tuned 40W 40MHz RF power amplifiers

[Winfield Hill]

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.?


--
Thanks,
- Win

 

[Winfield Hill]

Paul Burridge wrote...

Winfield Hill wrote:

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!

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


--
Thanks,
- Win

 

[Joel Kolstad]

"Paul Burridge" <pb@notthisbit.osiris1.co.uk> wrote in message
news:eisho01i5p9tmimv1m4e3l2aebb47u8u61@4ax.com...

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...

 

[Joerg]

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

http://www.analogconsultants.com

 

[Tim Wescott]

Joerg wrote:

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.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Clarence]

"Tim Wescott" <tim@wescottnospamdesign.com> wrote in message
news:10ojdtps024f3e3@corp.supernews.com...

Joerg wrote:


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.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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

 

[Tim Wescott]

Clarence wrote:

"Tim Wescott" <tim@wescottnospamdesign.com> wrote in message
news:10ojdtps024f3e3@corp.supernews.com...

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.


Yup. I doubt that I'll ever actually do it, but it's fun to think about.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Paul Burridge]

On 5 Nov 2004 07:25:39 -0800, Winfield Hill
<whill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:

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.

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.
--

"What is now proved was once only imagin'd." - William Blake, 1793.

 

[Rich Grise]

On Fri, 05 Nov 2004 07:25:39 -0800, Winfield Hill wrote:

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...

Surplus VHF TV driver modules are another option.

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

 

[SioL]

"Joerg" <notthisjoergsch@removethispacbell.net> wrote in message
news:JOQid.38841$QJ3.31942@newssvr21.news.prodigy.com...

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

 

[Watson A.Name - \"Watt Su]

"Winfield Hill" <whill_a@t_rowland-dotties-harvard-dot.s-edu> wrote in
message news:cmg61j014s4@drn.newsguy.com...

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%20Power%20Transistor.htm

--
Thanks,
- Win

 

[Watson A.Name - \"Watt Su]

"Rich Grise" <rich@example.net> wrote in message
newsan.2004.11.05.18.05.23.579531@example.net...

On Fri, 05 Nov 2004 07:25:39 -0800, Winfield Hill wrote:

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...

Surplus VHF TV driver modules are another option.

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.

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. ;-)

Good Luck!
Rich

 

[Paul Burridge]

On Fri, 5 Nov 2004 21:09:39 -0800, "Watson A.Name - \"Watt Sun, the
Dark Remover\"" <NOSPAM@dslextreme.com> wrote:

This might be of some help.
http://www.pcs-electronics.com/schematics/RF%20Power%20Transistor.htm

That's a neat, compact little guide, I must say. Is there any other
you know of for 1Ghz+ power devices?
--

"What is now proved was once only imagin'd." - William Blake, 1793.

 

[Winfield Hill]

Paul Burridge wrote...

Winfield Hill wrote:

. 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

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%20Power%20Transistor.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.


--
Thanks,
- Win

 

[Paul Burridge]

On 6 Nov 2004 13:34:24 -0800, Winfield Hill
<whill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:

Paul Burridge wrote...

Winfield Hill wrote:

. 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

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.

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

"What is now proved was once only imagin'd." - William Blake, 1793.

 

[SioL]

"Joerg" <notthisjoergsch@removethispacbell.net> wrote in message
news:_fcjd.39228$QJ3.23319@newssvr21.news.prodigy.com...

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.

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

 

[Paul Burridge]

On Sun, 07 Nov 2004 22:13:38 GMT, Joerg
<notthisjoergsch@removethispacbell.net> wrote:

Mostly I just used 43 material because I have lots in the lab. This is
also the material most popular for EMI remedies, the cores you slip over
offending cables. For pulsing a FET you only need a very small core 1/4"
or so. Nowadays I often need my glasses to wind these.

Don't you find that stuff far too lossy for transformers?
--

"What is now proved was once only imagin'd." - William Blake, 1793.

 

[Paul Burridge]

On Mon, 08 Nov 2004 14:54:22 -0800, Tim Wescott
<tim@wescottnospamdesign.com> wrote:

It's interesting that you should cite Amidon -- to my knowledge they
just resell Fair-Rite ferrites and Micro Metals iron powder cores.

IIRC, MM's powder iron toroids are about the best thing out there to
wind RF transformers on. The lossier ferrites do enable high values of
inductance to be obtained from very few windings, though, if you can
live with that loss.
--

"What is now proved was once only imagin'd." - William Blake, 1793.

 

[Joerg]

Hi Tim,

It's interesting that you should cite Amidon -- to my knowledge they
just resell Fair-Rite ferrites and Micro Metals iron powder cores.

Yes, but Amidon is a great source of information for engineers who are
pretty new to ferrites and iron powder cores. It is also a place with a
tremendous variety of cores in one location. Their booklets were what
taught me the ropes a long time ago. When a client has EMI problems or I
design isolation amps for them I usually point their engineers to
Amidon. There they can buy a large variety of cores to try things out on
their own. Literature from the mainstream manufacturers is great as well
but for the uninitiated it is often too heavy on math and formulas they
were never used to.

In Europe it used to be even more difficult. Once I saw a whole crate of
cores at a company. Unfortunately we needed a different ferrite so I
asked them why they bought this many. It was the minimum quantity...

Regards, Joerg

http://www.analogconsultants.com