"Rene Tschaggelar" wrote in message ...
Paul Burridge wrote:
Hi all,

I was thinking about having a go at contriving (I won't say
"designing" for obvious reasons) a class 'C' RF amp using MOSFETs
instead of the usual BJTs/toobz. They seem - on the face of it at
least - ideally suited to the task. I'm just a bit concerned about
whether even the fastest ones would be fast enough, even given
adequate gate drive. I'd be surprised if they weren't good for at
least a few Mhz., but am quite frankly clueless as to MUF. Anyone
know?


Paul,
what is the frequency range and the amplitude ?
I assume a 50 Ohm system.

Yep, that's a good question for Win.

Suitable general purpose MOSFETs for RF finals (probably only feasible in the
shortwave range) and best way to drive them.

Let's say the more power the better (10-200W).

SioL

On Wed, 05 May 2004 13:25:52 +0200, Rene Tschaggelar
wrote:

Paul Burridge wrote:
Hi all,

I was thinking about having a go at contriving (I won't say
"designing" for obvious reasons) a class 'C' RF amp using MOSFETs
instead of the usual BJTs/toobz. They seem - on the face of it at
least - ideally suited to the task. I'm just a bit concerned about
whether even the fastest ones would be fast enough, even given
adequate gate drive. I'd be surprised if they weren't good for at
least a few Mhz., but am quite frankly clueless as to MUF. Anyone
know?


Paul,
what is the frequency range and the amplitude ?
I assume a 50 Ohm system.

Yeah, 50 in; 50-75 out say. As for frequency, that's what I'm trying
to establish: the highest useable frequency device for say 50W-100W
out. Amplitude's subservient to power. What kind of frequency upper
limit might be achievable? Say with the best gate driver arrangement
conceivable?
--

The BBC: licenced at public expense to spread lies.

Paul Burridge wrote:

I was thinking about having a go at contriving (I won't say
"designing" for obvious reasons) a class 'C' RF amp using MOSFETs
instead of the usual BJTs/toobz. They seem - on the face of it at

Paul,
assuming that I understand your question and assuming you want a
50ohm-50ohm HF amplifier for CW, I have seen several designs using an
IRF510 MOSFET for amplifiers up to 30 MHz.
On a QST article using 2 IRF510 in a push-pull fashion some 30W could be
achieved over the entire HF range. For a simpler design check the NB6M
Miniboots (http://www.amqrp.org/kits/miniboots/miniboot.htm).

To my understanding, switching MOSFETs can be used as HF C-class
amplifiers as long as their gate capacitance is low (few pF).

As an example, IRF530 has a way too high gate capacitance to allow
acceptable HF results. On 80m it might be good, thought.

HTH,
Paolo IK1ZYW

Paul Burridge wrote:

Hi all,

I was thinking about having a go at contriving (I won't say
"designing" for obvious reasons) a class 'C' RF amp using MOSFETs
instead of the usual BJTs/toobz. They seem - on the face of it at
least - ideally suited to the task. I'm just a bit concerned about
whether even the fastest ones would be fast enough, even given
adequate gate drive. I'd be surprised if they weren't good for at
least a few Mhz., but am quite frankly clueless as to MUF. Anyone
know?

Thanks,

p.

You are aware that there are power RF MOSFETs that go up to VHF,
probably higher? There's a number of manufacturers out there -- I know
Motorola makes them, probably any one that is in the cellular base
station business makes them. You can pretty much build an amp off of a
Motorola app note.

As far as using 'regular' power MOSFETs they tend to have very high gate
capacitance as a result of getting the Rds down. Since Rds isn't the
main efficiency driver in an RF amp the chips aren't well suited to RF
application.

I know the ARRL handbook has information on MOSFET amps from 'standard'
power MOSFETs, and there have been a few articles in QST and QEX.

--

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

Paul Burridge wrote in message . ..
Hi all,

I was thinking about having a go at contriving (I won't say
"designing" for obvious reasons) a class 'C' RF amp using MOSFETs
instead of the usual BJTs/toobz. They seem - on the face of it at
least - ideally suited to the task. I'm just a bit concerned about
whether even the fastest ones would be fast enough, even given
adequate gate drive. I'd be surprised if they weren't good for at
least a few Mhz., but am quite frankly clueless as to MUF. Anyone
know?

Thanks,

p.

Here's a link to a MOSFET transistor that will do RF.

http://www.macom.com/data/datasheet/DU1260T.pdf

Jim

"Jim Meyer" wrote in message
om...
Paul Burridge wrote in message
. ..
Hi all,

I was thinking about having a go at contriving (I won't say
"designing" for obvious reasons) a class 'C' RF amp using MOSFETs
instead of the usual BJTs/toobz. They seem - on the face of it at
least - ideally suited to the task. I'm just a bit concerned about
whether even the fastest ones would be fast enough, even given
adequate gate drive. I'd be surprised if they weren't good for at
least a few Mhz., but am quite frankly clueless as to MUF. Anyone
know?

Thanks,

p.

Here's a link to a MOSFET transistor that will do RF.

http://www.macom.com/data/datasheet/DU1260T.pdf

Jim

I think what we're looking for here is a cheap, garden variety MOSFET used
as RF power amp.

Yep, there are units out there that will do VHF, UHF and maybe even SHF, but
the price will be Ultra High as well

SioL

"Jim Meyer" wrote in message
om...
Paul Burridge wrote in message
. ..
Hi all,

I was thinking about having a go at contriving (I won't say
"designing" for obvious reasons) a class 'C' RF amp using MOSFETs
instead of the usual BJTs/toobz. They seem - on the face of it at
least - ideally suited to the task. I'm just a bit concerned about
whether even the fastest ones would be fast enough, even given
adequate gate drive. I'd be surprised if they weren't good for at
least a few Mhz., but am quite frankly clueless as to MUF. Anyone
know?

Thanks,

p.

Here's a link to a MOSFET transistor that will do RF.

http://www.macom.com/data/datasheet/DU1260T.pdf

Jim

I think what we're looking for here is a cheap, garden variety MOSFET used
as RF power amp.

Yep, there are units out there that will do VHF, UHF and maybe even SHF, but
the price will be Ultra High as well

SioL

Do you want a linear amp (per the subject line) or a class C amp?
They're not the same critter.

There was an article in QEX just one issue ago about a P-P power
MOSFET amplifier for 7MHz.

There are power MOSFETs designed specifically for RF service: just do
a Google search for 'RF power mosfet' and you'll get lots of hits.
Many are designed for service well beyond HF.

73 de k7itm


Paul Burridge wrote in message . ..
Hi all,

I was thinking about having a go at contriving (I won't say
"designing" for obvious reasons) a class 'C' RF amp using MOSFETs
instead of the usual BJTs/toobz. They seem - on the face of it at
least - ideally suited to the task. I'm just a bit concerned about
whether even the fastest ones would be fast enough, even given
adequate gate drive. I'd be surprised if they weren't good for at
least a few Mhz., but am quite frankly clueless as to MUF. Anyone
know?

Thanks,

p.

On 5 May 2004 09:34:17 -0700, (Tom Bruhns) wrote:


Okay, thanks all and keep it comin'. I had no idea there were power
FETs capable of UHF. I'll follow any links given and investigate.

Do you want a linear amp (per the subject line) or a class C amp?
They're not the same critter.

Technically you're right, of course. I want to make an amp that
operates in Class C - yeah, I know that mode's the antithesis of
linear but over here in Britain, these bolt-on after market boosters -
though running in Class C., are knowns as "linears." I don't
understand it either.
Class C RF amps are basically switching on and off at high speed, so
it seemed like a job better suited to a power FET since power RF BJTs
are pretty expensive. Of course, I can now gather than the same
situation pertains with FETs as well.

There was an article in QEX just one issue ago about a P-P power
MOSFET amplifier for 7MHz.

It'd be fun to see just how fast you can force current in and out of
an ordinary, GP power MOS FET gate to get it switching as fast as
possible, I reckon, before pressing ahead with the dedicated devices.
--

The BBC: licenced at public expense to spread lies.

Paul Burridge wrote in message . ..
....
It'd be fun to see just how fast you can force current in and out of
an ordinary, GP power MOS FET gate to get it switching as fast as
possible, I reckon, before pressing ahead with the dedicated devices.

Back when hexfets first came out (1981 or so), I was having trouble
with them self-destructing. Back then, at least, if you read far
enough in the fine print, you'd find a maximum drain dv/dt rating. I
was seeing close to 100V in about 5nsec just before the
self-destruction as I recall. And the Siliconix V-mos transistors
were good for RF power back in that era.

If you're thinking driving it "digitally", that's probably the wrong
answer. Resonate the input and output capacitances, and life will be
much easier. Charging and discharging capacitance through a resistive
source is quite inefficient. Remember, too, that tuned triode valve
amplifiers are generally neutralized. Be VERY careful to not exceed
the gate-source voltage rating! Have fun playing, but expect some
"surprises."

Cheers,
Tom