| 摘要 |
1533719 Automatic gain control MOTOROLA Inc 10 Aug 1976 [22 Oct 1975] 33293/76 Heading H3T An automatic gain control (A.G.C.) signal from the audio stage of a single side band radio receiver is slow acting when the audio signal is low but fast acting when the audio signal is above a predetermined level. As described, the ouput from the audio preamplifier is applied through a clipper circuit 40, 41 to a transistor 43 which amplifies the peaks. Negative peaks are removed by diode 54 and positive peaks are passed by transistor 61 to charge a capacitor 90, the voltage on which is the A.G.C. signal. When the output from the audio preamplifier exceeds about 2À2 volts, the clipped peaks are sufficient to turn on transistor 45 and capacitor 90 is charged rapidly through resistors 102, 103 in parallel. Capacitor 90 has a slow discharge path through resistors 91 and 92, but if the audio signal ceases, transistors 61 and 65 turn off and after a 0À8 sec. delay due to capacitor 82, transistor 70 turns on to discharge capacitor 90 rapidly. The delay prevents discharge in response to inter-syllable gaps in speech. The A.G.C. voltage on capacitor 90 is applied through germanium diode 110 and resistor 111 to control a fieldeffect transistor r.f. attenuator. When the A.G.C. voltage reaches a value corresponding to near the end of the attenuator range of control, transistor 93 turns on applying the A.G.C. signal through germanium diode 97 and resistor 96 to control the gain of the i.f. amplifier. At the same time, transistor 101 is turned off to remove resistor 102 from 11 connection with 103 to reduce the fast charging rate of capacitor 90. |
