| 摘要 |
PURPOSE:To limit only the DC voltage for a base and send out a superposed AC signal (modulated signal) without distortion, by inputting the drop voltage due to a resistor for current monitoring in a differential amplifier, and connecting the output with the base of an output transistor. CONSTITUTION:When the collector current of a transistor (Tr) 3 increases, the voltage drop of a resistor 10 increases, a Tr15 turns off, a Tr16 turns on, and the potential of a node point Va decreased. That is, when an input voltage VIN increases, the voltage of the node point Va increases, but the Tr16 turns on, so that the node point voltage decreases and stabilizes at a specified voltage. Hence the node point voltage is limited. When an Ac signal is inputted in this state, the point contact voltage of the resistor 10 and a laser diode(LD) 12 becomes an AC voltage, but it is smoothed by a mean value detecting circuit (the resistor 19 and a capacitor 23), and a DC offset voltage of an AC voltage is applied to the base of a Tr13. Hence the AC signal is outputted at the node point Va, and finally, an AC current flows through the LD12. Thereby a modulated signal can be sent out without distortion. |
