In This Chapter

The basic operational amplifier is discussed from a topological point-of-view. A step-by-step case study illustrates the basic building blocks in a monolithic op-amp. At the end of this chapter we ll consider some of the real-world limitations of operational amplifiers.

Basic Device Operation

The ideal operational (Figure 12-1) amplifier has the following characteristics:

• Differential inputs. The output is an amplified version of the difference between the + and terminals.

• Infinite gain. The gain is infinite.

• Infinite bandwidth. There are no bandwidth limitations.

• Infinite slew rate. There is no limit to the rate with which the output can change. In other words, there is no limit to dV _out/ dt.

• Zero input current. The input current to both inputs is zero.

• Zero output impedance. The output impedance is zero.

• Zero power dissipation. The ideal op-amp doesn t draw or dissipate any power.

• Infinite power supply rejection. The output is not dependent on variations in power supply voltage.

• Infinite common-mode signal rejection. The output doesn t depend on the value of the common-mode signal.

Figure 12-1: Ideal operational amplifier showing differential inputs V+ and V . The ideal op-amp has zero input current and a gain approaching infinity that amplifies the difference between V+ and V .

The ideal op-amp is, of course, nonexistent, but device manufacturers have done a better and better job over the years designing devices that approach the ideal. For instance, it is common to find devices with DC...