Understanding the WB5LUA GaAsFET Bias circuit

and using it for PHEMT preamps

or GaAsFET power amplifiers


Back in 1989, WB5LUA described1 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 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.

Circuit Description

The circuit shown below is the implementation I used for a two-stage2 10 GHz amplifier.

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 must determine the desired bias point. A good starting point is the data sheet NF specification, which usually includes the operating voltage and current for NF measurement. For instance, an MGF-1302 is measured at 3 volts and 10 ma.

The bias point is set by the resistors in the bias circuit: R4 sets the drain current, while R5 and R6 determine the drain voltage. The 2N2907 transistor (or almost any small PNP transistor) acts as a feedback amplifier which adjusts the gate voltage to maintain the desired drain voltage and current as determined by R4, R5, and R6. Values for these three resistors to set a particular Vdrain and Idrain may be calculated as follows:

R4 = (VDD - Vdrain) / Idrain

R5 = R6*(VDD / (Vdrain - 0.65)) - R6

The easiest way to solve the second equation is to plug in an arbitrary value for R6, say 1K ohms, and solve for R5. Then we may change the values of R5 and R6 so that they are both standard values as long as the ratio of R5 to R6 does not change, and adequate current, perhaps one milliamp, flows through R5 and R6. (Note: 0.65 is the approximate emitter-base voltage for the 2N2907).

The gate current in a preamp should be zero, so R7 and R8 may be large; 10K is a convenient value. Power amplifiers may draw gate current, and amplifiers with a stabilizing resistor from gate to ground also require current. In these cases, R7 and R8 should be significantly smaller so that this current does not upset the gate bias.


A simple example may help. Suppose that we wish to operate a GaAsFET preamp at 3 volts and 18 mA, with VDD = 5V and VSS = -5V. Then:

R4 = (5 - 3) / 0.018 = 111 ohms [110 is a standard value]

if R6 = 1000 ohms, then

R5 = 1000 * (5 / (3 - 0.65)) - 1000 = 1127 ohms [ not standard]

however, if we keep the 1127/1000 ratio, we can find very close standard values with R5 = 2.7K and R6 = 2.4K.


These calculations can get tedious, particularly trying to find standard resistor values. To make it easier, I made up a small Excel spreadsheet to do the calculations (hold down the shift key while clicking here to download), and entered three different examples as starting points: a GaAsFET preamp, a PHEMT preamp, and a GaAsFET power amp. With the spreadsheet, it is easy to fiddle things for the desired bias point with standard or available resistor values. A small adjustment in VDD is useful in adjusting R4, so R10 and R11 are included for setting VDD by changing R11.

Power Amplifiers

Use of an active bias circuit GaAsFET (or IMFET) power amplifiers can help stabilize operation and prevent disasters. Operation is the same as for preamps, but voltage and current levels can be much higher, so power dissipation in the components, particularly R4 and U1, is a consideration. Make sure that power ratings and heatsinking are adequate.

Safety Shutdown

Most preamps operate at low enough voltage and current so that loss of the negative bias voltage will not cause damage, but the noise figure will increase. However, power amps will quickly draw excess current without gate bias. To prevent failure, a safety shutdown borrowed from K6UQH3 may be included in the bias circuit. As seen in the schematic, this is a 2N2222 or other small NPN transistor which forces the LM317 regulator to its minimum output voltage (about 1.2 volts) if negative voltage is lost; at 1.2 volts, the power dissipated in the GaAsFET should be reduced enough to prevent damage. The shutdown circuit senses the negative voltage through zener diode D2, which should have the same voltage rating as D1.

Tuning Adjustments

I feel that potentiometer adjustments cause more trouble than they are worth, unless you are careful to limit the adjustment range, and to wire them so that intermittent contact doesn't cause a catastrophe. My preference is to install fixed resistors based on my initial calculations, then use clipleads to add large resistors in parallel with R5 or R6 to make small changes in total resistance (adding ten times as much resistance in parallel reduces the total resistance by roughly 10%). Optimization is a bit slower this way, but noise figure meters respond slowly anyway, and I rarely let the smoke out of a GaAsFET. If you use three pots, you'll probably blow up some GaAsFETS! Before making these changes, try them in the spreadsheet and see how they change the bias point. With power FETs, try to adjust for maximum power with minimum voltage and current; excessive current or voltage may have unpleasant consequences.


The fastest way to destroy a GaAsFET or PHEMT is to apply excessive voltage. Be sure that the VDD and VSS supplied to the active bias circuit do not exceed its ratings, and that the SOURCE terminal of the bias circuit is connected to the RF circuit ground. If you are using a circuit with a built-in negative voltage generator chip, such as a 7660, remember that the negative output voltage has the same magnitude as the positive input voltage. Finally, idiot diodes to prevent application of reverse voltage never hurt. The schematic above only shows the capacitors necessary to keep the three-terminal regulator from oscillating. Normal RF bypassing and ferrite beads on the wires is are assumed.


An active bias circuit will provide safe, stable operation even with varying temperature and supply voltage, with a total part cost less than any GaAsFET worth using.


  1. Al Ward, WB5LUA, "Simple Low-Noise Microwave Preamplifiers," QST, May 1989, pp. 31-36.
  2. Building Blocks for a 10 GHz Transverter (N1BWT), Proceedings of the 1993 (19th) Eastern VHF/UHF Conference, ARRL, 1993.
  3. Bill Troetschel, K6UQH, "Dual Power Supplies for Microwave GaAsFETs," Proceedings of the 37th Annual West Coast VHF/UHF Conference, ARRL, 1992, p41-51.

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