GB1516027A - Automatic signal level control circuit - Google Patents

Abstract

1516027 Automatic signal level control circuit GRUNDIG EMV 24 Sept 1976 [26 Sept 1975] 39839/76 Heading H4R An automatic signal level control circuit of the type shown in Fig. I in which a threshold circuit is supplied with the rectified output voltage of the control amplifier, the threshold circuit output feeding a capacitive store C whose voltage is used to control the control amplifier via setter D, has means for reducing the effect of interference pulses on the voltage stored in C comprising a store control circuit G, (branches I-IV Fig. 2). In the absence of circuit G an interfering pulse entering at F will pass through a diode at A causing a positive pulse on base of transistor 10 and a negative pulse on collector 10a, which passes via diode 12 to the storage capacitor 14, thereby reducing the stored voltage and causing a change in the control amplifier gain. With the circuit G, the negative pulse on 10a reduces the voltage on capacitor 17 via diode 24. Base drive for transistor 23 is via diodes 25 and 26, and for an interfering pulse of small magnitude, the reduction of voltage on capacitor 17 is insufficient to reverse bias diode 25, and thus the contribution to the base current oftransistor 23 via diode 25 is reduced in accordance with the magnitude of the interfering pulse, causing an increase in voltage on the collector of transistor 23, which results in a positive pulse being fed via diode 29 to capacitor 14 thereby tending to neutralize the negative interfering pulse. For large interfering pulses, diode 25 becomes reversed biased and so the base current to transistor 23 is limited to that flowing through diode 26 but does not drop below that value. Thus in this case the interfering pulse is only partly neutralized and the voltage on capacitor 14 can change and adjust the gain.