GB677126A - Improvements relating to thermionic valve amplifiers in television apparatus - Google Patents

Abstract

677,126. Valve amplifying circuits. ELECTRIC & MUSICAL INDUSTRIES, Ltd. March 16, 1950 [March 24, 1949], No. 8005/49. Class 40 (vi). In a signal channel for electrical signals whose amplitude is liable to vary and which comprises a number of thermionic valve stages, the operating conditions of the valves are so arranged that undesirable variations of the amplitude.- frequency characteristic of the channel due to variation in the valve characteristics with the applied signal are effectively mutually cancelling so that the response characteristic is substantially independent of the signal amplitude. Fig. 2 shows a negative feed-back amplifier in which the intervalve coupling circuits are of the type described in Specification 456,450 and in which the anode impedances of the valves are large compared with the anode load resistors 10, 11, 12. The value of resistor 10 is sufficiently large so that current variations in valve 4 and hence changes in its slope are small. The load resistors 11, 12 and 16 are small so that there are large current variations in valves 5 and 6. As the current variations in valves 4 and 5 are of opposite phase the valve characteristics may be adjusted so that the product of their slopes is substantially independent of the input signal level. If the feed-back path contains thermionic valves this also should have an amplitude-frequency response which is maintained separately independent of variations in the amplitude level of the applied signals. Fig. 3 shows a two-stage amplifier in which the anode loads have impedances which are comparable with the valve anode impedances 23, 25. The anode loads 19- 22 and 24 are constituted by filter networks and are such that for a given value of the valve anode impedances 23, 25 the filter has a desired amplitude-frequency response characteristic. If, due to variations in the level of the signal to be amplified, the anode impedances 23, 25, vary, the overall response of the amplifier is maintained unaltered by adjusting its operating conditions so that the impedances 23 and 25 vary in opposite directions and the effect of variation thus mutually balances out. The amplifier should preferably be such that the product gZp/Z+p is effectively independent of signal level, where g denotes mutual conductance, Z the anode load and p the valve anode impedance of each stage. The amplifier may be divided into two sections the changes in the amplitude-frequency response characteristic of one section with change in signal strength being balanced out by the response of the other section. Fig. 4 shows the application to a television transmitter in which signals from a pick-up tube 26 are amplified in unit 27 modulated on to carrier waves from source 29 in modulator 28 and the resulting modulated signal amplified in unit 30 and radiated at 31. The video amplifier 27 and R.F. amplifier 30 are designed to have oppositely varying frequency response characteristic with change in level of the video signal handled, Fig. 5 (not shown), so that an overall response which is substantially independent of the signal level is obtained. Specification 657,026 also is referred to.