Negative group delay and “fronts” in a causal system: An experiment with very low frequency bandpass amplifiers

A very low frequency bandpass amplifier is shown to transmit pulses with negative group delay. Pulses transmitted through a chain of such amplifiers are advanced with little distortion by several milliseconds. Behaviors of abrupt “fronts” and “backs” are demonstrated experimentally. These experiments show that the amplifier is causal, as does the amplifier Green's function.