To realize this simple project you will need just a few parts; two resistors, two 9V batteries, one famous and popular operational amplifier IC (Integrated Circuit a.k.a. chip), LM741, a breadboard and some wire to connect the parts on the breadboard.
Single and split power supply, inverting and non inverting operational amplifier theory. Ps
This article shall focus on operational amplifiers which will form the basis for active filters.
It is not at all difficult to make an op-amp using transistors. This page shows you how and discusses how they work.
op-amp rectifier operates from a single supply and provides a logic output and an LED that indicate the state of the input voltage
simple circuit board to make testing and prototyping opamp circuits