# Building power amplifier. Need some help with schematics

1. Mar 17, 2009

### bambbrose

Hi everyone,

I'm new to this forum. I'm working on a senior project where we are using ultrasonic transducers at very high frequencies. Problem is we only have some weaker 25V power supplies to drive the transducers.

To fix this problem we were going to purchase a power amplifier but due to the expense, we started looking for alternatives.

We found an article titled "Power Amplifier for Ultrasonic Transducer Excitation" which I have scanned and attached as a PDF below. It's a 7 page document.

http://www.bambuilt.com/images/Power%20Amplifier%20for%20ultrasonic%20transducer%20excitation.pdf [Broken]

The article describes how to build a power amplifier using 2 Mosfets, a driving circuit, and a transformer. I'm not very experienced with electrical diagrams and I'm having a hard time understanding the circuit schematics. The only experience I have with circuits outside of building them is an EE for non-majors course.

Below you'll find the basic schematic for the power amplifier that I'm trying to reproduce from page 4 in the article.

http://www.bambuilt.com/images/Basic%20Schematic.jpg [Broken]

I understand all of the basics, but I do not understand what the Vbias and Vcc do, or what they are for that matter. I also don't really understand the fundamentals of a transformer and how it fits into the schematic. Can anyone explain maybe in simpler terms how this circuit would function and what it would do?

Next is the detailed schematic used in the article found on page 5.

http://www.bambuilt.com/images/Detailed%20Schematic.jpg [Broken]

It is the exact same as the basic schematic, just with all of the details filled in.
From what I can tell, they are using two power supplies for the driving circuit set at 3.85V. I'm unsure what the 1.4 means. On from there they run into some basic circuit elements and then on to M1 and M2 mosfets. I still do not understand what the Vbias or Vcc are in this diagram. They look to be DC power sources, one at 3.8v and one at 1000V? I'm very confused on this part. Also as mentioned above, I don't understand how the transformer works in the schematic. Honestly I don't even understand why the entire section off to the right is not connected to the rest of the circuit. Does it have anything to do with the K1 coupling written in the upper right?

As you can probably tell, I'm a newb when it comes to this stuff. I'm not entirely concerned with understanding the deep down details of this circuit. I'm just trying to build a small power amplifier to drive our transducers at 500V or so.

Thanks in advance for any help you can give. Big or small, anything would probably help in my understanding. I'd love to just be able to place an order from DigiKey or similar and build this thing, but I do understand that I've got a bit of learning to do before I can start ordering components.

Last edited by a moderator: May 4, 2017
2. Mar 17, 2009

### Proton Soup

Vcc is the power source for driving the center-tapped primary of your transformer coil. in order to get power through a transformer, the signal must be changing, and that is what the IRF510's are for. you can open M1 and close M2, or open M2 and close M1 to send current in either direction and thus set up your AC signal on the primary. Vdc is obviously used to bias the gates of your transistors, and V1 and V2 appear to be opposing AC signals with a 3.85 volt DC offset and 1.4 volts of AC amplitude at 1MHz.

3. Mar 17, 2009

### Staff: Mentor

Welcome to the PF, bambbrose. That paper describes a pretty high power amplifier, putting out 400Vpp on the secondary. Are you or your advisors well-versed in high-voltage, high-power electronics? What kind of safety measures are you taking, both for the high voltage, and for the high power ultrasound?

4. Mar 30, 2009

### jeff morriss

bambbrose

I recently designed and built some high power ultrasonic drive amplifiers, and the design may be useful for your needs. However, I need to understand the characteristics of the transducer you are trying to drive and its application.

* What is the drive frequency
* What drive voltage do you need?
* Have you attempted to extract an equivalent HSPICE model for the transducer. This is
important in understanding the load that is presented to the drive amp.
* How much power do you estimate is required
* What environment is the transducer operating in. For example, is it in a fluid or in a gas?

The amplifier I designed operates from a bipolar supply from +/-15 to +/- 50 volts and is capable of peak currents in excess of 5 amps at frequencies up to 2.0 MHz. I even have some leftover PC boards which I would be willing to sell at a reasonable price. If you would like just the schematic, I can furnish that also. However, unless you are skilled at high power, high frequency layout techniques, I would recommend you save yourself the trouble and use a known good layout.

If you need drive voltages >100V, there is a company that makes HV power opamps I think they are called Apex). Google them, and they have lots of good app notes.

Jeff

5. Mar 30, 2009

### bipolar

I have a question for jeff: What is a good source for elements useful as transformers? Are they built in house, or do the transducers come as "a unit". I've seen piezo-transformers mentioned for anywhere from 500Hz to 20kHz+. When operated in the audible range, don't they make an intolerably big racket, or are they kept in a sealed/damped case somehow?

6. Mar 31, 2009

### jeff morriss

bambbrose,

The design of the transformer is dependent on its operating frequency range as well as power requirements, turns ratio, etc. Transformer design is quite an art, especially if the load is highly reactive. Core material selection is critical, and you will probably have to do the design from scratch.

If I has to design a 400 VPP piezo electric amplifier I would use off the shelf high voltage opamps: http://apex.cirrus.com/en/products/apex/index.html. Almost all the hard work is done for you. Using a pair of 200V opamps in a bridge configuration permits use of lower supply voltages and lower voltage devices.

Jeff

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