A Constant Input-Impedance RF Amplifier for VHF Television Receivers

Most conventional television tuners exhibit undesirable variations of input impedance at different frequencies within the pass bands and with varying bias voltages applied to the rf amplifier tube. An analysis of the input characteristics of such tuners and commercial television antenna systems, including transmission lines, shows the formation of selective mismatch under certain conditions which may form ``holes´´ within the pass band of any television channel. Such holes degrade the picture quality, particularly for color reception when a deep hole falls upon the color subcarrier. One solution to this problem is presented in this paper, using a constant input-impedance rf amplifier which consists of two stages of grounded-grid triodes in cascade with agc voltage being applied to the grid of the second stage. Other performance characteristics of this amplifier, such as noise factor, gain, overloading capability, cross modulation, etc., as compared to those of a conventional cascode rf amplifier, also are described.