This article originaly appeared at:
https://www.colomar.com/Shavano/intro_opamp.html
It was edited to make it more applicable to audio amp builders, and was reprinted by permission by the origianl author, Jens Moller -
Operational Amplifiers (Op-Amp) were originally developed to build Analog Computers (and many Computer systems used in Aircraft still use them for this purpose). They are pre-built amplifier modules that are general enough to be plugged in almost anywhere an amplifier is needed. The advantage is that a small Op-Amp can often replace 20 or more discreet components. For people who like to build custom audio gear, they are a quick way to put together a highly functional amplifier stage.
I won't spend much time on theory, I would rather tell you generally how they work and how you might use them in your projects.
Figure 1:
https://www.nik-martin.com/opamps/op-amp1.gif
The basic diagram for an Op-Amp is shown in Figure 1. I've left off the power supply connections for this disccusion, however there is always a Positive and a Negative power supply requirement for these devices. Its possible to find Op-Amps packaged 4 to a 14 pin integrated circuit, as well as dual and single Op-Amp packages. To keep it simple, we will talk about an individual Op-Amp.
There are many types of Op-Amps. The ones that we will discuss are the common voltage amplifier type, such as a '741', 'TL081' or 'TL082' that are packaged as 8 pin integrated circuits. You'll note that there are 2 inputs - an Inverting Input (marked with a - sign) and a Non-Inverting Input. There is a single output. How it works internally is not really important for this discussion.
Most analog applications use an Op-Amp that has some amount of negative feedback. The Negative feedback is used to tell the Op-Amp how much to amplify a signal. In Figure 2, this Op-Amp will not amplify at all, it operates at Unity Gain, also known as a gain of 1. Unity Gain arrangements are also called Voltage Followers since they track the input voltage at the exact same level at output. Sometimes, you will want an output that is Inverting, and sometimes you want one that is Non-Inverting - often, you don't really care which way it works as long as it provides-an output signal.
https://www.nik-martin.com/opamps/op-amp2.gif
If you apply an input to either the - (Inverting) or the + (Non-Inverting) input, the Op-Amps output basically maintains the input level, but in the case of applying an input to the Inverting (-) input - Figure 3, the output signal will be 180 degrees out of phase with the input. In Figure 4, you see that the signal comes thru unchanged.
https://www.nik-martin.com/opamps/wave1.gif
https://www.nik-martin.com/opamps/wave2.gif
This may not seem very exciting, but the output of the Op-Amp is often substantially higher than that of the Input. If an Op-Amp is an amplifier, how hard is it to get it to amplify the signal? Its very easy. The following 2 schematics show the 2 variations again, this time configured to amplify the signal.
https://www.nik-martin.com/opamps/op-amp4.gif
https://www.nik-martin.com/opamps/op-amp3.gif
The way that you define the gain is by setting the ratio of R1 to R2. Neither of these resistors will ever have much power going thru them, so these can be very tiny - often 1/4 or 1/8 watt resistors are used. As a rule of thumb, use the lowest value resistors that give you the gain you are trying to achieve. This makes the circuit quieter than using very high value resisters.
The gain equation is V=(R2+R1):R1. If R2 is 0, then we have Unity Gain, or a 1X Amplifier - This is a 1:1 ratio. If R2 is twice the resistance of R1, we have an Amplifier with a gain of 3 - a 3:1 ratio. To build the 3X gain amplifier, lets pick resistor values that will set the 3:1 ratio - R2 = 2,000 ohms and R1 = 1,000 ohms ((2000+1000):1000 = 3:1). That really wasn't that hard to do.
To make it a gain of 10X, set the ratio to 10:1. That would equate to R2 = 9,000 ohms and R1 = 1,000 ohms. To make it gain of 100:1, set R2 = 99,000 (100k) ohms and R1 = 1,000 (1k) ohms. This is probably easier than you thought.
https://www.nik-martin.com/opamps/wave3.gif
One side effect of high gains is that sometimes the Op-Amp is not fast enough to keep up with the voltage swings. There is a parameter called Slew Rate that defines how fast an Op-Amp is. A common '741' Op-Amp is pretty slow with a 1/2 volt per micro-second Slew Rate. This is fine for Voltage Followers, but if you push a 10X gain thru a '741' Op-Amp, you'll find that it can alter the tonal quality of your signal, usually attenuating the high frequency parts. There are much faster Op-Amps available, and such as the TL081 which is 13 volts per microsecond - which is plenty fast for anything up to a 100X gain.
Op-Amps do become a load on the input signal. If you want the lowest possible loading effect, you would use a Non-Inverting Input - typically, the impedance is 50k ohm or more. An Inverting amplifier has the loading effects of the resistors (which is why my examples use low values) - effectively the load will appear to be the same as R1. This could provide a substantial change in the tonality of the signal, if its very low level to begin with.
If you wanted to make the gain adjustable, its only a matter of providing a way to alter the ratio of R2 to R1. Use a Potentiometer (variable resistor). Always wire it up with the wiper arm connected to one side of the Potentiometer in case that the wiper arm ever fails to make contact (this will prevent the feedback loop from ever being able to open up).
Figure 8 shows the schematic. To give yourself a range of 0 Gain to 11X gain, use a 10,000 ohm Potentiometer (use liner taper if possible) for R2, and 1,000 ohms for R1.
https://www.nik-martin.com/opamps/op-amp5.gif
Op-Amps can be used for many things that are non-audio - you can even build a digital computer out of them (it would take quite a few), using them as on/off switches. If you neglect to provide the Negative feedback, you will turn it into a switch instead of an Amplifier. Make sure that you use Op-Amps that are for the type of function that you desire, otherwise, you may choose one that operates in a non-linear manner (ie. its not for the types of applications shown here).
Because there is nothing for discrete op-amps, I will ad some here.
1.
