Something is bothering me about passive systems in resonance. Consider the Bode plot of a second-order system, for example: http://en.wikipedia.org/wiki/File:Harmonic_oscillator_gain.svg Let's say we excite this system near its resonance, so the system has a gain > 1. I don't understand how this can be the case for a passive system like an RLC circuit. If the output amplitude is greater than the input amplitude, that energy has to come from somewhere. What is it that I'm misunderstanding here? It seems like in a passive circuit, the output amplitude should, at most, be the same as the input amplitude. So how is there a gain of greater than 1? Thanks.