Re: GaAs FET
bouchy,
Self bias refers to any biasing scheme where an external source of bias voltage (or current) is not required.
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In a FET amplifier, this is accomplished by using a resistor between the source and ground (common). Consider an N-channel FET, either JFET or MOSFET. AsS the drain current increases, the voltage across the source resistor increases, thus reducing the Gate-Source voltage, which tends to couneract the effect of the drain current increase. This is negative feedback.
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Consider a bipolar (NPN in my example) amplifier with a collector resistor. Connect another resistor (the bias resistor) between the collector and the base. Now, as th collector current increases, the collector voltage decreases, resulting in a lower voltage across the bias resistor, and, therefore lower base current. The lower base current couneracts the increase in collector current. Again, this is negative feedback.
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Regards,
Kral