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

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.

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.

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.

Hi Paul,

Is cost an issue or do you just want to see if it can be done with
MOSFETs? About 25 years ago I did a small amp (25 watts or so) above
100MHz with FETs. But I found that cost was much higher as if I'd done
it bipolar. When I looked at 100W my then pretty puny budget made the
decision for me to go bipolar.

Personally I haven't seen much happening in cost/performance and I'd
probably start out bipolar. Another cost saver is if you leave the
supply voltage level a wildcard even if this is for a mobile app. It's
no big deal these days to create whatever voltage the transistors like
best from whatever is there as a supply.

I am still a big fan of tubes, as long as they are popular enough and
thus cheap. Transistors are rather fickle and a tiny glitch in the
load connection can send them to the greener grass in a millisecond.
Tubes brush this off in stride.

For rock bottom bill of materials budget it has always paid off to
check what TV manufacturers use in the horizontal deflection finals.
That's as low cost as it gets. But I guess with the advent of big
plasma screens that may fizzle over the next 10 years.

Regards, Joerg
http://www.analogconsultants.com