Often these pins are left out of the diagram for clarity, and the power configuration microelectronics digital and analog circuits and systems jacob millman pdf described or assumed from the circuit. Circuit diagram symbol for an op-amp. Pins are labeled as listed above.
Op-amps are among the most widely used electronic devices today, being used in a vast array of consumer, industrial, and scientific devices. Op-amps may be packaged as components or used as elements of more complex integrated circuits. If predictable operation is desired, negative feedback is used, by applying a portion of the output voltage to the inverting input. When negative feedback is used, the circuit’s overall gain and response becomes determined mostly by the feedback network, rather than by the op-amp characteristics. An equivalent circuit of an operational amplifier that models some resistive non-ideal parameters.
The inputs draw no current. These rules are commonly used as a good first approximation for analyzing or designing op-amp circuits. None of these ideals can be perfectly realized. A real op-amp may be modeled with non-infinite or non-zero parameters using equivalent resistors and capacitors in the op-amp model. The designer can then include these effects into the overall performance of the final circuit.
Some parameters may turn out to have negligible effect on the final design while others represent actual limitations of the final performance that must be evaluated. Real op-amps differ from the ideal model in various aspects. Typical devices exhibit open-loop DC gain ranging from 100,000 to over 1 million. However, as long as these operational amplifiers are used in a typical high-gain negative feedback application, these protection circuits will be inactive. The input bias and leakage currents described below are a more important design parameter for typical operational amplifier applications.
When large resistors or sources with high output impedances are used in the circuit, these small currents can produce large unmodeled voltage drops. It is more common for the input currents to be slightly mismatched. This offset voltage can create offsets or drifting in the operational amplifier. This voltage, which is what is required across the op-amp’s input terminals to drive the output voltage to zero. In the perfect amplifier, there would be no input offset voltage.
However, it exists in actual op-amps because of imperfections in the differential amplifier that constitutes the input stage of the vast majority of these devices. Input offset voltage creates two problems: First, due to the amplifier’s high voltage gain, it virtually assures that the amplifier output will go into saturation if it is operated without negative feedback, even when the input terminals are wired together. Second, in a closed loop, negative feedback configuration, the input offset voltage is amplified along with the signal and this may pose a problem if high precision DC amplification is required or if the input signal is very small. A perfect operational amplifier amplifies only the voltage difference between its two inputs, completely rejecting all voltages that are common to both. However, the differential input stage of an operational amplifier is never perfect, leading to the amplification of these common voltages to some degree.
The output of a perfect operational amplifier will be completely independent from its power supply. All parameters change with temperature. Temperature drift of the input offset voltage is especially important. Real op-amp parameters are subject to slow change over time and with changes in temperature, input conditions, etc. The op-amp gain calculated at DC does not apply at higher frequencies.