# Building an AC amplifying circuit with a PZT

Discussion in 'Analog & Mixed-Signal Design' started by Ilya Gulko, Jul 13, 2016.

1. ### Ilya Gulko Thread Starter New Member

Jul 13, 2016
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Hi everyone,

I am trying to build an AC amplifying circuit using a piezoelectric transformer (PZT). I am using this PZT: https://www.steminc.com/PZT/en/single-layer-piezo-electric-transformer-68-khz.

I built a test setup just to confirm that my PZT would actually amplify the input voltage, but it seems like instead of amplifying the voltage, the PZT is reducing it. The voltage supplied to PZT (Vin) is within the specifications of the PZT and at resonant frequency. Here is a schematic of the test circuit:

On the schematic, the Vrms measured by Probe 1 (Vin to PZT) is less than the Vrms measured by Probe 2 (Vout from PZT). The signal at both probes is in phase.

I am new to PZTs, and I don't know why the PZT in my circuit does not increase the voltage of input signal. I couldn't find good examples of how to incorporate a PZT into the circuit. I would really appreciate any help!

Thanks so much,
Ilya

2. ### AlbertHall Well-Known Member

Jun 4, 2014
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448
What are the characteristics of probe 2?
The output capacitance of the PZT is 8pF. Is the probe capacitance much greater than that?

3. ### Ilya Gulko Thread Starter New Member

Jul 13, 2016
7
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Thanks, Albert.
The probe 2 capacitance is 3pF.

4. ### Alec_t AAC Fanatic!

Sep 17, 2013
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Do you have the PZT correctly mounted at the resonant vibration nodes, as per the datasheet?
What is your output probe impedance?

5. ### Ilya Gulko Thread Starter New Member

Jul 13, 2016
7
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Thanks Alec_t,
The probe 2 impedance is 100MOhm. I am not exactly sure what you mean by "correctly mounted at the resonant vibration nodes". The input AC signal is at the resonant frequency.

6. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,972
1,135
The first image in post #1 shows the PZT mounted on a pcb by silicone blobs 13.25 mm from the ends. Those blobs will be at the vibration nodes. Their positions are critical. If your mounting is different, the vibration will likely be heavily damped, resulting in a low output voltage.

7. ### Ilya Gulko Thread Starter New Member

Jul 13, 2016
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Oh, I see. Yes, my PZT is mounted differently. I just put it snug into a piece of styrofoam and clamped it to a stand. Do you think that has a significant effect?

8. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,972
1,135
Yes. The foam would damp the vibration and probably cause a significant shift in the resonant frequency.
This article demonstrates why the mounting is important.

Last edited: Jul 14, 2016
9. ### Ilya Gulko Thread Starter New Member

Jul 13, 2016
7
0
I think I found the issue. It seems like the PZTs need some kind of load to work properly - there needs to be at least a minute amount of current. I connected the HV output of the PZT to ground through a 1MOhm resistor and got the PZT to actually amplify the signal.