Op-Amp Amplifier

An op-amp with two resistors in its feedback loop multiplies a small signal by a precise gain.

Advanced
Analog Electronics
7 parts

Opens in your browser with the parts, wiring and code already in place. Press Run to watch it work, then change anything you like.

How it works

An op-amp has a huge gain and two inputs. With feedback it behaves very simply: it adjusts its output until the two inputs are at the same voltage. Here the signal goes into the plus input, and the output is fed back to the minus input through a divider made of a 4 kilohm resistor Rf and a 1 kilohm resistor Rg. To keep the minus input at the signal voltage, the output has to be larger by the divider ratio, giving a gain of 1 plus Rf over Rg, which is 5.

The generator supplies a small 1 kilohertz sine wave of 0.2 volts above and below a 1 volt level, because this op-amp runs from a single 12 volt supply and cannot output below ground. The oscilloscope shows the input in yellow, swinging between 0.8 and 1.2 volts, and the output in blue swinging between 4 and 6 volts: five times as large. If the output tried to go past the supply, it would flatten at the top, which is called clipping.

Parts

Function Generator
DC Supply
Op-Amp
2 × Resistor
GND
Oscilloscope

Wiring

Function Generator OUT → Op-Amp IN+
Op-Amp OUT → Resistor 1 A
Resistor 1 B → Op-Amp IN-
Op-Amp IN- → Resistor 2 A
Resistor 2 B → GND
DC Supply + → Op-Amp VCC
DC Supply - → GND
Op-Amp GND → GND
Function Generator GND → GND
Oscilloscope CH1 → Function Generator OUT
Oscilloscope CH2 → Op-Amp OUT
Oscilloscope GND → GND

Arduino code

// Op-Amp Amplifier
// Press Run: channel 2 (blue) is channel 1 (yellow) times the gain, 1 + Rf / Rg.
// There is no sketch to write: this is a circuit, so there is nothing for code to do.
void setup() {
}

void loop() {
}

Try changing it

Change Rf to 9000 so the gain becomes 10, and the output now swings between 8 and 12 volts, flattening on top where it hits the supply limit.
Change Rf to 1000 and the gain is 2, so the output is only twice the input.
Select the generator and raise the amplitude to 1. Now the output tries to swing beyond what the supply allows and you can see it clip.

Open this circuit and run it

Next challenge

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