| 摘要 |
PURPOSE:To balance the outputs of a gain control circuit, by providing the 3rd transistor (TR) pair in which the bases are mutually connected to the 1st and 2nd TR pairs coupled to a differential amplifying circuit. CONSTITUTION:TRs Q1, Q2, bias voltage source EB1 and constant current source IA form a differential amplifying circuit. On the other hand, the bases of TRs Q3, Q6 are mutually connected, and those of TRs Q4, Q5 are mutually connected, where a voltage controlling the gain is applied by a voltage source EC. The bases and emitters of TRs Q7, Q8 are mutually connected, then collector currents I7, I8 are respectively equal. Further, currents IR3, IR4 flowing to resistors R3, R4 are respectively expressed as equations (I), (II). Thus, the DC components of the currents IR3, IR4 are equal and constant and the AC components are equal but different from the polarity, allowing to balance the output. |
