| 摘要 |
A gain-controlled amplifier is provided in which emitter degeneration is varied without affecting the D.C. biasing of the amplifier. An amplifying transistor is resistively coupled in a common emitter amplifier configuration with a fixed collector load resistor. The emitter impedance of the amplifying transistor includes a controlled resistance device having a base electrode coupled to the emitter 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 emitter impedance and the degeneration of the amplifying transistor, which varies the gain of the amplifier. The collector-to-emitter path of the device is separate from the transistor biasing circuitry, and hence modulation of the device will not affect the D.C. biasing of the amplifying transistor. Under strong signal (maximum gain reduction) conditions, the controlled resistance device is turned off completely so that it will not introduce intermodulation distortion into the amplifier. The fixed collector load resistor provides a constant, high output impedance, which reduces power dissipation and signal currents flowing in the amplifier power supply. |
