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.
http://www.e-dsp.com/why-do-we-need-amplifiers-how-to-build-a-simple-one-lm-741/
Single and split power supply, inverting and non inverting operational amplifier theory. Ps
http://homepages.internet.lu/absolute3/tronic/opsup.htm
This article shall focus on operational amplifiers which will form the basis for active filters.
http://members.tripod.com/~baec/MARCH98/basic3.htm
It is not at all difficult to make an op-amp using transistors. This page shows you how and discusses how they work.
http://www.4qdtec.com/opamp.html
op-amp rectifier operates from a single supply and provides a logic output and an LED that indicate the state of the input voltage
http://www.edn.com/archives/1994/072194/15di2.htm
simple circuit board to make testing and prototyping opamp circuits
http://www.sound.au.com/project41.htm
Calculators
http://www.analyzethat.net/65_operational_amplifiers.php
http://margo.student.utwente.nl/el/analog/disc-oa.gif