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Superfast Power MOSFETs for a Linear Amp?

Discussion in 'Electronic Design' started by Paul Burridge, May 5, 2004.

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  1. 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



  2. Paul,
    what is the frequency range and the amplitude ?
    I assume a 50 Ohm system.

  3. SioL

    SioL Guest

    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).

  4. 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
  5. Tim Wescott

    Tim Wescott Guest

    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

    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.
  6. Jim Meyer

    Jim Meyer Guest

    Here's a link to a MOSFET transistor that will do RF.

  7. SioL

    SioL Guest

    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 :)

  8. Tom Bruhns

    Tom Bruhns Guest

    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
  9. On 5 May 2004 09:34:17 -0700, (Tom Bruhns) wrote:

    Okay, thanks all and keep it comin'. I had no idea there were power
    FETs capable of UHF. I'll follow any links given and investigate.
    Technically you're right, of course. I want to make an amp that
    operates in Class C - yeah, I know that mode's the antithesis of
    linear but over here in Britain, these bolt-on after market boosters -
    though running in Class C., are knowns as "linears." I don't
    understand it either.
    Class C RF amps are basically switching on and off at high speed, so
    it seemed like a job better suited to a power FET since power RF BJTs
    are pretty expensive. Of course, I can now gather than the same
    situation pertains with FETs as well.
    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.
  10. Joerg

    Joerg Guest

    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
  11. Tam/WB2TT

    Tam/WB2TT Guest


    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.

  12. Tom Bruhns

    Tom Bruhns Guest

    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

  13. John Larkin

    John Larkin Guest

    And the substrate diode made a nice step-recovery diode, making it
    possible to generate lethal dv/dt's in totally non-obvious ways!

  14. Ken Scharf

    Ken Scharf Guest

    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.
  15. Ken Scharf

    Ken Scharf Guest

    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).
  16. Joerg

    Joerg Guest

    Hi Ken,

    It depends on the TV but all this has been 20+ years ago. In those days I
    found a few transistors that made great shortwave amps. I believe their
    part numbers pretty much all started with BU. When one type fell out of
    favor with the TV manufacturers they showed up on markets by the carton.
    Usually at dump shops or discounters.

    The other types that worked even better were the video transistors that
    drove the CRT. You had to secure a cheap source, no scavenging out of TVs
    because each set only had three. But they were low power so that required
    a soldering marathon and it only made sense when you could buy them a dime
    a dozen. We used to do some crazy things such as running them oil cooled
    because these weren't easy to mount on a heat sink.

    Except for the CRT drivers the data sheets mostly didn't spec FT, just
    switching times. But that used to be the same with small signal types such
    as the BSS123 that I have used heavily in front ends.

    Regards, Joerg
  17. Paul:

    There is an interesting two part article in QST May & June 1997
    describing 300 W and 500 W output Class E (more efficient, non-linear)
    amplifiers using a single IRFP440 (300 W) or IRFP450(500 W) at 7 MHz.
    Since you are in the UK, there is a brief mention in Radcom (Technical
    Topics) August 1997.

    Ordinary garden-variety power FETs can also be used for HF linear
    amplification using appropriate bias settings - though exactly how
    linear they can be in practice I don't know.

    Steve VE3SMA
  18. Highland Ham

    Highland Ham Guest

    I was thinking about having a go at contriving (I won't say
    MOSFETS can be used for RF applications provided the input and output
    capacitance is sufficiently low.
    I recently bought a power module from a surplus store with 48 pcs HUF76143P3
    Hi-speed MOSFET in a TO220 package ,all for Euro 4.00 (just under US$5.-)
    The relevant spec sheet says Input cap.= 3900pf , Output cap.=1600pF
    ,useful up to 1 MHz.

    Most MOSFET studded RF power amps circuits I have come across are equipped
    with IRFxxx range of devices.

    Frank GM0CSZ / KN6WH
  19. Fred Bartoli

    Fred Bartoli Guest

    If the drain load is resonated and you want some control over the resonant
    frequency (i.e. you have external to the mosfet tuning Cs) I don't see much
    discontinuities in inductors current. Of course when all the stuff is well
    "wired" and that's an entirely different matter, isn't it Paul ? ;-)

    John, can you suggest some refs that nicely snap ?

  20. Tom Bruhns

    Tom Bruhns Guest

    Check out; SR706: 300 watts at 225MHz. SR705: 200
    watts at 400MHz. LX401: 60W at 1000MHz. Also some very terse "ap
    notes" and some somewhat less terse but very to the point technical
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