On the voltage amplification by electric wavefilters terminated in negative resistances

Abstract

Ideal wave-filters should have zero attenuation in the transmission band whereas in commercial filters consisting of several sections this attenuation cannot be reduced below 3 or 4 decibels. In multiplex communication systems, the filter attenuation becomes very large as the frequency bands of a channel have to pass through several filters and a high gain repeater has therefore to be inserted to overcome the total filter attenuation and the circuit loss. The paper relates to the author's experiments on wave-filters terminated in negative resistances formed by screen grid thermionic tubes under secondary emission condition. He has found out that if a wave-filter be terminated in a negative resistance -Z_o where Z_o is a pure resistance of the value of the characteristic impedance in the transmission band, a voltage gain of several decibels can be obtained for frequencies in the transmission band, the cut-off improves and the amplitude distortion is well within the limits of tolerance laid down by the C.C.I. This research is of great value in the field of electrical communications as the same device can be used to filter out a certain band of frequencies and to amplify it at the same time. A no-loss filter can be manufactured by inserting a negative resistance element between any two sections. By employing no-loss or gain filters in the channel, very high grade circuits can be obtained and only a low gain repeater will be necessary.