Superfast Power MOSFETs for a Linear Amp?

[Paul Burridge]

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.
--

The BBC: licenced at public expense to spread lies.

 

[Rene Tschaggelar]

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.


Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

 

[SioL]

"Rene Tschaggelar" <none@none.net> wrote in message news:4098cf73$0$719$5402220f@news.sunrise.ch...

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

 

[Paul Burridge]

On Wed, 05 May 2004 13:25:52 +0200, Rene Tschaggelar <none@none.net>
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.

 

[Tim Wescott]

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

 

[Jim Meyer]

Paul Burridge <pb@notthisbit.osiris1.co.uk> wrote in message news:<mjeh9093ioe56ucglp5ubcbeqa1l945usl@4ax.com>...

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

 

[SioL]

"Jim Meyer" <jmeyer@nektonresearch.com> wrote in message
news:21ede509.0405050823.38742d74@posting.google.com...

Paul Burridge <pb@notthisbit.osiris1.co.uk> wrote in message
news:<mjeh9093ioe56ucglp5ubcbeqa1l945usl@4ax.com>...
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

 

[Tom Bruhns]

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 <pb@notthisbit.osiris1.co.uk> wrote in message news:<mjeh9093ioe56ucglp5ubcbeqa1l945usl@4ax.com>...

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.

 

[Paul Burridge]

On 5 May 2004 09:34:17 -0700, k7itm@aol.com (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.

 

[Joerg]

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

 

[Tam/WB2TT]

Paul,

Check out the Advanced Power Technology web site. They have some ap notes on
using some more or less regular power FETs for RF. Nice thing is that the
higher voltage ones will run off straight 170 V rectified AC.

Tam
"Paul Burridge" <pb@notthisbit.osiris1.co.uk> wrote in message
news:mjeh9093ioe56ucglp5ubcbeqa1l945usl@4ax.com...

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.
--

The BBC: licenced at public expense to spread lies.

 

[Tom Bruhns]

Paul Burridge <pb@notthisbit.osiris1.co.uk> wrote in message news:<0e8i90pr3qsn5gjs0mdcllapvnssl8u2p6@4ax.com>...
....

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

 

[John Larkin]

On 5 May 2004 16:41:11 -0700, k7itm@aol.com (Tom Bruhns) wrote:

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.

And the substrate diode made a nice step-recovery diode, making it
possible to generate lethal dv/dt's in totally non-obvious ways!

John

 

[Ken Scharf]

Paul Burridge wrote:

On Wed, 05 May 2004 13:25:52 +0200, Rene Tschaggelar <none@none.net
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?
Mosfets in the MRF5xx series (511, 521 for example) have been used

up to the 10 meter band with good results. A pair of them can give
at least 50w pep output. Depending on the input/output circuitry
used and the transistor they require 12-28v power supply. Layout
is somewhat critical.

 

[Ken Scharf]

Joerg wrote:

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.

Horizontal deflection transistors generally have an FT way too low

to be usefull much higher in frequency than the 160 or 80 meter ham
bands. Sweep tubes are now rare and expensive bottles costing more
than 'common' 6146 types. Surplus 1625 tubes used to be only a few
bucks each, can be found for about $5-10 each at hamfests and fleabay
(Still cheaper than 6146's though). 807's cost a little more.
The best bargin in price vs power might be the 811A, or the 813.

BTW I have a bunch of 813's I'd be willing to sell. Someone make me
a good offer on a lot of 5 of them. (Used, but don't look too bad).

 

[Joerg]

Hi Ken,

It depends on the TV but all this has been 20+ years ago. In those days I
found a few transistors that made great shortwave amps. I believe their
part numbers pretty much all started with BU. When one type fell out of
favor with the TV manufacturers they showed up on markets by the carton.
Usually at dump shops or discounters.

The other types that worked even better were the video transistors that
drove the CRT. You had to secure a cheap source, no scavenging out of TVs
because each set only had three. But they were low power so that required
a soldering marathon and it only made sense when you could buy them a dime
a dozen. We used to do some crazy things such as running them oil cooled
because these weren't easy to mount on a heat sink.

Except for the CRT drivers the data sheets mostly didn't spec FT, just
switching times. But that used to be the same with small signal types such
as the BSS123 that I have used heavily in front ends.

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

 

[Steve Kavanagh]

Paul:

There is an interesting two part article in QST May & June 1997
describing 300 W and 500 W output Class E (more efficient, non-linear)
amplifiers using a single IRFP440 (300 W) or IRFP450(500 W) at 7 MHz.
Since you are in the UK, there is a brief mention in Radcom (Technical
Topics) August 1997.

Ordinary garden-variety power FETs can also be used for HF linear
amplification using appropriate bias settings - though exactly how
linear they can be in practice I don't know.

Steve VE3SMA

 

[Fred Bartoli]

"John Larkin" <jjlarkin@highSNIPlandTHIStechPLEASEnology.com> a écrit dans
le message news: mkvi90lglctjoe1k5lu0ng58c57j2dk6op@4ax.com...

On 5 May 2004 16:41:11 -0700, k7itm@aol.com (Tom Bruhns) wrote:


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.

And the substrate diode made a nice step-recovery diode, making it
possible to generate lethal dv/dt's in totally non-obvious ways!

If the drain load is resonated and you want some control over the resonant
frequency (i.e. you have external to the mosfet tuning Cs) I don't see much
discontinuities in inductors current. Of course when all the stuff is well
"wired" and that's an entirely different matter, isn't it Paul ? ;-)

John, can you suggest some refs that nicely snap ?

Thanks,
Fred.

 

[Highland Ham]

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?
=================================

MOSFETS can be used for RF applications provided the input and output
capacitance is sufficiently low.
I recently bought a power module from a surplus store with 48 pcs HUF76143P3
Hi-speed MOSFET in a TO220 package ,all for Euro 4.00 (just under US$5.-)
The relevant spec sheet says Input cap.= 3900pf , Output cap.=1600pF
,useful up to 1 MHz.

Most MOSFET studded RF power amps circuits I have come across are equipped
with IRFxxx range of devices.

Frank GM0CSZ / KN6WH

 

[Tom Bruhns]

Paul Burridge <pb@notthisbit.osiris1.co.uk> wrote in message news:<0e8i90pr3qsn5gjs0mdcllapvnssl8u2p6@4ax.com>...

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.

Check out www.polyfet.com; SR706: 300 watts at 225MHz. SR705: 200
watts at 400MHz. LX401: 60W at 1000MHz. Also some very terse "ap
notes" and some somewhat less terse but very to the point technical
papers.