Attenuators not very sensitive to temperature variations whose degree of attenuation is controlled by a voltage.

摘要

989,423. Automatic volume controls. INSTITUT FRANCAIS DE PETROLE DES CARBURANTS ET LUBRIFIANTS. Aug. 25, 1961 [Aug. 25, 1960], No. 30818/61. Heading H4R. A controllable attenuation network comprises two groups of diodes G 1 , G 2 each having an even number of diodes similarly poled within groups but oppositely poled with respect to each other, the ends of the groups and the common point S of the groups being at substantially the same A.C. potential, viz. that of the point N, the input to the network being between the point N and the mid-point of the group G 1 , and the output being between the point N and the mid-point of the group G 2 . The control voltages - u/2, + u/2 superimposed on a bias voltage Ep are arranged so as to increase the current through group G 1 and to decrease the current through G 2 . Provided the characteristics of the diodes (exponential as described) are matched it is shown that the attenuation can be made independent of changes of temperature. To obtain this matching, each group can consist of 2n diodes, chosen so that any individual variation in each subgroup of n diodes cancel each other out. Various arrangements of this network connected between the emitter and base of a transistor amplifier are described. In one arrangement, Fig. 6, the whole of the control voltage u is applied to a single point D, the input is transformer coupled to the base and the output Es is taken from the collector. It is shown that the gain of the arrangement is directly proportional to Ep, inversely proportional to Eq and varies exponentially with u. In a further embodiment, Fig. 7 (not shown), two such arrangements are used between the respective emitters and collectors of two transistors in a two-stage amplifier. In a further embodiment, Fig. 8, the control potentials for the two groups of diodes are derived from a potentiometer P to provide potentials of u and (1 - m)u for G 1 and G 2 . The gain in this case is an exponential function of mu, m being adjusted to obtain a suitable factor in the exponent and u being controlled according to a programme. Thus u may be made variable as a function of the time after a seismic explosion which is being detected. In the arrangement shown in Fig. 9, two variable attenuators GA, GB and amplifiers Ao, Bo are arranged in a loop with inputs e e and output e s , the control voltages for G A and G B being equal and opposite. The gain of the system between e e and e s is shown to be proportional to an exponential function of u, but the gain around the loop in open circuit conditions is independent of u and thus the system can be kept stable. By altering the connections to the rectifiers of one of the attenuators, the same control voltage source can be used for both.