Linear ampflifier for ultrasonic transmitter

[Maik Vogt]

Linear amplifier for ultrasonic transmitter

Hi guys,

I am trying to amplify a sine chirp signal with a frequency range of about 37-43khz which I want to use for correlation.

Prior to this I used a logic level mosfet to ampflify a square wave from a microcontroller which worked fine.
But now I want to amplify a sine signal. I tried using a TSH22 op amp that worked fine in my receiver circuit.
**broken link removed**

But it does not seem to be able to provide the necessary current for my ultrasonic transducer.
I use 24V supply voltage.

Would a TL072 op amp work?
http://pdf.datasheetcatalog.com/datasheet/SGSThomsonMicroelectronics/mXrxvrt.pdf

Or do I need to build a class A, AB, etc ampflifier using BJTs?

This is the transmitter I use:
**broken link removed**

I want to have a clean signal for correlation of multiple recievers for trilateration/multilateration.
Distance is < 1m.

Thanks!

 

[FvM]

I don't see a particular advantage of driving the transmitter with a sine wave, but TL072 doesn't provide suffcicient output current in any case. You would use either a power OP or a discrete power amplifier stage.

 

[Astrid]

Re: Linear amplifier for ultrasonic transmitter

TL072 does not have sufficient current.
Drive ultrasonic transducers sine signal does not have much significance.
For max performance, it is appropriate to follow through resonant circuit with transformer
Her is my driver for 200kHz US transducer in wind speed meter

 

[Maik Vogt]

Maybe I have a misunderstanding here.

I used a square wave before, but using 38khz or 42khz did only change the attenuation. The transmitter always responded with its resonant frequency of 40khz.
The step response looks like this:
https://upload.wikimedia.org/wikipe...mortie.svg/2000px-Oscillation_amortie.svg.png

So I concluded I had to use a sine wave if I wanted to create a frequency sweep. Is there a mistake in my thinking?

 

[kripacharya]

These 40Khz transducers are difficult to operate over a wider frequency range. They are in fact hi-Q very narrow band devices.

This means that even if you drive it with a sine wave, it will give max output only at 40Khz (or whatever the resonance freq is), and output will fall off rapidly as the frequency moves away.

There ARE some techniques for widening the bandwidth a little by using inductor/ resistance - you could search for this online.

Lastly, to be able to drive an ultrasonic Tx with sufficient power, note that the main 'load' is its capacitance. Approx 2nF in your case. So whatever circuit you use has to be capable of handling this load.

 

[FvM]

You can hardly have a 40 kHz response with e.g. a steady state 38 kHz stimulation. The transmitter bandwidth is however quite small, so you get large sensitivity variation over the intended sweep range. More than +/- 500 Hz is probably not useful at all.

 

[Maik Vogt]

Thanks so far for all these good answers.
I am still looking for a readily available power op amp that should have enough output current, etc.
After that I could experiment with it whether or not this approach is feasible with the given transducers, or if I need others with higher damping, etc.

I only found exotic ones like this:
https://www.analog.com/en/high-spee...ada4870/products/product.html#product-samples

Can anyone give a rough estimate of how much output current is needed? I currently lack data / expertise to calculate this.
If I model the transducer only as a 2nF capacitor then I´d only need:
U_max * omega * C = I_max
12V*(2*pi*40000/s *2*10^-9 F) ~= 6mA

TSH22 I failed using has 25mA output short circuit current.

Is there a readily available power op amp?

 

[Audioguru]

The 2200pF capacitance of the piezo transducer causes phase shift at the output of an opamp. The phase shift causes the opamp to oscillate at a high frequency, maybe 1MHz since the negative feedback shifts to become positive feedback.

You can add a pair of comlementary emitter-follower transistors to the output of an opamp for it to drive the capacitance at a high level.
Or try a little audio power amplifier that has a good response at 40kHz.

 

[chuckey]

That transducer data sheet is missing some data, like the maximum voltage drive and the real part of its impedance. i think you will have to get hold of a transducer and measure the impedance. The normal thing to do for single frequency working is to use a series inductor, this then gets rid of the capacitive reactance at the working frequency leaving the real part only which could be as low as 10 ohms. The real problem is that even if you damp the external tuned circuits to give you the required bandwidth, the transmission efficiency of the piezo element will have its own mechanical resonance. This could be altered with external tubes (a-la a church organ), or vents/grills etc. Silly reminder, all testing on the transducer must be done whiles its in water
Frank

 

[Dan Mills]

A good trick for piezo ceramics is to wind the transformer such that the fixed transducer capacitence resonates with the secondary inductance, probably at somewhere just lightly above whatever resonant mode you are trying to use, then a series R-L-C resonance somewhere below the mecanical resonance and you can get somewhere close to an octave out of a ceramic transducer (in water, in air the Q goes way high), you do tend to find that most of the power goes into that resistor however.

Measure the admittance/susceptance curves for your device in whatever operating conditions are normal (If underwater, don't forget to wipe the bubbles off first it actually makes a measurable difference), then you can calculate the device equvalent model and for there a day playing with spice is time well spent.

Most piezo ceramic devices for underwater service would laugh at 12V drive, a few hundered volts to a few KV is more like it, but 106dB@1m is also at least 100dB off the pace for serious underwater work so it may be that you are working small distance in air in which case 12V may be ok, but you will really struggle with bandwidth.

Given your operating range there is no reason not to use a squarewave drive, consider the harmonic series of a squarewave the first non zero harmonic is the third, which is outside your intended bandwidth, and a mosfet H bridge is MUCH more efficient then a linear amp.

My hunch is that you have completely the wrong device for a chirped system in air.

Regards, Dan (Who used to do sonar transducers to pay the bills).

 

[FvM]

The transducer shown in post #1 is a waterproof air, not an underwater transmitter. It looks like a clone of a Murata MA40Exx type.

Like other low frequency air transmitters the Murata transmitter is build with a relative thin piezoceramic disc and has a limited voltage rating of 10 Vrms. I agree with chuckey that the specified capacitance doesn't describe the impedance in resonance. If I remember right, it's in a 100 ohm range. Operating the sensor with a matched resistive termination achieves maximum bandwidth.