| 摘要 |
PURPOSE:To detect disability of constant-current operation by monitoring the output amplitude of a drive amplifier. CONSTITUTION:With a transistor Q2 being OFF, when the current flowing into a (-) terminal of an amplifier M through a resistor R4 and a diode D1 is made larger than that flowing into a (+) terminal thereof, the potential at the point A becomes 0 and a transistor Q1 is not conducted. With the transistor Q2 being ON, a diode D2 is not conducted and hence the output of the amplifier M is determined such that the feedback current through a resistor R2 becomes equal to the current from a resistor R1. When the voltage VS is sufficient and the constant-current operation is effected, the potential at the point A assumes VBE+R3.i. (The current amplification rate of the transistors is assumed to be sufficiently large). In the event the voltage VS is insufficient for the constant-current operation, the potential at the point A is raised up so as to substantially coincide with the source voltage Vp of the amplifier M, as indicated by a dotted line. Since both voltages Vp and VBE+R3.i can be set independently of each other, the difference value of Vp-(VBE+R3.i) can be selected to become sufficiently large. Thus, by monitoring the potential at the point A, it is possible to detect that the constant-current operation was made disabled. |
