Mysterious Op Amp

The op amp is a cheap, mystery integrated circuit. Here is a short introduction.

Op amps are good mainly for DC and audio frequency. They cannot do much above 200 kHz. Let me tell you a story, and you start discovering op amps by seeing what they can do.

It is common, when you are designing circuits, that you need some signal, like the temperature of a power transistor, a power-supply reference, or the wind speed, to be a voltage in the circuit. "Voltage" here means an ideal voltage source, with zero source resistance, not "loaded down" when other circuits draw current from it. This is one thing an op amp can do.

There are op-amp circuits that can sum or subtract voltages, just as if you were summing or subtracting in Excel. There are even simple op-amp circuits to integrate and differentiate, such as in calculus.

There is an op-amp circuit that does what an air-conditioning thermostat does, use "hysteresis" to control the air conditioner. Namely, the thermostat doesn't just turn on or off the air conditioner when the room temperature is a tiny bit below or above the "set point." It waits until the temperature goes about a degree too far, then it switches the compressor on or off. This is important because the compressor can be destroyed from the pressure surge when it is turned back on too soon.

There are complex op-amp circuits to do multiplication and division.

All these math operations gave the op amp its name, "operational amplifier." These things have been doing math operations since before World War II.

So, if you accept that math operations might be important in circuits, exactly how does it get done? It is with an integrated circuit, typically the LM324, which has four op amps in a 14-pin package for $.18 if you purchase 100. So, you see, you can have math in your circuit cheaply!

Before I show you the schematics for these neat math functions, let's go over some limits to the op amp. The two supply pins, + and -, must "bracket" the voltages you are calculating on. For example, if all your voltages are above ground, the minus pin can be ground. But if you have some negative voltages, or a difference can give a negative result, you connect the minus pin to -15V. Typically, your op amp is powered by 5V or 9V or 15V with the minus pin at ground, or you power it with +15V and -15V.

Another limit is speed. The output of the op amp takes some microseconds to get from one answer to another.

Another limit is accuracy. There are two inputs to an op amp, and the critical property is that they can be "out of balance" or "offset" by .003V, which is really a pretty tiny voltage. Another accuracy limit is that the inputs each need a little current, like .04 microamps, a tiny current indeed. But you can get an FET-input op amp that needs just .002 microamps.

Just before I show the schematics, let me say that only the smartest .001% of people can come up with the schematics just by thinking about them, and I am not one of these. Everyone else has to look up examples of what these smart people have done in the past. There are whole books published that have op-amp schematics.

The first three schematics just have gain: gain=1, gain negative, and gain positive. The last two schematics are operations: subtraction and addition. If these circuits look complex, they really aren't when you use surface-mount parts.

The little squiggle-line is a resistor, and the three-line triangle is ground.

The algebra to prove that the positive-gain circuit works is shown. There is algebra for each circuit, and it is really neat to see it work out.

The photo above shows an op amp, IC1, in a solderless, nylon breadboard with six LEDs and other parts. The maker of this circuit has done a neat job. Normal breadboarded circuits have long leads on the components and do not utilize the cheap surface-mount parts.

Typical op amps: 741, LM324. Texas Instruments has an amazing, new, $2.70 dual op amp, OPA1662. It is bipolar input and has low noise. It is great for low-noise audio. The slew rate is 17V/us!

It is possible for hobbyists to make their own op amp from transistors, with some guidance about the circuit design. You aren't stuck using integrated circuits, you can make your own. Combining JFETs at the input with BJT for the rest can get amazing performance. The only drawback is input offset voltage. But you can even add an "oven" to stabilize the input JFETs' temperatures, and trim Vio to zero. This would be a major project, worthy of a science-fair project.