| 摘要 |
PURPOSE:To obtain an automatic gain control circuit in which no TR is used by setting the envelope signal of a carrier and a desired level value digitally, making a comparison with a D/A-converted signal, and controlling an up/down counter. CONSTITUTION:When an input signal to a carrier input terminal 1 rises in level, the output level of an amplifier 3 and the DC component of the envelope signal of the output of a filter 22 also rise above the set value of a D/A converter 24 which is set by a level setter 23 and inputted to a comparator 25. The comparator 25 sends its output signal as a count-down command to the up/down counter 26. The gain switching operation of the feedback resistance network 28 of the amplifier 3 is performed automatically by the count-down operation so that the gain decreases, namely, the impedance decreases until the DC component of the envelope signal coincides with a set analog value, holding an output terminal stably at the set level value. When the input signal drops in level, the same operation is carried out. |
