Basics
Differential input
If the non-inverting input rises above the inverting input, the output tends to move positive. If the inverting input rises above the non-inverting input, the output tends to move negative.
Negative feedback
Negative feedback returns part of the output to the inverting input. If the output moves too far, feedback creates an error that pushes it back. External components then determine the closed-loop gain far more accurately than the op-amp's raw open-loop gain.
Positive feedback
Positive feedback reinforces a change. It is useful in Schmitt triggers, oscillators, and bistable circuits but is not the normal condition for a linear amplifier.
Output saturation
The output cannot exceed its supply rails and usually cannot reach them exactly. When a circuit asks for more output voltage than the amplifier can supply, the waveform clips.
Common-mode input range
Both inputs must remain within the permitted common-mode range. A circuit can have almost zero differential input and still malfunction because both inputs sit too close to an unsupported rail.
Input bias current
Real input transistors need or leak some current. Bipolar-input op-amps usually have larger bias current than JFET or CMOS-input types.
Input offset voltage
A real op-amp behaves as though a tiny unwanted voltage is inserted between its inputs. Closed-loop gain can magnify that error.
Slew rate
Slew rate is the maximum large-signal output change, commonly stated in volts per microsecond.
Voltage follower
Connect output directly to the inverting input and apply the signal to the non-inverting input. The closed-loop gain is approximately one. The circuit is useful as a buffer because it presents high input impedance and lower output impedance.