Op_amps - Operational Amplifiers (Op-Amps) Author: John M....

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Operational Amplifiers (Op-Amps) Author: John M. Cimbala, Penn State University Latest revision: 11 March 2008 Introduction An operational amplifier (abbreviated op-amp ) is an integrated circuit ( IC ) that amplifies the signal across its input terminals. Op-amps are analog , not digital , devices, but they are also used in digital instruments. Op-amps are widely used in the electronics industry, and are thus rather inexpensive – the ones used in the lab are about $0.25 each! In this learning module, no details are given about the internal structure of the op-amp. Rather, we summarize many useful applications of op-amps. V n + V o V + supply Op-amp V p V supply Description of op-amps A triangle is used as the universal symbol for an op-amp (in schematic circuit diagrams), as shown to the right. The supply voltage terminals are at the top and bottom of the schematic diagram. Supply voltage is necessary because the op-amp requires power to run its internal circuitry. Both a positive and negative supply voltage are required, typically +/ 15 V. In other words, V + supply = 15 V, and V supply = 15 V. In applications, any + and – voltage between about 10 to 20 V can be used for the supply voltage, depending on the manufacturer’s specifications. ( Note : We don’t usually draw the supply voltages on circuit diagrams, but they must be connected or the op-amp will not work!) The signal input terminals are on the left – a positive input terminal V p and a negative input terminal, V n . However, the actual input voltages do not need to be positive and negative for inputs V p and V n , respectively. In fact, the V p input is usually referred to as the noninverting input instead of the positive input, and the V n input as the inverting input instead of the negative input, respectively. The connection for the output voltage V o is on the right (pointed) side of the op-amp, as sketched above. Ideal versus actual op-amps An ideal op-amp has infinite input impedance , and therefore it has no effect on the input voltage . This is called no input loading . An actual op-amp has very high , though not infinite, input impedance (typically millions of ohms), so that it has little effect on the input voltage . This is called minimal input loading . A direct result of the high input impedance is that we may assume negligible current flowing into (or out of) either op-amp input, V p or V n . This result helps us to analyze op-amp circuits, as discussed below. An ideal op-amp has zero output impedance , so that whatever is done to the output signal farther downstream in the circuit has no affect on the output voltage V o . This is called no output loading . An actual op-amp has very low, though not zero, output impedance (typically a few Ω ), so that what is done downstream of the op-amp has little effect on the output voltage . This is called minimal output loading .
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Op_amps - Operational Amplifiers (Op-Amps) Author: John M....

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