AUTOMATIC GAIN ADJUSTING CIRCUIT

摘要

An automatic gain adjusting circuit (5a) includes a peak detecting circuit (10) detecting the peak voltage of output signals from a gain varying circuit (3), an average value detection/output amplitude setting circuit (11) detecting the average value voltage of the output signals from the gain varying circuit (3) and adding voltage of 1/2 the desired output amplitude of the gain varying circuit (3) to the average value voltage, and a high-gain amplifier (12) amplifying the difference between the output voltage of the peak detecting circuit (10) and the output voltage of the average value detection/output amplitude setting circuit (11) and controlling the gain of the gain varying circuit (3) using the amplitude results as gain control signals. The peak detecting circuit (10) is composed of transistors (Q1, Q2, Q3), a current source (I1), and a filter circuit. The filter circuit is composed of resistance (Ra) and capacitance (C1) connected in series.