ABSTRACT
Symbols and connections The symbol for an operational amplifier is shown
in Fig. 8.2. 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! In Fig. 8.2, one of the inputs is marked ‘*’ and the other is marked ‘+’. These polarity markings have nothing to do with the supply connectionsthey indicate the overall phase shift between each input and the output. The ‘+’ sign indicates zero phase shift whilst 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 8.3 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
Figure 8.1 A typical operational amplifier. This device is supplied in an 8-pin dual-in-line (DIL) package. It has a JFET input stage and produces a typical open-loop voltage gain of 200,000
Figure 8.2 Symbol for an operational amplifier
Figure 8.3 Supply connections for an operational amplifier
applied). The effect of providing negative feedback is to reduce the loop voltage gain to a value that is both predictable and manageable. Practical closedloop voltage gains range from one to several thousand but note that high values of voltage gain may make unacceptable restrictions on bandwidth, see later. Closed-loop voltage gain is once again the ratio of output voltage to input voltage but with negative feedback is applied, hence:
where AV(CL) is the open-loop voltage gain, VOUT and VIN are the output and input voltages respectively under closed-loop conditions. The closed-loop voltage gain is normally very much less than the open-loop voltage gain.
