An operational amplifier (OPV), Operational Amplifier (OpAmp), is characterized by three crucial features: its infinitely high gain, its infinitely high input impedance and its output impedance going to zero. It is a galvanically coupled differential amplifier for DC and AC voltages. Like the differential amplifier, the operational amplifier has two inputs: a positive or non-inverting and a negative or inverting, which can be balanced and unbalanced, and an output.
The differential voltage between the input voltages is available amplified as the output voltage. Since the gain of the operational amplifier approaches infinity, the slightest voltage differences at the differential inputs cause output voltage changes up to the positive and negative supply voltages by which they are limited. The gain itself can be reduced to a desired value by negative feedback
The operational amplifier as a quadrupole
If we consider the operational amplifier as a quadrupole, the parameters include gain, bandwidth, the product of gain and bandwidth, input and output impedances, common- mode rejection, DC offset, harmonics due to nonlinear distortion, rate of change, and rise time.
The operational amplifier is an analog integrated circuit formed by resistors, capacitors and some transistors. The amplification values are between 80 dB and 120 dB. Typical values for one of the best known operational amplifiers, type µA 741, are the common-mode rejection of 90 dB, the product of gain and bandwidth of 1 MHz, the slew rate( SR), which is the slew rate of the output voltage, of 0.5 V/µs, and the rise time of 0.3 µs.
The circuit technology for operational amplifiers is relatively simple and is accomplished by little external circuit modification. Depending on their use, operational amplifiers can be used as differential amplifiers, summing amplifiers, integration amplifiers, adders, inverting amplifiers, active filters, impedance converters, oscillators or function generators.