| 摘要 |
PURPOSE:To constitute an optical receiving circuit having the automatic gain control function of a high linearity, by changing the gain of an amplifier by plural switches and fixed resistances. CONSTITUTION:A photocurrent induced in a photodiode 1 is amplified by a low noise amplifier 2, and the output is taken out as the final output through an AGC amplifier 3, a buffer amplifier 6, and a pilot signal eliminating circuit 7. A part of the output of the buffer amplifier 6 is led to a signal amplitude detecting circuit, and a DC voltage proportional to the amplitude of the input signal is given to an AGC circuit 5. The AGC circuit 5 generates n-bit turn-on/off signal for gain control to turn on and off switches 130, 131-13n-1, and the resultant resistance value of resistors 120, 121-12n-1 is switched to 2 kinds of value by the combination of turning-on and off of these switches to set the amplification degree of the AGC amplifier to a prescribed value. |
