Patent GB-592555-A (GB)
Title: Improvements in or relating to automatic gain control arrangements for thermionic amplifiers
IPC: H03G3/22
Filed: 1945-05-17
Abstract: 592,555. Automatic volume control. STANDARD TELEPHONES & CABLES, Ltd., and JACOBSEN, B. B. May 17, 1945, No. 12475. Drawings to Specification. [Class 40 (iv)] [Also in Group XL] An automatic gain control system is described in which the gain of an amplifier is periodically increased and decreased under the control of a pilot current so that the level of the output pilot current oscillates between two predetermined limits sufficiently close for the level changes not to be objectionable at the receiving end. A portion of the pilot current output is shunted off through a band-pass filter, rectified and applied to a two-condition device controlling a gain adjusting means in the amplifier which has a definite time constant. The twocondition device causes the gain to rise until the desired maximum output level is reached, whereupon it changes over with a resulting decrease in gain until the desired minimum level is reached. The process is then repeated and the gain hunts between these two limits as long as the input pilot level remains constant. If the input level is decreasing, the output level will take longer to reach the upper limit so that the gain rises by a larger amount than that by which it falls in each cycle, the limits being independent of the input level and determined solely by the two-condition device. This principle may also be applied to A.G.C. arrangements in radio receiving sets, where the control is by the signals themselves, in which case the band-pass filter is replaced by a circuit which passes the whole of the frequency band of the signals. According to the invention, the output level fluctuation range is reduced by superimposing a periodic wave on the rectified control current which has the effect of varying periodically the rectified voltage necessary to operate the two-condition device. A similar result is obtainable by modulating the pilot current (where such is employed) with this wave at its source. The two-condition device may comprise a thermionic valve arranged as a simple oscillator, the potential of the control grid of which is varied by the rectified pilot voltage applied thereto, such that a fall in output level reduces the grid potential and stops oscillation. The output of the oscillator is rectified and applied to a thermistor which comprises the shunt element of a T-network in the input circuit of the amplifier so that cessation of oscillation increases the gain until oscillation recommences, whereupon the gain is decreased and the cycle is repeated. Alternatively, the thermistor may be the series element in a II-network in which case the sign of the rectified voltage is reversed, or a rectifier bridge may be employed as the shunt element of a T-network, being biassed by the rectified oscillations. The loss networks may be included in a negative feed-back path of the amplifier instead of in its input circuit, or if the amplifier comprises a variable mu valve, the output of the two-condition device may provide variable negative bias for the control grid. Alternative forms of two-condition device described include a gas-filled valve whose grid potential is varied by the rectified pilot current, a thermistor in its anode circuit being heated when the valve fires; and an ordinary mechanical relay operated under control of the rectified pilot current to open and close the heating circuit of a thermistor. The system may also be applied to equalization adjustment, the loss networks containing the thermistors being designed as variable equalizers, so that by the use of two pilot waves gain control and equalization adjustment may be simultaneously effected. Specification 541,922, [Group XL], is referred to.