Introduction to Op-Amps - Part 1
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.
NOTE: This series of articles are aimed at teaching musicians how to use Op-Amps. It leaves out things that I consider un-important for this specific topic. The goal is to keep things simple and to the point.
|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 (See Figure 10) 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.|
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.
This may not seem very exciting, but the output current of the Op-Amp is often substantially higher than that of the Input device. In a case where you need to have a single guitar connected up to 3 or 4 stomp box inputs, the guitar pickup output often doesn't have enough current (guitar pickups don't generate very much power) to drive things without altering the tonal qualities of the instrument. Adding a Voltage Follower will allow the input signal to be sent to many stomp boxes with degrading it. Some times, you simply need to flip a signal around so that its 180 degrees out of phase with the input - you can't do it much simpler than this.
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.
For the Inverting Op-Amp (Figure 6), 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. To keep power consumption down, as well as noise introduced by cheap carbon resistors, we will use resistors in a range of 10,000 Ohms thru 1 Meg Ohm.
If R2 is equal to R1, 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 2 - a 2:1 ratio. To build the 2X gain amplifier, lets pick resistor values that will set the 2:1 ratio - R2 = 20,000 ohms and R1 = 10,000 ohms (20000:10000 = 2:1). That really wasn't that hard to do.
For the Non-Inverting amplifier (Figure 7), the gain is the ratio + 1, so a 1:1 ratio is really a gain of 2. Otherwise, everything else remains the same.
Figure 7 shows the results of a 2X gain Non-Inverting amplifier stage. To make it a gain of 10X, set the ratio to 9:1. That would equate to R2 = 90,000 ohms and R1 = 10,000 ohms. To make it gain of 100:1 (99:1 + 1), set R2 = 990,000 (990K) ohms and R1 = 10,000 ohms. This is probably simpler than you thought.
Its easier to design for the Inverting Op-Amp - the ratio is the gain, is 100:1 - R2 - 1 meg ohms and R1 = 10,000 ohms. The side effect is that the inverted output signal is 180 degrees out of phase with the input signal.
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. A TL081 is available
for around $1.00 (US) at Radio Shack. A TL082 is 2 TL081s in an 8 pin integrated circuit,
usually at the same price as a TL082.
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 1 meg ohm or more. An Inverting amplifier has the loading effects of the resistors (which is why my examples use values no lower than 10,000 ohms - this is usually high enough to avoid any problems) - 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, which is why many input stages use a Non-Inverting voltage follower (Figure 10).
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 10X gain, use a 100,000 ohm Potentiometer (use liner taper if possible) for R2, and 10,000 ohms for R1.
Using an Inverting Amplifier, you can extend the number of inputs and create a mixer.
Figure 9 shows the schematic. The ratio method remains the same. To get a mixer with
a 10X gain, the relationship between R2 and R1-A defines its gain. This applies to
all the other inputs as well. You will want to maintain a 10:1 ratio between
the R2 value and each individual R1 value. In this case for a 10X
gain, R1-A, R1-B, R1-C and R1-D are all 10,000 ohms. R2 is 100,000 ohms.
Please note that you will need a few other components to allow you to connect this sort of mixer into your real world audio systems. I'll cover that interface in Part 2.
You'll find that this sort of mixer works quite well up to approximately 8 inputs.
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).
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