Smart Biasing Keeps RF Power Amplifier On Track

Despite recent improvements in the performance of RF LDMOS field-effect transistors (FETs), temperature drift and aging continue to affect the efficiency and linearization of power amplifiers using these transistors. In addition, the combination of high currents and high temperatures cause instability in these devices.

Designers of RF LDMOS-based power amplifiers implement discrete correction circuits and temperature-compensation techniques for dc bias. But these are costly and sometimes impractical and unreliable over a longer period of time.

Even the latest methods of combining voltage regulators with digital potentiometers and temperature compensation aren't alleviating device aging issues. Temperature drift can cause gate bias voltage to drift as much as 3 to 10 mV/°C, while aging can change drain current by 5% in 12 to 14 months. The problems continue to haunt basestation makers using such amplifiers in their communications infrastructure equipment.

Developers at Xicor Inc. have now addressed these issues. Their solution is a monolithic smart bias controller that regulates and controls the output power of the LDMOS RF power amplifier. The X9470 bias controller integrates all necessary analog and mixed-signal functions on a single CMOS chip to automatically control the gate bias voltage of an LDMOS power amplifier.

"Besides setting optimum gate bias values in milliseconds, it can make 0.01% accuracy adjustments to drain current variation due to gate voltage in real-time modes," says Tony Ochoa, product marketing manager for Xicor's mixed-signal group. "In essence, it solves the aging problems of RF LDMOS FETs cost-effectively. While it can be used for both continuous and discontinuous adjustment modes, it also protects the LDMOS FET against overcurrent and undercurrent conditions."

Multiple X9470s can be connected in parallel to control many FETs, as seen in single-carrier and multicarrier wireless basestation designs. "The use of multiple LDMOS FETs doesn't complicate bias control because only two pins are required to control all of the LDMOS FETs," Ochoa asserts.

Unlike open-loop methods, Xicor's smart bias controller offers an integrated closed-loop solution that permits dynamic calibration of the LDMOS' gate bias voltage (V_GS) during both manufacturing and real-time modes in the basestation.

The monolithic bias controller has all required components on-chip. These components include an instrumentation amplifier, a comparator, a 2-V digital-to-analog converter (DAC), EEPROM, temperature-compensation circuitry, overcurrent/undercurrent protection, and an I²C bus interface (Fig. 1).

The controller employs an external resistor to sense the transistor's drain current and automatically control the LDMOS' gate bias voltage. It's rated to handle LDMOS FETs using voltages of up to 35 V. A proprietary low-noise instrumentation amplifier with a high degree of accuracy was designed for this solution. The calibrated input offset for this patent-pending amplifier is only 100 µV at a 28-V supply.

The FET's drain current is converted into a voltage that's sensed by the external sense resistor and the accompanying precision instrumentation amplifier. The output of this amplifier with an external-voltage reference is fed to the inputs of a comparator.

The voltage reference is selectable with 6-bit resolution using an internal 64-tap digitally controlled potentiometer (DCP). A difference at the output of this comparator is used to increment or decrement a bias potentiometer, which provides the gate bias voltage to the external LDMOS FET. The bias potentiometer is also a DCP, offering 255 taps and 8-bit resolution.

The optimum bias voltage is stored in the registers of the on-chip EEPROM, which can be updated continuously or periodically, depending on system requirements. The on-chip digital I²C interface provides the flexibility of programming these bias points externally via the host controller.

The X9470 calibrates and dynamically sets the optimum bias voltage operating point of the external RF amplifier. Four states are involved in this setting. State 1 provides calibration or dc bias setting when no RF is present, and state 2 disables bias adjustments when RF is present. State 3 recalibrates the bias point for any drifts or temperature variations under dynamic operation, and state 4 turns off the power during the shutdown mode (Fig. 2).

Made on a 0.8-µm CMOS process, the X9470 is crafted to set the bias operating points for class A or AB amplifiers. While the drain current is constant in type A, it's always changing in class AB when RF is present. The unit can provide continuous-time readjustment during RF transmissions.

Other features include a power-saving mode, a bias-direction flag, and a built-in 160-kHz low-pass filter that eliminates all RF modulation components at low frequency. It offers separate digital and analog power-supply pins to permit better isolation of noise that may be present on digital supplies. Windows-based LabView software is available. For programming during manufacturing, the supplier offers low-level LabView drivers.

The integrated bias controller can be extended to other transistor types like GaAs FETs and bipolars. Xicor also is readying complete reference designs for GSM/EDGE and wideband-CDMA power amplifiers using Motorola's MRF9080 LDMOS FET.

Xicor Inc., 1511 Buckeye Dr., Milpitas, CA 94035; (408) 546-3345; www.xicor.com.