We now turn to an examination of the properties and uses of the operational amplifier or op-amp. A detailed analysis of this multi-stage amplifier circuit is beyond the scope of this text, so we will treat it as a black box device as we did earlier with the voltage regulator. Thus, to use the device, we need only learn and apply some simple rules and, later, the real-world limitations of the device.
In current usage, the operational amplifier is usually packaged as an integrated circuit with multiple leads or pins. While there are hundreds of different op-amps with different specifications, they all follow the same usage rules. To be specific, we will focus on a “classic” version: the 741 op-amp.
The circuit symbol for the op-amp is shown in Fig. 6.1. There are inputs for two power supply voltages (one positive and one negative relative to ground, labeled V+
cc and V-
cc, respectively). There are also two signal inputs: the inverting input, labeled with a minus sign, and the non-inverting input, labeled with a plus sign. Happily, there is only one output.
As we know, voltages are always between two points, but our description of the op-amp inputs seems to refer to voltages at one point, the various input pins. It is thus important to note that all of the voltages for the op-amp are referenced to ground (i.e., the second point is ground).