| 摘要 |
PURPOSE:To prevent a level of a DC component in an output signal and an input impedance from being changed even if the gain of a transistor (TR) is changed by applying an input signal to a base of the TR connected whose emitter current is constant at all times. CONSTITUTION:A current flowing to resistors R3, R4 by diodes D1, D2 changes by a value in response to a control voltage Vc. The DC component of a current flowing to the resistors R3, R4 is not changed but constant even when the base voltage of TRs Q3-Q6 changes in response to the change in the voltage Vc. Since the collector current of TRs Q1, Q2 is unchanged even when the voltage Vc is change, the impedance is also unchanged. Thus, even when an output of a filter is applied directly to the variable gain amplifier, an excellent selectiv ity is obtained. |
