| 摘要 |
A gain-controlled amplifier is provided having a broad range of gain control which is traversed by varying the collector impedance of a common emitter coupled amplifying transistor. The collector load impedance of the transistor includes a controlled resistance device having a base electrode coupled to the collector electrode of the amplifying transistor, an emitter electrode coupled to receive a variable gain controlling current and a collector electrode coupled to a point of reference potential. At signal frequencies, the base-to-emitter junction of the device acts as a resistance which varies inversely with the flow of gain controlling current through the collector-to-emitter path of the device. Varying the resistance of the base-to-emitter junction of the device varies the collector load impedance of the amplifying transistor, whereby the gain of the transistor is varied by changing its load line as a function of the A.C. resistivity of the device. The collector-to-emitter path of the device through which the gain controlling current flows is separate from the transistor biasing circuitry, and hence control of the device will not affect the D.C. biasing of the amplifying transistor.
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