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power amplifier for ultrasound?

M

mikro

Jan 1, 1970
0
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
 
R

Roger Hamlett

Jan 1, 1970
0
mikro said:
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
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
 
M

Michael Black

Jan 1, 1970
0
mikro" ([email protected]) said:
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.
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
 
J

Joerg

Jan 1, 1970
0
Hello Michael,
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.

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
 
P

Phil Hobbs

Jan 1, 1970
0
Joerg said:
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


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
 
M

mikro

Jan 1, 1970
0
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 ...
 
J

Joerg

Jan 1, 1970
0
Hello Mikael (hope that's spelled correctly):
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.

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.

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

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
 
M

mikro

Jan 1, 1970
0
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!!!
 
J

Joerg

Jan 1, 1970
0
Hello Mikael,
Hi Jörg, assuming that's the correct spelling ; )

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

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 : )

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.

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 : )

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

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

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
 
M

mikro

Jan 1, 1970
0
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!
 
D

Don Foreman

Jan 1, 1970
0
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 ...

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.
 
J

Joerg

Jan 1, 1970
0
Hello Mikael,
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!

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
 
M

mikro

Jan 1, 1970
0
Thanks, I'll check them out and have a peak at e-bay at the same time!
 
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