Why not just use a resistive voltage divider for bias?

A resistive divider sets VGS or VBE from the supply voltage, but does not compensate for temperature. A GaAs pHEMT's threshold voltage drifts by about minus 1 to minus 2 mV per degree C, causing quiescent current to increase 10 to 30 percent over a minus 40 to plus 85 degree C range. For a Class AB PA, this shifts the bias toward Class A at high temperature (wasting power and possibly exceeding thermal limits) and toward Class B at low temperature (degrading linearity). An active bias circuit with a current mirror or negative feedback loop holds Idq within plus or minus 5 percent over the same range.

How does a current mirror bias circuit work for RF?

A reference transistor (diode-connected or with a voltage reference) establishes a stable reference current through a precision resistor. A mirror transistor, matched to the RF device, replicates this current and drives the RF transistor's gate or base. Because both the reference and mirror transistors experience the same temperature shift, the VGS tracking cancels the temperature drift. The mirror ratio sets Idq independently of supply voltage. For best tracking, the reference transistor should be on the same die or in the same package as the RF transistor.

What is special about GaN HEMT biasing?

GaN HEMTs are depletion-mode devices with a negative threshold voltage (typically minus 2 to minus 3 V). They require a negative gate supply, which is usually generated by a charge pump or voltage inverter from the positive drain supply. More critically, GaN devices exhibit current collapse (trap-related Idq reduction after RF stress) and require a specific power-up sequence: the negative gate voltage must be applied before the positive drain voltage, otherwise the device conducts at full Idss and can be destroyed by the inrush current.

Active Components

Active Bias

A Class AB power amplifier's linearity depends on its quiescent current sitting at a precise value: too high and it wastes power like Class A; too low and crossover distortion spikes. A simple resistive voltage divider cannot hold that current steady because the transistor's threshold voltage drifts with temperature. At +85°C, a GaAs pHEMT's V_th has shifted by 100 mV and the quiescent current has risen 25%. An active bias circuit uses a current mirror or feedback loop with a temperature-tracking reference transistor to cancel this drift, holding I_dq within ±5% from −40°C to +85°C.

Category: Active Components

I_dq Stability: ±5% over temperature

Key Challenge: V_th drift (−1 to −2 mV/°C)

When Resistors Are Not Enough

Bias Approach Comparison

Approach	I_dq Stability (−40 to +85°C)	Supply Rejection	Complexity	Best For
Resistive divider	±15 to 30%	Poor	2 resistors	Non-critical, fixed V_DD
Self-bias (source resistor)	±10 to 20%	Moderate	1 resistor	Small-signal, Class A
Current mirror	±3 to 7%	Good	Mirror FET + R_ref	MMIC integration, PA
Feedback with op-amp	±1 to 3%	Excellent	Op-amp + sense R	High-reliability, base station
Digital DAC control	±0.5 to 1%	Excellent	DAC + ADC + MCU	Adaptive bias, DPD systems

GaN-Specific Bias Challenges

Negative gate voltage: GaN HEMTs are depletion-mode (V_th = −2 to −3 V). The gate supply is typically generated by a charge pump or inverter from the positive drain supply.
Power-up sequencing: The negative V_GS must be applied before V_DS. Without this, the device conducts at full I_DSS and the inrush current can destroy it. Sequencing circuits add 2 to 5 ms of controlled ramp time.
Current collapse: Trapping effects in the GaN buffer layer cause I_dq to drop after RF stress. Active bias circuits must monitor and compensate for this drift in real time.
Self-heating: GaN devices run at junction temperatures of 150 to 200°C. The bias circuit must track this extreme temperature, not the ambient.

Common Questions

Frequently Asked Questions

Why not use a resistive divider?

A resistive divider does not compensate for V_th drift (−1 to −2 mV/°C). Over −40 to +85°C, quiescent current can shift 15 to 30%, pushing Class AB toward Class A at hot temperatures and Class B at cold. Active bias holds I_dq within ±5%.

How does a current mirror work for RF?

A reference transistor matched to the RF device establishes a stable current through a precision resistor. The mirror transistor replicates this to the RF device's gate. Both transistors track temperature equally, cancelling V_th drift. Best tracking requires the reference FET on the same die or package.

What is special about GaN biasing?

GaN HEMTs need negative gate voltage (generated by a charge pump), strict V_GS-before-V_DS sequencing (or the device self-destructs), and compensation for current collapse (trap-induced I_dq droop after RF stress).

Related Terms

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Bias Design

GaN Sequencing and Protection Reference Design

Complete schematic, BOM, and layout files for a GaN HEMT bias controller with negative voltage generation, power-up sequencing, I_dq monitoring, and over-temperature shutdown.

View Reference Design