# tuned 40W 40MHz RF power amplifiers

W

#### Winfield Hill

Jan 1, 1970
0
I need a few 40W 40MHz RF power amplifiers and can think of a
half-dozen ways to do it, from RF transistors to commercial
modules, but I'm not that happy with any of my ideas; there
has to be something better out there. The load is tuned, but
I'd like a Q of no more than 40. Suggestions for easy-to-get
transistors or modules, etc.?

P

#### Paul Burridge

Jan 1, 1970
0
I need a few 40W 40MHz RF power amplifiers and can think of a
half-dozen ways to do it, from RF transistors to commercial
modules, but I'm not that happy with any of my ideas; there
has to be something better out there. The load is tuned, but
I'd like a Q of no more than 40. Suggestions for easy-to-get
transistors or modules, etc.?

That's a radio control frequency (here in the UK at any rate) with a
Po limit of 100mW! Are we near sunspot maxima? If so, kindly drop me
an warning e-mail if you're planning on using that thing!

W

#### Winfield Hill

Jan 1, 1970
0
Paul Burridge wrote...
That's a radio control frequency (here in the UK at any rate) with
a Po limit of 100mW! Are we near sunspot maxima? If so, kindly
drop me an warning e-mail if you're planning on using that thing!

This will be inside a coax, generating 500V on a 50pF crystal,
after resonant step-up of course..

J

Jan 1, 1970
0
Paul Burridge said:
That's a radio control frequency (here in the UK at any rate) with a
Po limit of 100mW! Are we near sunspot maxima? If so, kindly drop me
an warning e-mail if you're planning on using that thing!

He might be trying to build a plasma drive or something -- there are plenty
of uses for RF (and RF power amplifiers) that don't involve broadcasting.

Now if he'd ask for a 500W transmitter at 27MHz I'd be worried...

J

#### Joerg

Jan 1, 1970
0
Hi Winfield,

If it has to be quick and easy you could look at ham radio solutions.
There are plenty of transistor amps described in books such as the ARRL
Handbook that do 3MHz-30MHz at 100W or more. They may need a little mod
for 40MHz or will operate sluggishly there but should be able to produce
40W. It might even be possible at less cost to modify one of those 'not
so legal' CB booster amps. Surplus VHF TV driver modules are another option.

If it's just a tuned circuit you want the best bet may be to take a
large enough FET, a tuned circuit at the drain and drive it hard from a
digital oscillator, buffer and resonant step-up transformer.

I'd probably do it with a tube, just for nostalgia. If you need really
high impedances that would be the ticket but the plate voltage makes
this approach quite dangerous if students are involved.

Regards, Joerg

T

#### Tim Wescott

Jan 1, 1970
0
Joerg said:
I'd probably do it with a tube, just for nostalgia. If you need really
high impedances that would be the ticket but the plate voltage makes
this approach quite dangerous if students are involved.

Antique radio supply has 1625's for $5 a pop, plus sockets and grid caps if you need 'em. I toy with the notion of building a transmitter with a 6V6 oscillator, 807 or 1625 final, 1940 all the way, then modulate it with and a class-D amplifier. C #### Clarence Jan 1, 1970 0 Tim Wescott said: Antique radio supply has 1625's for$5 a pop, plus sockets and grid caps
if you need 'em. I toy with the notion of building a transmitter with a
6V6 oscillator, 807 or 1625 final, 1940 all the way, then modulate it
with and a class-D amplifier.

I guess you know to watch the filter for the Class 'D' modulator, else you will
be a very wide signal.

T

#### Tim Wescott

Jan 1, 1970
0
Clarence said:
I guess you know to watch the filter for the Class 'D' modulator, else you will
be a very wide signal.
Yup. I doubt that I'll ever actually do it, but it's fun to think about.

W

#### Winfield Hill

Jan 1, 1970
0
I have taken the liberty of crossposting this to alt.ham-radio.hf,

Joerg wrote...

If it has to be quick and easy you could look at ham radio solutions.
There are plenty of transistor amps described in books such as the
ARRL Handbook that do 3MHz-30MHz at 100W or more. They may need a
little mod for 40MHz or will operate sluggishly there but should be
able to produce 40W.

This seemed a good idea, and I have an 18" bookshelf of ham books,
handbooks, and article compendiums. But looking through them I
was unable to find much of use - I'm not interested in using tubes.
The power amp chapter in the 2002 ARRL Handbook (unchanged in this
respect from the 1998 issue) suggested the MRF464 bipolar RF power
transistor for a 30MHz 80W amplifier, but this is an obsolete part,
no longer available at the distributors I checked. They also don't
present a detailed design to go with this suggestion.
It might even be possible at less cost to modify one of those 'not
so legal' CB booster amps.
Hmm.

Surplus VHF TV driver modules are another option.

Interesting, where does one get these?
If it's just a tuned circuit you want the best bet may be to take a
large enough FET, a tuned circuit at the drain and drive it hard
from a digital oscillator, buffer and resonant step-up transformer.

Checking the MRF464 reminded me to look at Motorola's other newer RF
MOSFET power transistors. They still offer some high-frequency parts,
but most of their RF power MOSFET line was sold to M/A-COM. The data
sheets and app notes are unchanged from the Motorola versions.

In the case of an RF MOSFET, where the device is "on" for only a small
part of each cycle, say 30 degrees (which is 2ns at 40MHz), it's not
possible to "hard drive" the FET and turn it on and off, in the sense
we are used to. Instead the gate is presented with a sine wave wave
from a tuned matching stage (the gate's high capacitance looks like a
low RF impedance), and it's DC biased to be on for a small time at the
tip of each cycle. Of course it's not going on and off, instead its
channel conductance is just increasing and decreasing for a few ns.

These RF FETs are not made with the VMOS groove construction we're used
to in common power MOSFETs. With high capacitance and poor high thermal
resistance, not to mention high gate spreading resistance, ordinary power
MOSFETs are badly suited for applications other than on/off switching.
Instead the RF types are lateral FETs, sort of large versions of the old
small enhancement-mode FETs such as the 2n4351 we talked about in AoE.
This means that despite the RF power MOSFET's large die area, it has a
comparatively low-capacitance, which is well suited for RF work.

The M/A-COM parts are stocked by Richardson, and three of them look
interesting for my inventory. Does anyone recognize any of these FETs?

. type Vdss Pd Ciss Crss cost ea
------- ---- ---- ---- ---- -------
MRF134 65V 18W 7pF 4.5pF $21.42 MRF171A 65 115 80 8 (at 28V) 37.40 MRF148A 120 115 50 8 (at 50V) 35.70 By comparison, a modern VMOS type power MOSFET rated at 100V and 127W has Ciss = 1150pF and Crss = 62pF (at 25V), which is about 10x higher. Of course, it's able to handle much higher DC currents when fully on. P #### Paul Burridge Jan 1, 1970 0 Interesting, where does one get these? Checking the MRF464 reminded me to look at Motorola's other newer RF MOSFET power transistors. They still offer some high-frequency parts, but most of their RF power MOSFET line was sold to M/A-COM. The data sheets and app notes are unchanged from the Motorola versions. In the case of an RF MOSFET, where the device is "on" for only a small part of each cycle, say 30 degrees (which is 2ns at 40MHz), it's not possible to "hard drive" the FET and turn it on and off, in the sense we are used to. Instead the gate is presented with a sine wave wave from a tuned matching stage (the gate's high capacitance looks like a low RF impedance), and it's DC biased to be on for a small time at the tip of each cycle. Of course it's not going on and off, instead its channel conductance is just increasing and decreasing for a few ns. From what I've seen, these power MOSFETs aren't much use for anything beyond HF. Check these out, though: 2SC1969 2SC1971 2SC1972 These are BJTs but some sort of arrangement of them should get you near the mark. R #### Rich Grise Jan 1, 1970 0 I have taken the liberty of crossposting this to alt.ham-radio.hf, rec.radio.amateur.homebrew, and alt.ham-radio.vhf-uhf Joerg wrote... Interesting, where does one get these? AFAIK, the station engineer. I read a construction article many years ago about 2 x 813s in a push-pull 1KW final. The author said that stations routinely replace their driver tubes (which these days could very well have been replaced with SS), just to avoid failures, and the pulled tubes are really cheap, like they toss them. I have no idea if anything like this is going on today. Good Luck! Rich J #### Joerg Jan 1, 1970 0 Hi Winfield, For some reason I can't see my own post on this thread. Anyway, RF transistors become obsolete quite quickly so a 2002 schematic may need the transistor(s) to be substituted. Same for low noise preamp transistors where my old Motorola favorite simply vanished. For bipolar RF power transistors you might want to check BLX15 and things like that. You probably have to settle for a transistor you can get easily and not for the dream transistor, the market is so small that there may be no stock. If the budget allows to simply buy the amps, their 6m amp might work: http://www.mizuhoradio.com/personal/k7vo/thp/qpamps.html Look on the web for "6m" amps since that is the newer 50MHz ham gear which, if done broadband with toroid transformers, may work at 40MHz without mods. As for surplus TV driver amps the best bet would be to call the local electronics surplus dealers and check EBay. Regarding FETs, we did drive them hard a lot. Mainly in ultrasound where frequencies were 15MHz or below but where we achieved very steep transition times in the 25nsec range and could set power by controlling the pulse width. But it does require very stiff drivers which are also becoming scarce when you want a chip. Also, sometimes we just paralleled a whole lot of smaller FETs for cost and performance reasons, and because we didn't want a heat sink. For RF stuff you can also run four or eight smaller amps with combiner toroids Regards, Joerg S #### SioL Jan 1, 1970 0 Joerg said: Hi Winfield, Regarding FETs, we did drive them hard a lot. Mainly in ultrasound where frequencies were 15MHz or below but where we achieved very steep transition times in the 25nsec range and could set power by controlling the pulse width. But it does require very stiff drivers which are also becoming scarce when you want a chip. Also, sometimes we just paralleled a whole lot of smaller FETs for cost and performance reasons, and because we didn't want a heat sink. For RF stuff you can also run four or eight smaller amps with combiner toroids Regards, Joerg Hi Joerg! What FET's did you use up to 15MHz? If garden variety, how did you drive them (which chip/small fets). Just applies to a small project of mine so I thought I might ask. Regarding the amplifier, look at Advanced Power Technology ARF family of fets. They're a bit expensive, but high voltage - they work off 100V or so and are RF fets in fact. A couple of these gets you into the KW range very quickly up to about 30, maybe 40MHz. They have application note of a complete 27MHz amplifier for industrial applications, I think it is powered from mains directly without any transformer, just rectified US mains. SioL W #### Watson A.Name - \Watt Sun, the Dark Remover\ Jan 1, 1970 0 Winfield Hill said: I have taken the liberty of crossposting this to alt.ham-radio.hf, rec.radio.amateur.homebrew, and alt.ham-radio.vhf-uhf [snip] The M/A-COM parts are stocked by Richardson, and three of them look interesting for my inventory. Does anyone recognize any of these FETs? . type Vdss Pd Ciss Crss cost ea ------- ---- ---- ---- ---- ------- MRF134 65V 18W 7pF 4.5pF$21.42
MRF171A 65 115 80 8 (at 28V) 37.40
MRF148A 120 115 50 8 (at 50V) 35.70

By comparison, a modern VMOS type power MOSFET rated at 100V and 127W
has Ciss = 1150pF and Crss = 62pF (at 25V), which is about 10x higher.
Of course, it's able to handle much higher DC currents when fully on.

This might be of some help.
http://www.pcs-electronics.com/schematics/RF Power Transistor.htm

W

#### Watson A.Name - \Watt Sun, the Dark Remover\

Jan 1, 1970
0
Rich Grise said:
AFAIK, the station engineer. I read a construction article many
years ago about 2 x 813s in a push-pull 1KW final. The author
said that stations routinely replace their driver tubes (which
these days could very well have been replaced with SS), just
to avoid failures, and the pulled tubes are really cheap, like
they toss them.

I have no idea if anything like this is going on today.

Reminds me of the time we went scrounging in the dump, which happened to
be at the base of a 50kW AM transmitter and antenna. We came across a
large pile of glass light bulbs, the ones that were used in street
lamps. Of course many of these were broken, but a few were still okay.
They were 1500W lights, with the big old mogul screw base. The street
light maintenance crews would go through a whole neighborhood and
replace all the lights because it was cheaper to do, labor-wise than to
go around replacing the bad lights. So of the several that we snagged,
most worked just fine. We screwed them into a big old floor lamp and
tried to find a heavy extension cord to use, and put the floor lamp up
on the roof. We had it lit up inside the house, but it was an unfrosted
light and Yikes! It was like having a mini sun inside the house, too
much light and glare! So instead we lit up the whole neighborhood. ;-)

W

#### Winfield Hill

Jan 1, 1970
0
Paul Burridge wrote...
From what I've seen, these power MOSFETs aren't much
use for anything beyond HF.

Not at all.

E.g., bottom of the page at Watson's link, specifying 225MHz.
http://www.pcs-electronics.com/schematics/RF Power Transistor.htm
Actually, the datasheet shows useful gain to over 1000MHz.
Check these out, though:
2SC1969
2SC1971
2SC1972

These are BJTs but some sort of arrangement of them
should get you near the mark.

Nice high-frequency capability, but pretty wimpy collector
dissipation ratings, surely the MOSFETs can do a better job,
more easily. They are expensive though.

J

#### Joerg

Jan 1, 1970
0
Hi SioL,
What FET's did you use up to 15MHz? If garden variety,
how did you drive them (which chip/small fets). Just
applies to a small project of mine so I thought I might ask.
It is too long ago but I could find out if you really want to know. I
believe that the FETs we used as well as the drivers (from National's DS
Series) have to a large extent gone to lala-land, unobtainium.

Nowadays I'd use BSS123 or similar which are much cheaper than what we
used. ON-Semi also makes larger varities of these. BSS84 would be the
p-channel version. For driving I'd use paralleled bus drivers, and small
toroids for a zippier transistion. In the old days we couldn't as bus
drivers were either prohibitively expensive or too slow. You can usually
parallel stages from one chip for more drive but never between different
chips. A parallel configuration of four or eight bus driver pins results
in a whole lot of oompf.

Regards, Joerg

P

#### Paul Burridge

Jan 1, 1970
0
Paul Burridge wrote...

Not at all.

E.g., bottom of the page at Watson's link, specifying 225MHz.

Oh bugger. I didn't see the bottom part. :-(

S

#### SioL

Jan 1, 1970
0
Joerg said:
Hi SioL,

It is too long ago but I could find out if you really want to know. I believe
that the FETs we used as well as the drivers (from National's DS Series) have
to a large extent gone to lala-land, unobtainium.

Probably not worth the effort than, not that important.
Nowadays I'd use BSS123 or similar which are much cheaper than what we used.
ON-Semi also makes larger varities of these. BSS84 would be the p-channel
version. For driving I'd use paralleled bus drivers, and small toroids for a
zippier transistion....

Hmm, this sounds interesting. How exactly did you do this toroid part?

Thanks Joerg!

SioL

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