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Op Amp Function
Most analog circuits need to amplify signals, either to increase the voltage of a weak input signal or to compensate for signal losses in components. The op is itself a complex circuit, though one packaged with only a few external terminals and operating by fairly simple rules. In addition to power supply terminals, it has two signal inputs and an output. One of the inputs inverts the signal, turning a negative voltage into a positive one and vice-versa. With a pair of resistors and the right power supply, an op can, for example, increase a weak input voltage by a factor of 100.
Supply Voltage Effects
An op cannot output a greater voltage than its power supply. This fact restricts its input voltage range. For example, if it is connected to a 20-volt power supply, you cannot input 1 volt into an op configured with a gain factor of 100. The maximum is less than 20/100, or .2 volts; the op amp's circuitry needs to work with a certain amount of voltage "overhead," so the actual input maximum is between .1 to .15 volts. This effect is most noticeable with audio signals. A 1-volt audio input will not damage the op amp, but the circuit will "clip" the peaks of the signal waveform, introducing severe amounts of distortion into the sound.
Absolute Maximum Ratings
The op amp's data sheet lists maximum ratings, including those for supply and input voltage. These ratings are guidelines for engineers and designers; they indicate the limits an op can handle without damage. The op will continue to work as long as the input and supply voltages stay within the published ratings; however, if the input voltage and the circuit's gain exceed the supply voltage, it will give abnormal outputs.
Single and Dual Supplies
Some op amps, such as the 741, use a dual-voltage power supply. This supply provides positive and negative voltages plus a ground reference connection. The positive and negative voltages connect to the op amp's supply terminals and the ground terminal connects to one of the op amp's inputs, either directly or through a resistor. With this type of supply, the op accurately processes both positive and negative input voltages. The rule applies for maintaining a higher supply voltage than the expected input voltages.
The 358 type op can run on a single-voltage power supply if necessary, but in that configuration can amplify only positive voltages at the input. Since its supply goes from zero to a positive voltage, it interprets negative voltages as "zero." This does not harm the device, but it produces an unexpected output.