CAUSALITY AND NEGATIVE GROUP DELAYS IN A SIMPLE BANDPASS-AMPLIFIER

We demonstrate a bandpass amplifier which can be constructed from comm on electronic components and has the surprising property that the grou p delay is negative in most spectral regions. A pulse propagating thro ugh a chain of such amplifiers is advanced by several milliseconds: th e output waveform precedes the input waveform. Although striking, this behavior is not in conflict with causality, as demonstrated by experi ments with pulses which start or end abruptly. (C) 1998 American Assoc iation of Physics Teachers.