GB991685A - Circuit arrangement for altering the level of a video signal - Google Patents

Abstract

991,685. Amplifiers. FERNSEH G.m.b.H. Aug. 29, 1963 [Sept. 1, 1962], No. 34198/63. Heading H3T. A variable attenuator circuit comprises a resistor 8 by way of which the input signal is applied to the junction of two like non-linear resistors 11, 12 connected in series across a variable resistance element, such as variable resistor 17, the ends of the parallel combination of the non-linear resistors and variable resistance element being connected through equal resistors 13, 14 to the terminals of a direct-voltage source so that by variation of the variable resistance element, the potential difference applied across the resistors 11 and 12 is varied to vary the resistance presented by said resistors to the signal applied thereto through the resistor 8 and hence the attenuation of the output signal appearing at the junction point 9. Owing to the symmetrical form of the circuit, alteration of variable resistor 17 does not alter the direct-voltage level at junction point 9 so that the input and output transistors may be directly coupled. A common-emitter coupling for the transistors 3 and 18 is decoupled by capacitor 27 to prevent spurious oscillation. The equal series resistors 13, 14 are decoupled by resistors 15, 16 respectively. In a modification, Fig. 2 (not shown), the variable resistor 17 is replaced by the emitter-collector path of a transistor whose resistance is controlled by a voltage applied to its base electrode, taken either from a potentiometer connected across the directvoltage source or from some other controlling signal source.