| 摘要 |
PURPOSE:To make a DC level constant even if the gain of an amplifier is changed by constituting a gain control circuit by a differential amplifier circuit, the 1st and 2nd differential pairs and a current mirror circuit so as to flow a DC component in balancing state at all times. CONSTITUTION:The differential amplifier circuit comprising transistors (TRs) Q4, Q5 is provided to the gain control circuit and a TR pair comprising TRs Q6, Q7 and TRs Q8, Q9 is connected to each of the TRs Q4, Q5. Further, a current mirror circuit comprising TRs Q10, Q11 is connected between emitters of the TRs Q7, Q8 of the TR pair, and a power supply Vcc is connected to the collector of the TRs Q6, Q9 and the emitter of the TRs Q10, Q11. Moreover, the bases of the TRs Q6, Q9 are connected in common, a voltage from a voltage source comprising TRs Q12-Q14 is applied thereto and a control voltage from a terminal P3 is applied to the common connecting point of the base of the TRs Q7, Q8. Then, the DC component is made to flow to the differential pair is balancing state at all times by the mirror circuit, allowing to make the DC level constant when the gain of the amplifier circuit is changed. |
