Patent GB-407317-A (GB)
Title: Improvements in thermionic valve amplifier circuits
IPC: H03G7/02
Filed: 1932-08-13
Abstract: 407,317. Valve circuits for wireless reception. ROBINSON, E. Y., Garvary, Dryden Road, Bush Hill Park, Enfield, Middlesex, and ASSOCIATED ELECTRICAL INDUSTRIES, Ltd., Crown House, Aldwych, London. Aug. 13, 1932, No. 22772. [Class 40 (v).] In a system in which automatic gain control is applied to an amplifier the signal is rectified and the direct current component of the voltage produced is amplified and applied to effect the gain control through a smoothing network incorporating a device having a unilateral characteristic so that the control voltage is effective only when it is negative with respect to the amplifier valve cathodes. Arrangements are also described in which the output is suppressed when the input is below a pre-determined minimum. Fig. 2 shows a diode detector 12, the direct current component of the voltage of which controls the bias of the amplifier 13 in which an adjustable amount of the audio frequency component is amplified. The direct current load resistance 15 is connected to a negative voltage relatively to that of the cathodes of the high frequency amplifier 10 as by including resistance 16 or a loud-speaker field supply in the high tension negative supply lead. The cathode voltage of the valve 13 is used to grid bias the high frequency amplifier 10, the voltage drops being so proportioned that with no signal this bias tends to be positive but is short-circuited by the diode, copper oxide or other unilateral conductor 19. Incoming signals apply negative bias to the grid of the amplifier 13 and thus the drop of voltage in the load 15 is reduced and a negative bias is applied to the high frequency amplifier 10, giving " delayed " automatic volume control. The audio-frequency output may be taken from a resistance as shown in the anode lead or this may be replaced by a choke or transformer and may be inserted in the cathode lead between the load 15 and the cathode (Fig. 4 not shown). In a modification (Fig. 5 not shown) the unilateral conductor is arranged in series in the grid bias supply lead and shunted by a resistance. Fig. 6 shows a supersonic heterodyne receiver in which the rectifier in the smoothing network is constituted .by the unilateral conductivity of the grid circuits of the controlled valves which may be valves of the kind described in Specification 408,256, or screened pentodes with the suppressor grid brought out to a separate terminal and connected to the control voltage. Alternatively the anode of the high frequency amplifying valve may be perforated and an internal supplementary anode provided and used as the anode of the diode. When valves of the kind described in Specification 408,256 are employed the control may be applied to the gain control grid only or to both this grid and the input grid. The circuit comprises a high frequency amplifier I, detector II, intermediate frequency amplifier III, and a separate oscillator 35. The second detector IV operates as a leaky grid detector with an audio load resistance 40 while resistance 41 shunted by condenser 43 supplements the direct current load circuit from which the control voltage is derived. Volume control is provided by varying the initial voltage of the cathode of the detector IV when no signal is received by varying resistance 41 which may be ganged with a resistance volume control in the input. Alternatively the control voltage may be taken from a variable tapping on resistance 40 which may be ganged to an aerial potentiometer. In a modification (Fig. 8 not shown) the load resistance is provided with two tappings and connected in the cathode lead, one of the tappings varying the automatic control voltage and the other the audio frequency output. Fig. 10 shows an arrangement in which the detector is inoperative when the signal strength is below a predetermined value. With no signal negative bias is applied to the amplifier 13 and to the detector 12 and when the signal exceeds the detector bias, additional bias is applied to the amplifier 13 which reduces the detector bias giving a cumulative effect and removing the detector bias completely. Fig. 12 shows a further arrangement in which the detector 77 is inoperative when the signal is below a predetermined value in which the full delay voltage is available for noise suppression. An auxiliary detector 30 controls the bias voltage of the amplifier 13. When the cathode of the valve 13 is positive with respect to the earth line 29, the diode 78 is non-conductive so that the automatic gain control voltage and the anode of the detector are maintained at earth potential through the resistance 80. When the signal strength reaches a value at which the bias applied to the valve 13 renders its cathode negative with respect to the earth line 29 detector diode 77 is rendered conductive and the direct current component flowing through diode 78 and resistance 80 causes a negative potential to be applied to the lead 20 connected to the grid of the high frequency amplifier 10. In a modification the diode 78 is dispensed with and a diode is connected between the lead 20 and the earth line 29 (Fig. 11 not shown). The valve 13 and the diodes 30, 77, 78 may be mounted in one container and provided with a common cathode sleeve, and an additional diode may be provided so that full wave rectification may be employed. If the gain of the high frequency amplifier is reduced the detector is held inoperative until a larger input signal voltage is reached. The gain of the high frequency amplifier may be adjusted by varying the bias resistance of the valve 10 or the coupling of the aerial. The gain of the low frequency amplifier may also be controlled automatically and more than one directcurrent amplifying valve may be used and this valve may be of the multi-electrode type. In a circuit in which a plurality of high frequency amplifying valves are used the network device having unilateral conductivity may comprise the control or input grid of the last of the high frequency amplifying valves. With this arrangement the damping of the preceding grid circuit produced by grid current, provides an effect of signal suppression when the input is below a predetermined value.