# Piezo signal amplification circuit question

#### incunabula

Joined Jun 30, 2017
2
Hi all. I am using this circuit (not mine) to amplify the signal output by this piezo element. The circuit works fine for my purposes, but I've been using it to learn some basic electronics and had a few questions. Not pictured is the 5v power supply, and the 100uF capacitor between the positive and negative outputs of it.

1) R1 and R5 are in a configuration that I recognize as a voltage divider, but R5 is so big compared to R1 that it looks like there will effectively be no change in voltage. I assume it's either not what I think, or it's serving a different function in this case. What is the purpose of these resistors in this configuration?

2) If I understand correctly, the piezo outputs AC, but when I hook the output of the second op-amp up to an oscilloscope, I only see pulses in one direction. Although this is what I want, I'm confused about how this transformation is happening.

3) What purpose does the 100uF cap bridging the power supply inputs serve. What about the .1uF cap before the second op-amp?

I apologize for linking to the image. For some reason flash crashes every time I try to upload a file here. Thanks for your help!

#### RichardO

Joined May 4, 2013
2,270
1) R1 and R5 are in a configuration that I recognize as a voltage divider, but R5 is so big compared to R1 that it looks like there will effectively be no change in voltage. I assume it's either not what I think, or it's serving a different function in this case. What is the purpose of these resistors in this configuration?
The 1Meg resistor is there to make sure the input of the op-amp does not float if the transducer is not connected. If the input is not connected, the output can go to full scale voltage. The 10K resistor is probably there to protect against over voltage on the input. For instance if the transducer is impacted it might produce a high voltage.

2) If I understand correctly, the piezo outputs AC, but when I hook the output of the second op-amp up to an oscilloscope, I only see pulses in one direction. Although this is what I want, I'm confused about how this transformation is happening.
You are running the op-amps on a single power supply polarity. The inputs to the amplifiers are biased at ground so the output can only swing positive from ground. (There is no power supply voltage to swing the output negative).

3) What purpose does the 100uF cap bridging the power supply inputs serve. What about the .1uF cap before the second op-amp?
Both of these are power supply bypass capacitors. The 100uF cap can supply a pulsed high current load for a short period so the power supply voltage does not drop.
The 0.1uF cap does the same thing at high frequencies.

The 100uF cap should be placed near where the power supply enters your circuit. The 0.1uF cap should be placed right next to the op-amps and have short leads.