 Valve amplifiers have a tuned network that serves as both the low pass harmonic filter and impedance matching to the output load.In either case, both solid state and valve devices need such filtering networks before the RF signal is output to the load.Unlike audio amplifiers, in which the analog output signal is of the same form and frequency as the input signal, RF circuits may modulate low frequency information (audio, video, or data) onto a carrier (at a much higher frequency), and the circuitry comprises several distinct stages.