An operational amplifier (op-amp) is a device with two inputs and a single output. The output of the amplifier is given by the formula:
Where A is the open-loop voltage gain of the amplifier, is the non-inverting input voltage and is the inverting input voltage. Both and are node voltages with respect to ground. Typically, the open-loop voltage gain A is on the order of . A resistor is placed between the output node and the inverting input to provide feedback and adjust amplification. When an op-amp circuit behaves linearly, the op-amp adjusts its output current such that the voltage difference between the two inputs is nearly zero.
Another important feature of the op-amp is that its input resistance is very large and may be taken as infinite in many applications. The most common type of op-amp is the 741 which has an input resistance of 2 M. This is large enough to be considered infinite in most applications. Because of the high input resistance, only a very small current flows into either input of an op-amp. In practical op-amp circuits, the current flowing into either of the inputs is usually on the order of . In the case of an ideal op-amp, where the single assumption is made that the open-loop voltage gain A goes to infinity,
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