6.1 Introduction to Operational Amplifiers (Op Amps)

The operational amplifier, or op amp, is an electronic device used for many applications including signal amplification, filtering, comparing voltage values, buffering, or isolating components of a circuit, and creating timing oscillators. Op amps are active devices, meaning that power needs to be supplied to them, in the form of unipolar or bipolar DC voltage sources, to operate. An op amp is a difference amplifier that produces an output voltage proportional to the difference between two inputs, the non-inverting input, denoted by or  , and the inverting input, denoted by or . The schematic symbol for the op amp is a triangle having two inputs and one output.

The op amp is constructed from dozens of transistors; the details of such construction are not of concern to us here. Instead, our interest is in using op amps for different circuit applications and we therefore need to understand how the op amp behaves when it is used as a circuit element. Op amps  generally require a bipolar pair (one positive voltage and one negative voltage) of  external DC voltages to operate, labeled and as shown in Fig. 6.2. Note that all voltages shown in this diagram are node voltages, and they are measured relative to a common reference or ground node which is established by the power supplies, as described below.

The power supply voltage requirement can either be met with either a unipolar power supply, such as  , =ground or a bipolar supply, such as , as shown in Figs. 6.3 – 6.5.

It is common to draw the circuit of Fig 6.3 without a connecting wire between and the terminal of the battery, but using ground symbols instead as shown in Fig. 6.4. Note that ground is the negative terminal of the power supply as shown explicitly in Fig. 6.4. Since the ground was not shown explicitly in Fig 6.3, it is understood that the reference for the input voltages and is the negative terminal of the power supply.

In the bipolar power supply configuration, ground is the node shown between a pair of series-connected  power supplies as shown in Fig. 6.5.  Note that the power line is connected to the terminal of one of the batteries.  This will be made more clear later in this chapter.

A bipolar voltage supply is used when the op amp takes AC differential input voltages and  produces AC output voltages. An example application requiring a bipolar voltage supply would be an op amp circuit that amplifies an audio signal, such as the oscillating voltage on a microphone.

An example of an application requiring a unipolar voltage supply would be an op amp circuit that generates a control voltage to drive an LED on or off, based on a comparison of two input values. This example will be developed further in this chapter.  Our applications in this book will primarily involve unipolar voltage supplies. Note that in the schematics shown on this page, the op amps are shown having  five terminals or connections: the differential inputs, and , the output, , and the power supply rails. Unipolar supply rails are often labeled and the ground symbol. whereas bipolar supply rails are labeled and . (Like much of the labeling in electronics, and have historic significance, referring to the positive voltage at the collector, , and the negative voltage at the emitter, , of a transistor. Such labeling conventions  tend to stick around and become part of common usage.) The power supply voltages are often referred to as voltage supply rails. Schematic drawings of op amp circuits sometimes show the voltage supply rails, but not always.

Op amp saturation. Voltage rails typically range between and volts, depending on the particular op amp selected and specified in the data sheet for the device. The output voltage of an op amp is not capable of exceeding the power supply voltage. If the product of the differential input voltage and the op amp gain exceeds the voltage rail, the output voltage will be saturated, or clipped, to the rail voltage. Consider the illustration of an amplifier circuit below, which has a gain of (we will cover this in a subsequent section) and a unipolar supply voltage rail of . The inverting input is grounded and an input voltage is applied to the non-inverting input, so that the output voltage is .  Input voltage levels < would be amplified by a voltage gain of in this circuit producing output voltages < , while input levels exceeding would exhibit clipping or saturation, owing to the inability of the op amp to deliver an output voltage exceeding . This is illustrated in Fig. 6.7.

A characteristic feature of any  op amp is a very large open-loop gain, ,  which is typically to or even higher. A consequence of this large gain is that op amps readily saturate for very small differential input signals; this occurs unless negative feedback paths are introduced to reduce the overall gain of the circuit. This is developed further the next two sections.

Op Amps are often supplied as dual in-line package (DIP) integrated circuits. A photo and pin-out drawing of the LM358 integrated circuit (IC) is shown in Fig. 6.8. This is a dual op amp IC with two independent op amps. The figure shows pin 4 as GND but a negative voltage can be supplied to that pin for bipolar operation.