Understanding the WB5LUA GaAsFET Bias circuit

Posted on Feb 6, 2014

WB5LUA described GaAsFET preamps for several microwave bands which included an active bias circuit for the GaAsFET. Many devices have come along since then which offer better performance, but require a bias point with different current and voltages. It isn`t obvious how to modify the active bias circuit for a different bias point, so

Understanding the WB5LUA GaAsFET Bias circuit
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many folks simply resort to a potentiometer, which usually works but may drift or fail completely. Since I`ve never seen a desciption of how the active bias circuit works, I worked it out - it`s actually pretty clever. I`ll try to describe the operation so that others can utilize this circuit, not only for GaAsFET preamps, but also PHEMT preamps and GaAsFET power amplifiers. All component references will refer to this schematic. Since it is for a two-stage amplifier, there are two active bias circuits, each with a 2N2907 transistor and five resistors, R4 thru R8, which set the bias point of each stage independently. Everything in the bias circuit is referenced to the input voltage VDD, so it is important that this is a well-regulated voltage. Normally this is provided by a three-terminal regulator, U1, which can be a 78L05 to provide 5 volts for a normal low-noise GaAsFET, or an LM317 for other voltages. The input voltages, VDD and VSS, MUST be less than the maximum voltage rating for the GaAsFET or PHEMT or you risk burnout of an expensive device. Some of the newer PHEMT devices with very low NF have a 3 volt maximum rating, so VDD must be regulated at 3 volts or less by an LM317, and VSS limited to 3 volts or less by an appropriate zener diode. Zeners may be hard to find for this low voltage, but a string of four forward-biased diodes in series would work fine; readily available 1N914 or 1N4148 diodes are fine. Next we...

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