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I need to drive an ultrasonic transducer with a high center frequency (25 MHz) using my function generator that outputs a 25 MHz sine wave at only 5 Vpp. The problem is that the transducer needs at least 50 Vpp (preferably over 150 Vpp). I tried to use a few amplifier I had around (designed for piezo driving) but they all stopped "amplifying" after around 1 MHz...

What is the cheapest but cleanest way to amplify a waveform voltage which is at such a high frequency? I am willing to buy or build but I need to complete this for less than $500.

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  • \$\begingroup\$ Have you worked out the maximum \$\frac{\text{d} V}{\text{d} t}\$? It's shockingly high. This will require some careful design and construction work, I fear. Probably even an active design with feedback -- perhaps even using an MCU monitoring integrated results on the fly for impedances. A transformer is in your future I also suspect. But this frequency is way over anything I've looked at. \$\endgroup\$
    – jonk
    Mar 4, 2021 at 8:12
  • \$\begingroup\$ They actually often use transformer for this job. But there are also (expensive) ICs. Or just use a linear amplifier for the CB band (but he didn't say how much power he needs, that would the choosing point) \$\endgroup\$ Mar 4, 2021 at 8:39
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    \$\begingroup\$ An 807 would probably do. en.wikipedia.org/wiki/807_(vacuum_tube) \$\endgroup\$
    – user16324
    Mar 4, 2021 at 12:33
  • \$\begingroup\$ Thumbs up for the valve solution. I was thinking something more modern but that would fit the bill nicely. Looking for RF transistors in online catalog usually yield very high end components, when an IRF520 could be probably coaxed to work (it's popular for hams) \$\endgroup\$ Mar 4, 2021 at 13:56
  • \$\begingroup\$ @LorenzoMarcantonio If I am driving the transducer at its resonance frequency then wouldn't the avg. power (given that its impedance is 50\$\Omega\$) be \$P_{avg}\approx\frac{Vpp^2}{R}=\frac{(100)^2}{(50)}=200 W\$? \$\endgroup\$
    – Landon
    Mar 4, 2021 at 20:00

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Ultrasonics are driven by peculiar means. If you also need to receive (I assume you don't) you also need to worry about the T/R switches.

There are two kind of transducer: resonant and wide band. A resonant transducer is made for working at exactly one frequency, like sonar sensor or atomizers. These are usually driven either in self oscillation mode (they work like crystals or, more exactly, ceramic filters) or with some pseudo class C amplifier: you load an inductor and then make it discharge thru the transducer in a transient, it will then oscillate at his own frequency (given the right LC network around it, of course).

These are also called 'pulsers' since you give a pulse and wait for it to do it job (maybe it get received by something else or do some mechanical work).

25MHz seems more a frequency used by imagers (that's why I asked if you needed the receiver too), and you drive them mostly like big speakers (which they are, in fact).

The old school called for a matching transformer: you made your signal at, like 12V and a substantial current, then put it to a step up transformer (often specified or even supplied by the transducer manufacturer) that outputted the 100-800V required (yes, there are piezo stack working at more than 800V).

These day is often 'simpler' (quotes are a must here) to have a +50V -50V (or whatever) split power rail and then make a suitable class B or AB amplifier. 25MHz is usually tough as RF so you need to shop for an RF transistor (luckily there is the 27MHz ISM band near); power isn't really a big issue since they make these BJT (maybe some MOSFET too) in all the sizes.

Depending on your particular application there are also ICs dedicated for the application. For example Supertex (now Microchip) has a wide range of pulsers and T-R switches, look in their portfolio to see if there's something you can use (for example the MD2131 is a somewhat high end driver, but there are simpler parts too).

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  • \$\begingroup\$ It is for an immersion pulse-echo measuring setup but, since I couldn’t find a combination pulser/receiver for less than $1k, I decided to use separate function generator and oscilloscope with 2 silicon diodes since I already had all of those. I’m not against the idea of making my own pulser/receiver if you think it is doable? \$\endgroup\$
    – Landon
    Mar 4, 2021 at 18:35
  • \$\begingroup\$ In that case I'd look for the supertex parts, they have in shop mostly everything \$\endgroup\$ Mar 5, 2021 at 6:59

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