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DIY Op Amps
(c) Nelson Pass, Pass Labs presumption that simpler circuitry is better, particularly when applied to audio applications. How Op Amps Work First, a little tutorial on how op amps work. Fig. 1 shows an op amp, which has five connections to the outside world. Two of these are power supply rail pins, +V and -V, which for monolithic op amps usually want to be attached to supply voltagesin the range of about 5 to 15 volts each for commercially available chips. Specialized products work outside this range, from as low as 1 volt to as high as 500 volts. There is a presumption here that +V and -V are referenced to a ground potential, but this is not required by the op amp as such. The op amp only needs positive voltage on the +V pin relative to the voltage on the -V pin to operateproperly. Introduction Carl Sagan observed, "If you want to make an apple pie from scratch, you must first create the universe." If you want to build an audio circuit, you will make the decision as to how much you will actually build yourself, and how much you will buy fabricated. The line is always drawn somewhere. Will you be melting down some sand to make your transistors? Probably not, but itis always in the mind of the hobbyist to do as much as possible. Many of the project articles in AE and elsewhere employ commercially available integrated circuits, usually operational amplifiers. Monolithic op amps are the gum drops of linear electronics. The Digikey catalog shows hundreds of such parts with prices ranging from about fifty cents to over twenty dollars, depending on performance,package, and manufacturer. Virtually all of these parts would serve in an audio project calling for an op amp. It is very cheap and easy to use monolithic op amps. In contrast we would suppose that constructing our own op amps from scratch using transistors (or even tubes) would be a daunting task, to be tackled only by seasoned engineers, with no guarantee of satisfaction. This is not the case atall. Simple high-performance op amps are easy to make out of as few as six discrete components whose total cost is less than a dollar. It is the purpose of this article to show how easy it is. Besides the satisfaction of doing it yourself, are there any other reasons to construct your own op amps? Yes. First you may want some special characteristic such as very high voltage, or ultra low noise, orhigh output current, that might be available commercially but at very high cost. Apex, for example, offers high voltage and high current op amps, but you should be prepared to pay as much as $682 for a PA03A which can deal with 75 volt rails and 30 amp output currents. For audiophiles with a subjective orientation, there is another reason for building your own op amps which is revealed when youlook at the internal schematics of the commercially available chips. Most of them are designed to achieve specifications which are not of great importance to audiophiles, such as ultra high gain, or very low DC drift, or very low dissipation. Their complex topologies reflect these requirements, so you see many transistors and gain stages in series. The output stages of these op amps are operatedClass B or AB, so they are not as linear as might be achieved with Class A operation. We see also that monolithic fabrication techniques do not necessarily deliver the best semiconductor or resistor for each part of the circuit, and by making our own choices in device selection and testing we can achieve high linear performance with very simple circuits. I take it as a Pass D.I.Y Project: DIY Op AmpsAn op amp has two input pins, designated the positive and negative inputs (+In and -In), which control the voltage at the output pin. For linear operation we will generally want the voltages appearing at +In and -In to be in the range between the power supply voltages, and in many applications we will see these pins operated near ground potential, midway between +V and -V. The output of the...
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