| 摘要 |
A gain-controlled amplifier is provided, including first and second common emitter-coupled amplifying transistors for receiving differential input signals. The emitter circuits of the amplifying transistors include first and second controlled resistance devices, having base electrodes coupled to each other and to a source of gain-control potential, collector electrodes coupled to a source of reference potential, and respective emitter electrodes coupled to respective emitter electrodes of the amplifying transistors. Each of the controlled resistance devices includes a region of high resistivity (intrinsic) semiconductor material between the base and emitter electrodes, which gives the base-to-emitter junction of the device a PIN diode-like characteristic. The application of a gain-control potential to the bases of the devices may be used to vary both the small base-to-emitter current and the relatively larger emitter-to-collector current in the devices. Variation of the base-to-emitter current will vary the PIN diode junction resistances and hence the resistances in the emitter circuits of the amplifying transistors, and therefore which control the emitter degeneration of the amplifying transistors. The source of emitter-to-collector current for the devices is the D.C. emitter current of the amplifying transistors. As this current flow is varied, the D.C. biasing of the amplifying transistors is varied, which alters the transconductance of the amplifying transistors. Gain control is thus provided by the two techniques of variable transistor transconductance and variable emitter degeneration.
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