| 摘要 |
PURPOSE:To obtain the gain according to the exponential function of control voltage applied externally, by providing the driver circuit changing the common emitter current of two differential amplifiers in opposite phase each other and operating the difference of each output of the differential amplifiers. CONSTITUTION:The common emitter of the differential amplifiers 22, 23 is connected to the collectors of the transistors 29, 35 of the driver circuit 25, the input signal in voltage Vi of the input terminal 24 is fed to the emitters of the transistors 29, 35 in opposite phase, accordingly the collector voltages V1, V2 of the transistors 28, 33 are changed in opposite phase, and the difference of the both voltages V1, V2 is operated with the subtractor 26 and output. The collector current of the transistors 28, 33 is changed with the voltage Vc fed to the control input terminal 37 and the collector voltage is changed in phase, but it gives no effect to the output. The relation between the output voltage Vo and the gain A of the variable gain amplifying circuit 21 is obtained with equation 22, the gain A is determined with the exponential function of the control voltage Vc, and the circuit 21 can obtain greater S/N at small signal output. |
