[Amps] HV MOSFETs for RF

[Shane Youhouse]

The efficiency goes to hell because all solid state amps I know of in ham
TX are broadband.

Therefore, they will ONLY have decent efficiency at the designed for level.

12 volt final, running at 50 watts pep will have a load of 1.4 ohms

Same final xister running at 15 watts pep will have 4.8 ohm collector
impedance.

Also, broadband binocular cores suck.  Go to transmission line xformers.
Mas Grande better.

Shunt feed the DC to keep DC off the binocular cores if that's what your
going to use.  Get the DC out of their, only serves to heat the brass and
ferrite.

Other tricks are available, as we talked about off line years ago.  Just
like tubes, you need a stiff bias supply, that can handle PEAK current
draw, not just idle. Meaning, as Manfred pointed out, it must source
multiple amps PER transistor. And your B+ must be rock stable as well.

And lots of dissipation.

--Shane
KD6VXI

On Tuesday, May 9, 2017, Jim Thomson <jim.thom at telus.net> wrote:

> Date: Tue, 9 May 2017 07:59:45 +0930
> From: "Leigh Turner" <invertech at frontierisp.net.au <javascript:;>>
> To: "'AMPS'" <amps at contesting.com <javascript:;>>
> 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
> 600watt 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 lets 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 its 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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