Operational Amplifiers with Single Ended Outputs

In the previous three chapters, the most important circuit building blocks utilized in analog integrated circuits (ICs) have been studied. Most analog ICs consist primarily of these basic circuits connected in such a way as to perform the desired function. Although the variety of standard and special-purpose custom ICs is almost limitless, a few standard circuits stand out as perhaps having the widest application in systems of various kinds. These include operational amplifiers, voltage regulators, and analog-to-digital (A/D) and digital-to-analog (D/A) converters. In this chapter, we will consider monolithic operational amplifiers (op amps) with single-ended outputs, both as an example of the utilization of the previously described circuit building blocks and as an introduction to the design and application of this important class of analog circuit. Op amps with fully differential outputs are considered in Chapter 12, and voltage-regulator circuits are considered in Chapter 8. The design of A/D and D/A converters is not covered, but it involves application of the circuit techniques described throughout the book.

An ideal op amp with a single-ended output has a differential input, infinite voltage gain, infinite input resistance, and zero output resistance. A conceptual schematic diagram is shown in Fig. 6.1. While actual op amps do not have these ideal characteristics, their performance is usually sufficiently good that the circuit behavior closely approximates that of an ideal op amp in most applications.

In op-amp design, bipolar transistors offer many advantages over their CMOS counterparts, such as higher transconductance for a given current, higher gain (gmr0), higher speed, lower input-referred offset voltage and lower input-referred noise voltage. (The topic of noise is considered in Chapter 11.) As a result, op amps made from bipolar transistors offer the best performance in many cases, including for example dc-coupled, low-offset, low-drift applications. For these reasons, bipolar op amps became commercially significant first and still usually offer superior analog performance. However, CMOS technologies have become dominant in building the digital portions of signal-processing systems because CMOS digital circuits are smaller and dissipate less power than their bipolar counterparts. Since these systems often operate on signals that originate in analog form, analog circuits such as op amps are required to interface to the digital CMOS circuits. To reduce system cost and increase portability, analog and digital circuits are now often integrated together, providing a strong economic incentive to use CMOS op amps.

In this chapter, we first explore several applications of op amps to illustrate their versatility in analog circuit and system design. CMOS op amps are considered next. Then a general-purpose bipolar monolithic op amp, the 741, is analyzed, and the ways in which the performance of the circuit deviates from ideality are described. Design considerations for improving the various aspects of monolithic op-amp low-frequency performance are described. The high-frequency and transient response of op amps are covered in Chapters 7 and 9.

The assumption that V, = 0 is called a summing-point constraint. A second constraint is that no current can flow into the op-amp input terminals, since no voltage exists across the input resistance of the op amp if V, = 0. This summing-point approach allows an intuitive understanding of the operation of the inverting amplifier configuration of Fig. 6.3a. Since the inverting input terminal is forced to ground potential, the resistor Ri serves to convert the voltage Vs to an input current of value Vs/R\. This current cannot flow in the input terminal of an ideal op amp; therefore, it flows through R2, producing a voltage drop of VSR2IR\. Because the op-amp input terminal operates at ground potential, the input resistance of the overall circuit as seen by Vs is equal to R]. Since the inverting input of the amplifier is forced to ground potential by the negative feedback, it is sometimes called a virtual ground.

6.1.3 Noninverting Amplifier

The noninverting amplifier is shown in Fig. 6.3b.1,2'3 Using Fig. 6.1, assume that no current flows into the inverting op-amp input terminal. If the open-loop gain is a, Vr = V,Ja and

The approximation in (6.12) is valid to the extent that aR\l(R\ + R2) » 1.

In contrast to the inverting case, this circuit displays a very high input resistance as seen by Vs because of the type of feedback used. (See Chapter 8.) Also unlike the inverting case, the noninverting connection causes the common-mode input voltage of the op amp to be equal to Vs. An important variation of this connection is the voltage follower, in which Ri oo and R2 = 0. This circuit is shown in Fig. 6.3c, and its gain is close to unity if a » 1.

6.1.4 Differential Amplifier

The differential amplifier is used to amplify the difference between two voltages. The circuit is shown in Fig. 6.4.1,2 For this circuit, /,i = 0 and thus resistors R\ and R2 form a voltage divider. Voltage Vx is then given by

Rearranging (6.11) gives aRi

The current 1\ is

The output voltage is given by yo = l/v - I2R2

resistor R serves to convert the input voltage Vs into a current. This same current must then flow into the collector of the transistor. Thus the circuit forces the collector current of the transistor to be proportional to the input voltage. Furthermore, the transistor operates in the forward-active region because VCb — 0. Since the base-emitter voltage of a bipolar transistor in the forward-active region is logarithmically related to the collector current, and since the output voltage is just the emitter-base voltage of the transistor, a logarithmic transfer characteristic is produced. In terms of equations, exp

Vbe VT

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