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The trend toward the use of operational amplifiers as general-purpose analog building blocks began when modular, solid-state discrete-component designs became available to replace the older, more expensive vacuum-tube circuits that had been used primarily in analog computers. As cost decreased and performance improved, it became advantageous to replace specialized circuits with these modular operational amplifiers.
This trend was greatly accelerated in the mid 1960s as low-cost monolithic integrated-circuit operational amplifiers became available. While the very early monolithic designs had sadly deficient specifications compared with discrete-component circuits of the era, present circuits approach the performance of the best discrete designs in many areas and surpass it in a few. Performance improvements are announced with amazing regularity, and there seem to be few limitations that cannot be overcome by appropriately improving the circuit designs and processing techniques that are used. No new fundamental breakthrough is necessary to provide performance comparable to that of the best discrete designs. It seems clear that the days of the discrete-component operational amplifier, except for special-purpose units where economics cannot justify an integrated-circuit design, are numbered.
In spite of the clear size, reliability, and in some respects performance advantages of the integrated circuit, its ultimate impact is and always will be economic. If a function can be realized with a mass-produced integrated circuit, such a realization will be the cheapest one available. The relative cost advantage of monolithic integrated circuits can be illustrated with the aid of the discrete-component operational amplifier used as a design example in the previous chapter. The overall specifications for the circuit are probably slightly superior to those of presently available general-purpose integrated-circuit amplifiers, since it has better bandwidth, d-c gain, and open-loop output resistance than many integrated designs. Unfortunately, economic reality dictates that a company producing the circuit would probably have to sell it for more than $20 in order to survive. General-purpose integrated-circuit operational amplifiers are presently available for approximately $0.50 in quantity, and will probably become cheaper in the future. Most system designers would find a way to circumvent any performance deficiencies of the integrated circuits in order to take advantage of their dramatically lower cost.
The tendency toward replacing even relatively simple discrete-component analog circuits with integrated operational amplifiers will certainly increase as we design the ever more complex electronic systems of the future that are made economically feasible by integrated circuits. The challenge to the designer becomes that of getting maximum performance from these amplifiers by devising clever configurations and ways to tailor behavior from the available terminals. The basic philosophy is in fundamental agreement with many areas of design engineering where the objective is to get the maximum performance from available components.
Prior to a discussion of integrated-circuit fabrication and designs, it is worth emphasizing that when compromises in the fabrication of integrated circuits are exercised, they are frequently slanted toward improving the economic advantages of the resultant circuits. The technology exists to design monolithic operational amplifiers with performance comparable to or better than that of the best discrete designs. These superior designs will become available as manufacturers find the ways to produce them economically. Thus the answer to many of the "why don't they" questions that may be raised while reading the following material is "at present it is cheaper not to."