GB564821A - Improvements in or relating to line amplifiers for high-frequency electric signals such as television signals - Google Patents

Abstract

564,821. Valve amplifying circuits; television. WHITE, E. L. C. April 9, 1943, No. 5690. [Classes 40 (iii) and 40 (v)] A line amplifier for television, adapted for feeding a plurality of loads (e.g. television receivers fed in parallel from a single input) comprises two valves with a common cathode impedance having the signals to be amplified applied to the control electrode of the first valve and to beat signals at the cathode of the second valve, the third valve being driven from the second valve and having a load in its cathode circuit. In such an amplifier, feed-back potentials are applied to the control electrode of the second valve in phase with the cathode potential, so that the third valve has a low output impedance. Fig. 1 shows a valve 1 to which the signal is applied having a cathode resistance 11 in common with a second valve 2 which amplifies a signal and feeds the main amplifier 3. The main amplifier may consist of an appropriate number of valves in parallel depending on the number of loads fed. The loads may consist of concentric lines 5 which may have series resistances inserted in them equal to the characteristic impedance of the section of line and matched at the-other end to the input of the device so that when the load is short-circuited the amplifier cannot consume more than twice the normal load. The negative feedback is taken from the output line 4 and may include a potentiometer 32, Fig. 3, which may be so adjusted that the output level in the cable 5 is the same as the input level. In addition, the feedback connection may include a network to correct for distortion in the input signals. To ensure that the potential of the output shall be positive for vision signals and negative to earth for synchronising signals, a resistance may be inserted between the output 4 and the high-tension supply; preferably, however, as shown in Fig. 3, further valves 20 are provided to furnish the negative signal. Such valves may be fed as shown from the anode of the main amplifier 3 or from the anode of the first valve, Fig. 2 (not shown). A diode 29 may be provided in shunt to the input to the valves 29 to reinsert the D.C. component. Fig. 4 shows a further arrangement in which the auxiliary valves 20 are energized from resistance 31 in the output circuit of the main amplifier 3. If the input capacities of the valves 3 and 20 are excessive due to the use of many valves in parallel, a cathode follower stage may be inserted in the inputs to these valves. The amplifiers may be duplicated if a balanced system is required in the output circuit.