Deliver a 40kHz & >40V cosine wave to a transducer for communication

Question

I'm trying to do on/off keying (OOK) to transmit messages with ultrasound.

This is the transducer.

This is the schematic of the signal generator and amplifier:

It seems that the class A amplifier doesn't work.

• Have I made an error with the operating point?
• Should I instead use opamps?

If you know how to fix this approach or if you have any better ideas please let me know.

Answer 1

If you are going to use a class A amplifier on such a low voltage supply then might I suggest you configure the collector to drive a step up transformer. You can arbitrarily deliver what ever voltage you need to the secondary (within reason) but you might need some heat sinking on the output transistor.

The next problem is delivering a sine wave to the load. At the moment, the 555 produces a square wave and that’s roughly what you’d get in your current circuit (with or without an output transformer) so, you might be advised to use an op-amp low pass filter between 555 and the amplifier. You could use a pi network filter of course if you knew how to design it.

It’s unclear about how you would intend to perform OOK in the circuit but I would think that some logic input to the 555 and another transistor could be persuaded to give this functionality.

Answer 2

Your DC blocking capacitor forms a high pass filter in combination with the input impedance of the amplifier.

The input impedance is less than 100 ohms, giving a cutoff of over 200kHz.

Your DC blocker is filtering out your ultrasound signal.

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Other problems:

1. A simple class A amplifier can't put out a voltage higher than its supply voltage. You have 5V in, so 40V out is not possible with this design.
2. The amplifier presents a heavy load to the 555. The 555 may have trouble delivering enough current to the BD139 base.
3. You've already mentioned that you know that your signal is a square wave and that you'll have to filter it to make a sine wave out of it. Keep the low input impedance of your amplifier in mind when you design the filter. You might do better to useva buffer between the filter output and the amplifier input.

Answer 3

Ultrasound transducers require only very small currents. Instead of using an audio transformer, you might want to try a CMOS H-Bridge running at 15V. The H configuration gives you 30Vss accross the transducer.

^{simulate this circuit – Schematic created using CircuitLab}