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?
Best Answer
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.