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

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" 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.
--

The BBC: licenced at public expense to spread lies.

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" 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.
--

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

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

On 5 May 2004 16:41:11 -0700, (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

On 5 May 2004 16:41:11 -0700, (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

Paul Burridge wrote:
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?
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.

Paul Burridge wrote:
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?
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.

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).