OPTICAL DEMODULATION CIRCUIT

PURPOSE:To prevent the frequency characteristic of a video signal attended with a negative feedback circuit from fluctuation by not using a switch for changeover the negative feedback quantity in said circuit, but by switching a post-stage preamplifier to prevent an excess input. CONSTITUTION:When weak light is received, the output of a preamplifier A1 enters a 2nd preamplifier A2, in which the signal is amplified, and the output of the preamplifier A2 enters an automatic gain control amplifier A3 via a contact (a), the terminal COM of a negative feedback changeover switch SW2. When strong light is received, the negative feedback changeover switch SW2 is switched and the output of the preamplifier A1 enters the amplifier A3 directly via a contact (b) and the terminal COM of the negative feedback changeover switch SW2 without amplification, thereby evading an excess input. In such a case, a contact of a negative feedback changeover switch SW3 interlocking with the switch SW2 is opened to interrupt the power supply for the preamplifier A2. Thus, the fluctuation of the frequency characteristic of the video signal attended with the changeover of the negative feedback circuit.