linear amplifier with low IMD
performance. These CW/FM spec'd LDMOS devices running in nonlinear deep
saturation must be substantially de-rated for such use to cater for the low
efficiency and associated massive power dissipation when biased in broadband
linear mode for SSB and low spectral spread / regrowth digital modes.

The thermal resistance between the die and the spreader/heatsink is not so
conducive to getting the heat out and away...to maintain the absolutely
critical junction temperature for ensuring long device life and reliability.
The total systemic junction-to-air thermal resistance is the rather severe
practical limitation of LDMOS.

All is not quite as rosy as it might first seem on paper...  :-(

The remedy: de-rate the devices and use several parallel-combined modules to
achieve high PEP / low distortion output with good MTBF.

Many designers and builders of commercial ham amplifiers misinterpret the
Data Sheets and get carried away in the folly of shoehorning these
deliciously enticing devices into supposed reliable and bullet-proof HF
linear amplifier service...an area of application where rugged tubes have
the edge and still prevail.

The current breed of commercial LDMOS amps have too many short-cuts in their
design; the principal Achilles heel is their thermal aspects and device SOE
under adverse load VSWR and heavy duty cycle conditions.

Solid-state QRO HF linear amps can be made to work very nicely particularly
when conservatively driven and backed-off paralleled modules operating well
clear of their 1 dB power compression points are combined and harmonic
terminating diplexer LPF output filters are used to yield stable performance
and good IMD specs. And this is achievable without resorting to esoteric
pre-distortion and linearity correction techniques.

Leigh
VK5KLT

-----Original Message-----
From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Jim Thomson
Sent: Sunday, 23 April 2017 11:39 AM
To: amps at contesting.com
Subject: [Amps] Price per Watt Conversation

Date: Sat, 22 Apr 2017 16:53:50 +0000 (UTC)
From: Catherine James <catherine.james at att.net>
To: <amps at contesting.com>, Manfred Mornhinweg <manfred at ludens.cl>
Subject: Re: [Amps] Price per Watt Conversation

Manfred,

   So would it be better to use 4 or or more devices with somewhat lower
ratings for linear operation?

   Is there a cheaper option to use simpler heat sinks that don't have to be
carefully machine, but push them less hard by spreading the power over a
larger number of devices?  What's the practical upper limit of devices that
could be used before creating other design problems?

73,
Cathy
N5WVR


##  why not use 4 x BLF188XR or 4 x MRFX1K80H ? Is it even possible to use 2
x devices in parallel in each half of a push-pull amplifier ? If it is
possible, then the heat could be extracted over 4 x devices.
The TX imd, with 4 x devices run at 1.5 kw, on paper, should be good, since
each device is running at 375 w pep output.   Toss in pre-distortion, and
IMD could be reduced further. 

##  As is, I believe the SS amp manufacturers will be using pre-distortion
techniques to try and get acceptable IMD out of fewer devices.  Meanwhile,
the pair of devices will still be running too hot.  

##  A 4 x device SS amp, if feasible, IMO, would not increase the total cost
of the amp by very much...but the benefits would be huge. 

Jim  VE7RF    


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