On 6 May 2004 21:13:57 GMT, "Walter Harley"
wrote:

THanks, very interesting. I wonder if anyone's tried paralleling up
half a dozen of 'em for more power?

Directly parallelling the semiconductors have several drawbacks. First
of all, the capacitances are in parallel thus limiting the frequency
response. Also load sharing between the semiconductors can be hard to
achieve.

Modern commercial radio transmitters (including FM band, 88-108MHz) by,
e.g., Harris use MOSFETs. So clearly it is possible to get up to 25kW. I
believe they also use them in 100kW AM band transmitters. A bit of
meandering on Harris' web page will tell you more.

They are complete amplifier modules with well specified output
impedances that are combined. If e.g. Wilkinson dividers/combiners are
used, these have a limited bandwidth, but this is usually not a big
issue in broadcasting, in which the frequency remains the same and if
changes are needed, the relative frequency range is limited.

However, Wilkinson dividers and combiners would not be suitable for
1.8-50 MHz amateur linear amplifier due to the huge relative frequency
range.

Paul OH3LWR

Look at this example for starters.

http://www.picks.force9.co.uk/throb.htm


--
Leland C. Scott
KC8LDO

Wireless Network
Mobile computing
on the go brought
to you by Micro$oft

"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.

Look at this example for starters.

http://www.picks.force9.co.uk/throb.htm


--
Leland C. Scott
KC8LDO

Wireless Network
Mobile computing
on the go brought
to you by Micro$oft

"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.

I've successfully used a complementary-symmetry driver for an MRF510 at
20 and 15 meters, class C, 5 watts out. The driver is essentially
digital, being driven to the rails.

Roy Lewallen, W7EL

Paul Burridge wrote:
Indeed, but there must be some current-pumping circuit that might
assist here?

I've successfully used a complementary-symmetry driver for an MRF510 at
20 and 15 meters, class C, 5 watts out. The driver is essentially
digital, being driven to the rails.

Roy Lewallen, W7EL

Paul Burridge wrote:
Indeed, but there must be some current-pumping circuit that might
assist here?