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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. |
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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 |
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