| 摘要 |
PURPOSE:To attain a low voltage and a high S/N by using an operational amplifier to control the supply current of the lst and 2nd transistors (TR) in response to an input signal. CONSTITUTION:The operational amplifier 26 is provided in place of a bias voltage source in a conventional variable gain circuit. A collector of a TR 21 is connected to a noninverting input of the operational amplifier 26 and an inverting input 27 is connected to a bias voltage source. Then an output terminal is connected to a common base of the TRs 21,22. Both emitters of the TRs 21,22 are connected to ground. In the variable gain circuit of the constitution as above, it is required to connect a bias voltage source to the input terminal 27, since the input terminal 27 is an input terminal of the operational amplifier 26, there is no limitation of its impedance. Since a voltage applied to a resistor 15 corresponds accurately to an input signal, the voltage/current conversion is applied without any distortion by the resistor 15. Since no diode connection is employed for the TR 21, no effect of base current is caused thereby decreasing the offset. Since the input impedance is linear, the ratio of the signal current to a bias current is increased and the S/N is increased.
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