Superfast Power MOSFETs for a Linear Amp?
 

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:
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

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

"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

"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

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:

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

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.

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:

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