Patent GB-743481-A (GB)
Title: Improvements relating to a.c. coupled amplifiers especially for oscilloscopes
Inventor: MANSFORD HUGH LYON
IPC: H03K5/00
Filed: 1952-05-03
Abstract: 743,481. Automatic control systems. ELECTRIC & MUSICAL INDUSTRIES, Ltd. April 20, 1953 [May 3, 1952], No. 11228/52. Class 38 (4). [Also in Groups XXXVI and XL (c)] The peaks of pulses applied to an A.C. coupled amplifier are arranged to be substantially symmetrical about the mean operating level by superimposing a level changing signal on the pulse train, the signal being removed in intervals between pulses. The circuit is described as applied to an oscilloscope amplifier in which the positive input pulses 2 trigger a time base 19 to generate a sawtooth with fast-forward stroke and slow flyback, applied to the X-plates. The first stage of the amplifier comprises two valves 3, 6, the valve 6 being rendered operative and the valve 3 being cut-off by gating signals derived from a pulse generator 24 controlled by time base 19 and applied to the suppressor grids during the return stroke. Valve 3 may be biased negatively so that the pulses 2 are symmetrically placed on the linear part of the characteristic, the output being fed through valve 15 to the main A.C. coupled amplifier 16 connected to the Y-plates of a C.R.T. during the forward stroke of the time base. In order to prevent the amplifier drifting, due to the coupling capacitors charging or discharging, a high frequency square wave 11 from oscillator 10 is fed through valve 6 during the return stroke. The troughs and crests of this wave appear as cursor lines during the flyback period and their distance apart may be adjusted by operation of the control 9 so that a pulse 2 appears between the lines and its amplitude may be read off the calibration of the control. Automatic operation. Two thyratrons 25, 26 are arranged to operate as high frequency relaxation oscillators, small capacitors 32, 34 discharging into large capacitors 27, 28. The mean bias is set by the cathode potential of valve 35, the grid of which is coupled to the penultimate stage of amplifier 16. A connection from a point in the final stage of the amplifier giving the same gain is made to the inner grid of valve 26 so that the oscillators 25, 26 are frequency modulated in accordance with the degree of asymmetry existing in the amplifier. The average potentials of the cathodes is amplified by the push-pull pair 43, 44 to feed a bias in the correct sense to valve 3. The output from valves 43, 44 modulates the wave 11 in valves 53, 55 having a common load 57, the output being demodulated by rectifier 61 to cause valve 64 to conduct and thus lengthen the flyback of time base 19 and reduce the asymmetry. The push-pull pulse waveform from generator 24 is fed over differentiating circuits to valves 67, 68 biased beyond cut-off so that short negative pulses appear across resistor 69 when either valve 3 or 6 is switched on. The negative pulses are longer than any switching surge which may occur in amplifier 16 and prevent any spurious modulation of the oscillation thereby. In order to calibrate the device, a key is depressed to operate switches 58, 59 connecting valve 3 to a voltage source 82 such as a standard cell, the time base being allowed to run free. The control 9 may then be adjusted until a cursor line coincides with the trace provided by the voltage source. Specification 716,442, [Group XL (c)], is referred to.