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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 |
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 |
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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). |
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