| 摘要 |
PURPOSE:To amplify a photocurrent of large variation into desired value, by varying the constant current level ratio at the time of logarithmic compression and elongation of photocurrent. CONSTITUTION:A photo diode SPD in a logarithmic compressing circuit 11 supplies a photocurrent Ii depending on the intensity of radiated light to the collector of a transistor Q1, and the emitter-base voltage in the transistor Q1 becomes VBE1, while the emitter-base voltage of a transistor Q2 of identical characteristics in which constant current IREF is supplied to its collector becomes VBE2. The output end of an operational amplifier A1 which compares the anode and cathode voltages of the diode SPD is connected to the emitters of the transistors Q1, Q2 through resistance R1, and a voltage VB2 corresponding to the photocurrent Ii logarithmically compressed depending on the constant current IREF according to formula I is generated in the base of the transistor Q2. When this voltage VB2 is similarly elongated logarithmically depending on the constant current IREF, in an elongating circuit 12, the photocurrent Ii is amplified depending on the constant current ratio at the time of compression and elongation, so that a photocurrent of large variation may be amplified to a desired value. |
