The symbol for an operational amplifier is shown in Figure 11.2. There are a few things to note about this. The device has two inputs and one output and no common connection. Furthermore, we often don’t show the supply connections—it is often clearer to leave them out of the circuit altogether!
Figure 11.2 Symbol for an operational amplifier
In Figure 11.2, one of the inputs is marked “-” and the other is marked “+”. These polarity markings have nothing to do with the supply connections—they indicate the overall phase shift between each input and the output. The “+” sign indicates zero phase shift while the “-” sign indicates 180° phase shift. Since 180° phase shift produces an inverted waveform, the “-” input is often referred to as the inverting input. Similarly, the “+” input is known as the non-inverting input.
Most (but not all) operational amplifiers require a symmetrical supply (of typically ±6 V to ±15 V) which allows the output voltage to swing both positive (above 0 V) and negative (below 0 V). Figure 11.3 shows how the supply connections would appear if we decided to include them. Note that we usually have two separate supplies; a positive supply and an equal, but opposite, negative supply. The common connection to these two supplies (i.e., the 0 V supply connection) acts as the common rail in our circuit. The input and output voltages are usually measured relative to this rail.
Figure 11.3 Supply connections for an operational amplifier
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