GB947467A - Improvements in or relating to radio receiving circuits having automatic volume control - Google Patents

Abstract

947,467. Automatic gain control. PHILIPS ELECTRICAL INDUSTRIES Ltd. Sept. 12, 1962 [Sept. 15, 1961], No. 34840/62. Headings H3T and H4R. The invention relates to an automatic gain control circuit for a radio receiver, in which signals are fed from an aerial coupling circuit 1 to a bridge circuit, comprising the centretapped secondary of a transformer 3, a non- linear resistor comprising a diode 4 and a further resistor 5. This circuit feeds into a transistor 9 in a frequency-changing stage which may be self-oscillating or supplied from a separate local oscillator 10. An automatic gain control voltage is applied to terminals 6, 7 and biases the rectifier 4 towards an impedance value which tends to balance the bridge as the signal increases. Such an arrangement is subject to overloading if the signal is suddenly increased, i.e. before there is time for the gain control voltage to follow the change. To prevent this, according to the invention capacitors 12, 13 are connected across the diode and resistor 4, 5 respectively and the diode arranged to be biased to a low impedance state for small signals. A suddenly increased signal is then rectified by the diode, which is thereby selfbiased to a state of higher impedance, so tending to balance the bridge and automatically reduce the signal applied to the receiver. Capacitors 12 and 13 and blocking capacitors 15, 21 form a low impedance path for local oscillations. A modification is also described in the Specification, in which the bridge is reversed (Fig. 2, not shown), the signal being applied to the join of diode and resistor and fed out via a transformer with a centre-tapped primary. In a further modification (Fig. 3, not shown), the transformer with tapped winding is replaced by a pair of capacitors.