op amp with single supply - non inverting circuit gain not being produced

Question

I'm trying to boost a 12V signal to around 100-120V using an opamp from a single 12V 5A supply voltage (gain of about 10). I can't seem to get it to work though. Right now I'm just trying to make a piezo element louder (with the final hopes of making a pump out of it).

I've tried 3 different opamps, but for the sake of simplicity, I'll just talk about one of them, the LM324 (For a pump, I assume I will need one that has a higher slew rate)

I'm trying to follow the non-inverting gain schematic from this PDF (section 2.1 on page 6).

Here's a list of everything in my circuit (a Fritzing image is below)

• Arduino uno
• 12V 5A power supply
• Opamp (LM324)
• Piezo element
• Ceramic capacitor (100nF)
• R1 = 1K ohm resistor
• R2 = 10K ohm resistor
• R3 = 10K ohm resistor
• R4 = 10K ohm resistor

If the arduino pin just sends a high signal, when I measure the voltage coming out of the opamp, I get 11.01V from Vout. With R2 changed to 2K ohms, I get 10.9V.

If I change the adruino code to a modified blink example with the "delayMicroseconds" command in place of the "delay" one....

void setup() {
pinMode(3, OUTPUT); }

void loop() {
digitalWrite(3, HIGH); delayMicroseconds(800);

digitalWrite(3, LOW);
delayMicroseconds(800);}

... with a 10K resistor I measure 5.7V, and with a 2K one I get 10.8V.

Have I wired something up wrong, is the value of the capacitor not high enough, or am I demanding too much from this opamp?

Answer 1

You can't get more voltage out of an op-amp than it's supply voltage.

^{simulate this circuit – Schematic created using CircuitLab}

You may consider the output stage of the opamp as a pair of transistors with their emitters connected to the output.

If Q1 turns on it pulls Vout towards V+. If, instead, Q2 turns on the output will be pulled towards V-. It should be clear from this that the maximum possible output voltage is V+ and minimum is V-. In practice most op-amps can't even manage this but some can and these are called "rail-to-rail" type but the pull-up and down are weak and can't drive much of a load.

Answer 2

You are demanding too much from your power supply.

Op-amps aren't magic. All they do is to connect the output more or less strongly to the voltage rails depending on the strength of the input voltage.

So, to get 120V out of an opamp, you have to power it off of at least 120V.

But you CANNOT operate an opamp on that voltage. Most are only rated up to maybe 20V. The LM324 is only rated for up to 32V.

Also, please note that those voltage levels are dangerous. Do not try to operate this circuit by powering it from 120V. Given your skill level as indicated by the question you've had to ask, you don't have the skills to work with high voltages safely.