[Amps] HV MOSFETs for RF

[Leigh Turner]

Yeah Peter's Class-A amp is likely run with considerable back-off so the
expected efficiency figures seem worse. Nevertheless I'm sure it runs
squeaky-clean with very low IMD at 25 Watts PEP.

The "average operator" to whom you refer is often clueless in adjusting
their rig and amp combos for attaining good end-to-end Tx IMD.

New SS amps like the OM-600 etc are primarily designed with modern SDR
exciters in mind, those owners being their target audience rather than
conventional 100/200W rigs. The latter requiring a power attenuator pad to
properly interface and achieve the systemic operational sweet-spot and no
nasty power overshoot possibilities. In some respects this is not unlike the
situation with a high-gain grid-driven AB1 tetrode linear amplifier.

In the ham HF context I would regard IMD levels attained by the classic
Yaesu FT-767GX or the Icom IC-751A rigs as perfectly acceptable with their
2-tone IMD performance and cleanness of their on-air signal. Sadly few
successive modern ham rigs managed to attain / retain this good benchmark in
respect of IM3 and curtailing the more troublesome higher order stuff :-(

Marine HF radios of virtually any era routinely achieve this good level of
IMD performance because all Tx user controls are removed from the front
panel so the rig always stays in its sweet spot mode no matter what.

Similarly the commercial and military HF radio manufacturers like Codan, etc
routinely attain excellent Tx IMD performance from the very same families of
RF power devices used by the ham radio manufacturers; so it can be done if
the will and commitment to do it is there.

Leigh
VK5KLT

-----Original Message-----
From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Jim Thomson
Sent: Wednesday, 10 May 2017 3:10 AM
To: amps at contesting.com
Subject: [Amps] HV MOSFETs for RF

Date: Tue, 9 May 2017 07:59:45 +0930
From: "Leigh Turner" <invertech at frontierisp.net.au>
To: "'AMPS'" <amps at contesting.com>
Subject: Re: [Amps] HV MOSFETs for RF

Good point Peter; not an appealing scenario for your average punter...where
pragmatism and practicality rule the day.

Perhaps limiting the Class-A PEP to 10 Watts and having a PA bias mode
switch would be a good compromise for keeping the purists happy.

I think there's an unwarranted obsession with low transmitter IMD numbers in
the non-channelized HF ham bands where above a certain respectable number no
practical advantage is realized. In many cases poor Tx signals heard on air
are attributed to bad operator practice and maladjustment rather than
inherent transceiver design issues in respect of IMD; although a few
notoriously bad rigs are out there.

73

Leigh
VK5KLT

##   Leigh, no class A amp runs a lousy 8.9% eff. I see that the new OM-600
SS 600 watt amp is the only ss amp on the market without an ALC jack on the
back of the amp.  The average operator will have fun times trying to get his
100-200 watt xcvr..
.....down to just a few watts to drive it. Dunno if Henry radio is still
selling their pads or not.  Ideally, you find the imd sweet spot on the
xcvr...then insert the correct pad for the SS amp drive level.  A k3s sweet
spot for imd is 30 w pep out.  A 3-6db pad is then used, so 7.5 - 15 watts
to drive the ss amp. 

##  what do you call acceptable IMD ?? I mean well away from the other
fellows freq. And let's not make the classic mistake of adding the RX BW to
the TX BW.  IE: Tx on say 3800 LSB.  RX station listens on 3803 + 3797..and
thinks the TX station is 3 khz up, and 3 khz down...using his 3 khz wide ssb
filter. If the RX station had used a 200 Hz filter, he would hear the TX
station down 3 khz.... but only 200 Hz up. 

##  baffles me why I / K / Y cant produce ham gear with IMD thats as good as
their marine gear. Sure, the marine ssb gear does not have a mic gain
control + processor, but it's still tested with a 2 tone signal, same as the
ham gear.   Its not like its gonna cost them more $. 

## The old Yaesu 767 ham xcvr is a magnitude better than modern ham gear.
50% eff, and 100 w pep out from a pair of MRF-422s.  My 1000-D uses the same
pair of MRF-422s...and runs 200w pep out at 50%.  And 100w pep out at 35%
eff. So what happened there ? 

The 767 was designed to operate at 100w pep out + 50%...and real low IMD,
like -46db pep IMD3 
The 200w yaesus have good imd...at 100w out....but then eff sucks...at 35%. 
They already had it right the 1st time..with the 767.   If they wanted a
200w xcvr, they should used 4 x MRF-422s. 

Jim    VE7RF
    

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