GB928779A - Improvements in or relating to control circuits using diodes - Google Patents

Abstract

928,779. Volume control. PHILIPS ELECTRICAL INDUSTRIES Ltd. Aug. 23, 1961 [Aug. 26, 1960], No. 30401/61. Class 40 (4). [Also in Group XL (c)] An alternating current control circuit comprises two similarly poled diodes 1, 2 providing parallel paths for a high impedance alternating current source i, control currents I 01 , I 02 being provided to vary the impedance of one or both of the diodes to vary the distribution of the alternating current between them, the diodes having an exponential voltage-current characteristic, over a range in which the current may vary a hundredfold. It is shown that in these conditions the current is divided in the two branches in the ratio:- where Is is the saturation current of the diodes and the currents are thus substantially free from distortion. In one embodiment a variable gain stage 12, Fig. 3, in an amplifier comprises an input point 3 receiving alternating current from a preceding transistor stage having a high output impedance. The alternating current divides between diode 1 and diode 2 the latter being in series with the low impedance input circuit of transistor 5. Variable D.C. potential, obtained from a manually variable potentiometer across the + and - leads, is applied via a high resistance 18 to the diode 2, the control circuit being completed over resistance 20. Thus as the control current through diode 2 increases, the increased drop of potential across resistance 20 decreases the current through diode 1 so enhancing the gain variation. In a modification, Fig. 5 (not shown), the emitter-base rectifying contact of the transistor 5 is used instead of the diode 2. In a further modification, Fig. 6 (not shown), the transistor 5 may be replaced by two transistors in cascade, the first being of the grounded emitter type but with negative feed-back from the second transistor to reduce the input impedance. In a second embodiment, Fig. 4, a balanced arrangement is provided, two pairs of diodes 31, 32 and 33, 34 being provided. In this case the control current c which is obtained from the rectified output of a subsequent stage affects only the diodes 31, 33 ; the diodes 32, 34 receiving an invariable current. The circuit may also be used for amplitude modulation of a carrier, or for frequency modulation of an oscillator, the control circuit being included together with a 90 degrees phase shift circuit in the feed-back circuit of the oscillator.