Operational Amplifier Circuits

An operational amplifier (op-amp) is a high-gain direct-coupled amplifier with two inputs, inverting and non-inverting, and a single output. Its open-loop gain is very large, so in linear use it is almost always surrounded by a feedback network that sets the actual, predictable gain of the circuit. The ideal op-amp draws no input current and amplifies only the difference between its two inputs, which makes its behaviour easy to analyse using a few simple rules. The 741 is a long-established general-purpose op-amp often used to teach these principles.

With negative feedback, part of the output is returned to the inverting input, and the circuit settles so that the two inputs are held at almost the same voltage. This gives the familiar configurations: the inverting amplifier, whose gain is set by the ratio of two resistors and which inverts the signal; the non-inverting amplifier, which keeps the signal in phase; and the voltage follower, which buffers a signal at unity gain. A difference amplifier amplifies the difference between two signals while rejecting any voltage common to both.

Op-amps also perform mathematical operations and decisions. An integrator uses a capacitor in the feedback path to produce an output proportional to the time integral of the input, and a differentiator does the reverse. Without feedback, or with positive feedback, the op-amp becomes a comparator that swings its output to one extreme or the other depending on which input is higher, useful for detecting thresholds. Related devices such as the operational transconductance amplifier (OTA) produce an output current rather than a voltage, extending the family to applications such as variable-gain and filter circuits.

Frequently asked questions

Why do op-amps almost always use feedback?

The open-loop gain of an op-amp is very large and imprecise. Negative feedback trades some of that gain for a stable, predictable closed-loop gain set by external components.

What is the difference between an op-amp and a comparator use?

As an amplifier the op-amp runs with negative feedback in its linear region. As a comparator it runs open-loop or with positive feedback, switching its output fully high or low based on which input is larger.

What does an op-amp integrator do?

It produces an output voltage proportional to the accumulated integral of the input over time, achieved by placing a capacitor in the feedback path.