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INTEGRATED CIRCUITS AN165 Integrated operational amplifier theory 1988 Dec Philips Semiconductors Philips Semiconductors Application note Integrated operational amplifier theory AN165 INTRODUCTION The operational amplifier was first introduced in the early 1940s. Primary usage of these vacuum tube forerunners of the ideal gain block was in computational circuits. They were fed back in such a way as to accomplish addition, subtraction, and other mathematical functions. Expensive and extremely bulky, the operational amplifier found limited use until new technology brought about the integrated version, solving both size and cost drawbacks. Volumes upon volumes have been and could be written on the subject of op amps. In the interest of brevity, this application note will cover the basic op amp as it is defined, along with test methods and suggestive applications. Also, included is a basic coverage of the feedback theory from which all configurations can be analyzed. DEFINITION OF TERMS Earlier, the ideal operational amplifier was defined. No circuit is ideal, of course, so practical realizations contain some sources of error. Most sources of error are very small and therefore can usually be ignored. It should be noted that some applications require special attention to specific sources of error. Before the internal circuitry of the op amp is further explored, it would be beneficial to define those parameters commonly referenced. INPUT OFFSET VOLTAGE Ideal amplifiers produce 0V out for 0V input. But, since the practical case is not perfect, a small DC voltage will appear at the output, even though no differential voltage is applied. This DC voltage is called the input offset voltage, with the majority of its magnitude being generated by the differential input stage pictured in Figure 2. An operational amplifier's performance is, in large part, dependent upon the first stage. It is the very high gain of the first stage that amplifies small signal levels to drive remaining circuitry. Coincidentally, the input current, a function of beta, must be as small as possible. Collector current levels are thus made very low in the input stage in order to gain low bias currents. It is this input stage which also determines DC parameters such as offset voltage, since the amplified output of this stage is of sufficient voltage levels to eclipse most subsequent error terms added by the remaining circuitry. Under balanced conditions, the collector currents of Q1 and Q2 are perfectly matched, hence we may say: V+ THE PERFECT AMPLIFIER The ideal operational amplifier possesses several unique characteristics. Since the device will be used as a gain block, the ideal amplifier should have infinite gain. By definition also, the gain block should have an infinite input impedance in order not to draw any power from the driving source. Additionally, the output impedance would be zero in order to supply infinite current to the load being driven. These ideal definitio