Active Bias Networks for FETs

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Biasing a voltage-controlled-current-source active device (MESFET or HEMT amplifiers) in a module should arguably be done so that the final assembler or end-user don't have to think about it. The poor man's bias network is a voltage divider using fixed resistor, strapped across a regulated supply such as -5V, but this will require setting one resistor value at test.  One step up from that is using a miniature potentiometer, set with a screwdriver. In both cases you might want to consider creating a network that is only adjustable over a small range, so that it is idiot-proof during adjustment.  Potentiometers may have long-term stability problems and may be susceptible to vibration.

Active bias networks are an elegant solution, but they have some drawbacks  For pulsed amplifiers, be sure to think about settling time. For GaN devices, make sure your controller can handle whatever drain voltage you need (these days that could mean 48 volts!).

There are perhaps three levels of active bias networks. We will review these on this page, and perhaps expand the content in the future.

• Make it out of discrete parts
• Buy a ready-made analog solution
• Buy a digital solution with a micro-controller

Grow-your-own active bias network

Growing your own active bias network is legal in all fifty states.  You can employ discrete transistors, or op-amps, or a little bit of everything.

Here's someone else's description of an active bias network (it's pretty good), contributed by Fred L.

Here's an op-amp circuit you can use. In this case, R1234 is the sense resistor that sets the drain current to whatever is connected to VD_OUT0. Because the op-amp (ADA4092 is strapped across positive and negative supplies, the output could rail to either extreme. Three diodes are configured across the gate output VG0 to restrict the voltage between 01.3 and +0.3V. note that if the amplifier drain is not hooked up, the gate will peg at +0.3V (which should not harm it). Note that this circuit, and most of the others on this page, could be used for enhancement-mode devices with maybe a little tweaking.

Integrated circuit active bias controllers

One of the original bias controller networks was the HBC-1 from Phase IV Components LLC of Huntsville Alabama. They seem to have disappeared, after they got bought out in 2004.

Analog Devices has three options you should be aware of:  HMC920, HMC980 and HMC981 They have a lot of features in common, you can compare them on this app note, or download all three data sheets. Or you can study this table we borrowed from that app note:

HMC920

Below is a description of the HMC920 that someone sent us a while back. Apologies for losing a name we could credit this to!

The Analog Devices HMC920 is an interesting part for active bias application to set operating point of an amplifier MMIC.   Let’s take a look at its features.

• Single Positive Voltage Supply (5V to 16.5V) – convenient as it produces negative voltages needed.  May be useful for only positive rail supplies.
• Adjustable Drain Voltage 3V to 15V – if rail voltage is used for drain bias, then not an advantage.
• Adjustable Drain Current up to 500mA – typical of active bias circuit setpoint.
• Internal Negative Voltage Generator – advantageous if there isn’t a negative voltage rail.
• Active Bias Current Regulation – what it’s all about
• Stable Bias Current Over Temperature and Process Variation and Aging – the reason to use active bias topology, which is a closed-loop circuit solution.
• Short Circuit Protection (Power Foldback) – nice feature.  Could be added to discrete active bias solution.
• Adjustable Over/Under-Current Alarm – nice feature.  Need monitoring.  Typically, the drain current should be monitored directly, so knowledge of the magnitude of the current could be another means to know over/under currents.
• Adjustable Low Vdd Voltage Alarm – this is UVLO.  The voltage rail that powers the part can be monitored to determine OVLO/UVLO.
• Automated Power-up Sequencing – this is nice at the MMIC level.  Typically, the rail voltages for a system are evaluated and controlled in regards to voltage sequencing.  If this is a one part application, then this single IC has a lot of functionality.  If there are other amplifiers, then some of these features are realized at the system level.
• Enable Input for Active Bias Control – can enable/disable active bias operation while protecting MMIC.  Puts Vd=0 and Vg=max negative.
• Trigger-out Output for Daisy Chain Power-Up and Power-Down Sequencing – for using more of these IC parts.

This IC part is useful for silicon, GaAs, or InP amplifiers.  Is cannot be used for GaN, unless drain voltage is backed off to 15V.  The max gate current available is +/-4mA, pointing more to LNA, gain block, and driver type of amplifier.  The application circuits show two potentiometers.  I’d hope that fixed resistors for a specific application could be inserted in their place.  With the IC part, the supporting cast of parts look to take up a significant area of circuit board.

HMC980

Here we offer a schematic of this part in use, and a circuit layout. R2 and R3 are what sets the drain current, following this relation:

ID=150/Rsense (units are amperes). In this case the target was 480 milli-amps. The negative charge pump requires external diodes D1 and D2 and capacitors C9 and C7.

Digitally controlled bias networks

This is the most versatile bias network of all, but you will need a micro-controller and some knowledge of firmware to pull it off. It is beyond our skill set!

A good example is Texas Instruments AMC7834. Here's a link to its data sheet for now, we'll try to add something more descriptive in the future.

https://www.ti.com/product/AMC7834