| 摘要 |
An I.F. amplifier is provided in which gain control is accomplished by varying the A.C. impedances of variable impedance devices, which devices are coupled as load and emitter degeneration impedances for amplifying transistors. Variable D.C. gain control currents are applied to the variable impedance devices to vary their impedance. Substantially all of the D.C. gain control currents flow through the variable impedance devices in current paths which are separate from the amplifying transistors, thereby avoiding variation of the operating points of the amplifying transistors. The I.F. amplifier includes a D.C. feedback path which does not require the use of feedback amplification, due to the lack of any changes in the operating points of the amplifying stages during gain control. Elimination of the need for feedback amplification also prevents the introduction of noise into the amplified I.F. signals.
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