| 摘要 |
PURPOSE:To eliminate generation of distortions and noises, by controlling each current value in response to the level of an input signal while keeping a fixed current ratio of a common current source of a pair of differential amplifiers. CONSTITUTION:A pair of differential amplifiers 2 and 4 are provided. Then I1- I2=2I in+(2IA-IS) is satisfied, where I1 and I2 are collector currents of transistors TRQ1 and Q2, IA is the current of a current source 6 and IS is the current of a common current source 1 of the TRQ1 and TRQ2 respectively. Then I1-I2= 2I in is satisfied by obtaining 2IA=IS. Thus ¦I1-I2¦ decreases when the input current Iin decreases, and therefore both IS and the current IC of a common current source 3 decrease. As currents IC and IS are controlled in the same direction, both IC/IS and a gain G=2IC/IS have no change. While the gain control voltage VC is supplied to the base of a TR3 via operational amplifiers 15 and 14 and changes the common circuit IC of the amplifier 4 and then the gain G. The output impedance of an operational amplifier 12 is extremely low, and the voltage VC is not supplied to the base of the amplifier 2. Thus it is possible to control currents IC and IS independently of each other. |
