OpAmps 101

Dave at EEVBlog explains what Operational Amplifiers (OpAmps) are and how they work. The concepts of negative feedback, open loop gain, virtual grounds and opamp action. The comparator, the buffer, the inverting and non-inverting amplifiers, the differential amplifier, and the integrator circuit configurations are also explained.
Then a practical breadboard circuit to demonstrate a virtual ground and the effect of voltage rail limitations.

Source: eevblog