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power amplifier for ultrasound?

Discussion in 'Electronic Design' started by mikro, Nov 8, 2005.

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  1. mikro

    mikro Guest

    Hi!

    I just found this group and noticed that there seem to be a lot of
    people that know much about amplifiers here. Thought I'd pester you
    with some question and see if you could give me some design hints.

    I'd like to build a power amplifier that works up to 50 MHz and can
    deliver maybe 50 Watts. The input will be more or less a pure
    sinusoidal signal and no spikes or distorsions are allowed.

    I've not very experienced in amplifiers so I'm not even sure where to
    start looking and what kind of techniques are available that might be
    interesting for me.

    If anyone could please give me som clues or good places to find more
    information I'd be very grateful : )

    Thanks!!!

    /Mikael
     
  2. You need to specify a couple of other things before a sensible reply can
    exist. 50W, into _what_ load?. This changes massively the sort of voltages
    and currents involved. Also does the range go 'down' to DC?.

    Best Wishes
     
  3. 50MHz, if that's what you mean, is not "ultrasound". It's radio frequencies,
    and is in the low VHF, ie Very High Frequency range.

    And you've not really specified what you want, because you say "works
    up to" but gave no indication of where else you expect it to work.

    And, as is too often the case, you are asking for a solution you
    think you need, while revealing that you don't know much about that
    end solution. If you don't reveal the real purpose of this amplifier,
    the solution will be long in coming. Because it may turn out you don't
    need the amplifier, it may turn out that you've missed some detail
    and it's not 50MHz but 50KHz (a 1000:1 error), it may turn out some
    other solution will come into play based on the end game. Or, if
    this is really a DC to 50MHz amplifier you really need, it may turn
    out the solution is a collection of amplifiers, covering portions of
    that range.

    Michael
     
  4. Joerg

    Joerg Guest

    Hello Michael,
    50MHz can be ultrasound. The highest frequency in ultrasound I designed
    stuff for so far was 40MHz but the trend is upwards, for more
    resolution. 20-30MHz is nowadays kind of a 'normal' ultrasound
    frequency. The days that it was all under 7.5MHz are long gone.

    Regards, Joerg
     
  5. Phil Hobbs

    Phil Hobbs Guest


    The first thing I worked on in grad school was a 2 GHz acoustic
    microscope. The group down the hall was trying to do it at 100 GHz in
    superfluid helium.

    Cheers,

    Phil Hobbs
     
  6. mikro

    mikro Guest

    Hello again!

    Wow, that's fast input from you. thanks for your answers and opinions.
    If I'm not being very specific it's intentional : )

    The application is in ultrasound but the frequencies it will be used
    can vary a bit. This is one of the reasons I wanted to post the
    question here, I know a bit about ultrasound but when it comes to radio
    I'd rather not say too much : ) Wouldn' it be correct to say that it is
    ultrasound as long as it's a longitudinal pressure wave above hearing
    range? 2 GHz sound pretty far up though : )

    What I want to do is to amplify a signal from a standard waveform
    generator in the range of between 2 MHz up to as of now a maximum of 14
    MHz. The 50 MHz maximum limit is probably a bit higher than I'll ever
    go but it would be nice to be able to go a bit further than 20 MHz just
    to be on the safe side.

    Concering the load it's a bit trickier. I'll be using the amplifier
    with different piezomaterials and sometimes you drive at a low
    impedance, around 2 Ohms, and sometimes you're working more around 50
    Ohms. We're trying to keep the piezos as close to 50 Ohms as we can but
    it's not always controllable. The voltages I guess would be between
    10-50 V.

    As you say, it might be that I should be looking for a RF-amplifier
    rather than searching for amplifier designs suitable for ultrasound ...
     
  7. Joerg

    Joerg Guest

    Hello Mikael (hope that's spelled correctly):
    2ohms would be rather low unless you are working with huge transducer
    disks. For amplifier design you need to narrow that range down somewhat.
    Once you have picked a certain transducer design and have it made at the
    usual places like TRS or Parallel Design they won't stray that much.
    +/-40% maybe, at the most. Unless you do what we did with elements that
    are barely wider than the crystal sizes ;-)

    Does the amp really have to be linear? I have never driven any PZT with
    a linear amp in about 20 years except for R&D testing. If it doesn't
    have to be linear it'll be much easier, FET switches etc. In case it
    must be linear be prepared for a larger amp and lots of heat
    dissipation. You can do PWM and stuff like that if it has to be small
    but the engineering efforts for that only pay off if it all goes into
    volume production or where cost doesn't matter.

    ENI makes nice RF power amps. 100W is not big deal but they are huge and
    heavy. At that power level it would be about the size of carry-on
    luggage. You could rent one to try it out. These are normally used for
    EMI susceptibility measurements.

    Regards, Joerg
     
  8. mikro

    mikro Guest

    Hi Jörg, assuming that's the correct spelling ; )

    2 Ohms is very low and that's maybe another problem that should be
    adressed separately. I think it would be ok to assume an impedance
    range of may 30-50 Ohms. I'm usually working with rather small
    transducers, around 600 µm square but probably not anywhere near the
    sizes you seem to have used : )

    I guess that it wouldn't have to be linear. It's always easier but it
    really shouldn't matter. The only ultrasonic amplifiers I've seen are
    huge, expensive, unstable and maybe even more important very old. One
    of them were described as a "time capsule" by a guy that opened it and
    found 10 years of collected dust in it : )

    Assuming that it doesn't have to be linear, what should I have a look
    at apart from fET switches?

    many thanks!!!
     
  9. Joerg

    Joerg Guest

    Hello Mikael,
    Yes! Nice to see that umlaut back in there once in a while.

    Oh, some of ours were a lot smaller. Like an array of 64 elements in a
    cirular arrangement and the whole thing being under 1.5mm in diameter.

    That sounds like old NDT stuff or amps designed for scientific projects.

    You'll have to make a decision between unipolar or bipolar pulsers.
    Unipolar only requires one FET per channel, inductors, some diodes.
    Bipolar requires push-pull and that usually comes with a pulse
    transformer. In your case probably something on a #43 ferrite toroid
    core. The driver is often a challenge because FETs need a good 10V and
    lots of pulse current into the gates to be snappy enough, no matter what
    the ads say.

    No idea what you want to do after the pulse. But if you have to detect
    weak echoes or even Doppler make sure the supplies for the pulsers and
    their drive logic are clean like a whistle. There is an old saying that
    truly applies to ultrasound: What you shout into the forest will come
    back to your ears. IOW, any noise that's on the TX pulse is tough to get
    rid of afterwards.

    The drive logic should be free of jitter which usually means that the
    uC, FPGA or whatever is triggering the pulse should not be shouldering
    any other realtime tasks that could cause its substrate and busses to
    become modulated.

    Then there is the T/R switch but that shouldn't be rocket science. It
    just has to be squeaky clean as well on the supply rail. The receive amp
    must be able to cope with a huge overload peak without doing any weird
    stuff such as ringing.

    Well, that about sums it up. Sounds easy but it isn't, at least not when
    your Doppler has to be 2-3dB more sensitive than the competition's ;-)

    Regards, Joerg
     
  10. mikro

    mikro Guest

    Thanks for all the info! Actually I'm only interested in transmitting
    and not receiving anything at all. To make it even better I'm only
    running continous ultraound. It might sound a bit strange but it's
    quite nice : ) We're using the ultrasound to position cells and
    particles within fluids and are only using the transmitted wave to
    create a standing wave. Thankfully this makes the electronics a bit
    simpler as well!

    Due to the low polarising voltage required on the transducers I'll have
    to use bipolar driving. The FETs I've looked at before had problems in
    that the either weren't fast enough or they simply couldn't deliver
    enough current.

    Since it's time to stop working for today I'll have to wait till
    tomorrow to start looking for more FETs. Thank you very much for all
    the input, you've been very helpful!
     
  11. Don Foreman

    Don Foreman Guest

    Look for a "linear amplifier" for amateur radio work. Many of those
    would go from 160 meters to 10 meters, which is about 2 MHz to 30 MHz,
    in power levels up to a KW. You will want a pi matching section
    between the amplifier and your load, like the old Johnson Matchbox.

    EBay is your friend. This stuff is out there.
     
  12. Joerg

    Joerg Guest

    Hello Mikael,
    Check IRF. They have one of the largest selections for that purpose. You
    can always parallel some if they are on the wimpy side. I have even used
    the rather tiny BSS123 and BSS84 in some smaller applications. Old as
    Methusaleh but hard to beat at five Cents a pop (at our quantities...).

    Regards, Joerg
     
  13. mikro

    mikro Guest

    Thanks, I'll check them out and have a peak at e-bay at the same time!
     
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