Superposition theorem

The superposition theorem says that in a linear circuit driven by several sources, the response (any voltage or current) is just the sum of the responses you'd get from each source acting alone. It's like figuring out the height of overlapping ripples on a pond by adding the wave from each pebble separately — each cause contributes independently, and the effects pile up.

To use it, you take one source at a time, switch off all the others (replace each ignored voltage source with a short and each ignored current source with an open), solve the simplified circuit, then add up the partial results with their signs. This only works because the circuit is linear: doubling a source exactly doubles its contribution. Superposition is invaluable for circuits mixing DC bias with an AC signal, since you can analyze the steady bias and the wiggling signal completely separately.