The inverting summing amplifier using op amp outputs a voltage equal to the sum of the voltages it has in its inputs. The explanation is based on an adder of three inputs, but applies to an adder of any number of inputs

Window comparator using op amps A window comparator lets you know if a signal is within or outside a previously defined acceptable voltage limits.

How to find the voltage gain (formula) in a non-inverting Op. Amp. How to find the Input impedance (Zi) and output impedance (Zo) of a non-inverting operational amplifier.

Operational Amplifier as Comparator (voltage comparator). Op-Amp can work as comparator when the differential gain in open loop is high and the speed is not a critical factor

Op-amp voltage follower (op-amp voltage buffer). A buffer has an output that is exactly like the input. This feature is very useful for solving impedance matching problems.

Op Amp Open Loop Gain – Operational Amplifier Open Loop gain is given when there is no feedback path between the output and one of the two inputs

How to use op amps with a single rail power supply?
Method #1: Voltage divider using two resistors. Method #2: Voltage divider using two Zener diodes.