Telecoms, Datacoms, Wireless, IoT


Basestation RF power amplifier biasing

3 November 2004 Telecoms, Datacoms, Wireless, IoT

Power amplifiers used in basestations require biasing for proper RF performance. This article explains the two classes of biasing that are prevalent in the RF industry, analyses their characteristics, and shows implementations with existing ICs.

The power device of choice for basestation amplifiers today is the lateral DMOS (LDMOS) MOSFET, and is used in this article to illustrate biasing techniques. However, as future generations of devices become available, such as GaN FETs, HFETs, or SiC devices, they too will benefit from the following implementations.

RF classes and biasing

LDMOS amplifiers used in RF circuits exhibit varying degrees of nonlinearity, depending on the DC-bias level upon which the input RF waveshape rides. That is, while maintaining a constant RF gating signal, the output current's (Iout) harmonic content varies as the DC bias at the gate of an LDMOS device (Figure 1) changes. The harmonic content of the LDMOS amplifier's current is important because, in the RF load, it creates power interference with the local bandwidth (in-band interference) or with adjacent bandwidths (out-of-band interference).

Figure 1. LDMOS device gating is shown with an uncontrolled DC bias
Figure 1. LDMOS device gating is shown with an uncontrolled DC bias

The best linearity occurs when the output current tracks the input voltage - a 360° conduction angle. Operating the MOSFET in this manner (ie, class-A operation) creates less distortion than when biasing it in any other way. From a power-dissipation perspective, however, class-A operation is least desirable because it consumes the most DC current.

At high RF power, given a nominal power-supply voltage of 28 V, the DC power dissipated in the amplifier is prohibitive. For this reason, RF engineers use class-AB biasing in the last stage of an amplifier chain, while they favour class-A operation in the preceding stages where power dissipation is smaller by orders of magnitude. In class-AB stages, the output current does not track the input voltage entirely, and thus the amplifier's conduction angle is lower than 360°.

Distortion of the RF signal in class AB is more significant than in class A. The spectrum of this distortion is wider and more densely populated than that of class A. However class-AB power dissipation is lower because the average current into the amplifier is lower. In short, the basis for choosing a given class of commercial RF amplifiers is a tradeoff between linearity and efficiency.

Biasing requirements and LDMOS behaviour

Biasing requires managing the DC content in the LDMOS current across temperature and supply variation. The ultimate objective is to ensure that the amplifier RF gain, as well as its distortion levels, varies within limits consistent with requirements. In this respect, proper biasing can assist linearisation techniques to minimise distortion.

The equation governing LDMOS's gain is Iout = K (Vgs - Vth)², where K is a constant reflecting gain due to electron mobility and Vth is the FET's threshold. Both K and Vth are temperature dependent. In Figure 2, LDMOS characteristics are shown across temperature. In class AB, designers tend to operate the bias to the left of the crossover region where the gain has a positive temperature coefficient. In class A, operation occurs to the right of the crossover region.

Figure 2. LDMOS characteristics are shown across temperature
Figure 2. LDMOS characteristics are shown across temperature

Controlling A and AB bias with the DS1847

Figure 3 shows a DS1847 dual, temperature-controlled variable resistor controlling the gate of an LDMOS amplifier. The DS1847's internal temperature sensor provides a temperature reading to its look-up tables. These look-up tables adjust the IC's two 256-position variable resistors so the amplifier's gate receives the proper bias voltage. The user programs the look-up tables to generate a constant LDMOS-amplifier output current. Refer to Figure 2 (or to manufacturer-specific data curves) for LDMOS characteristics. By using the two resistors to attenuate the reference voltage, a temperature-insensitive voltage is maintained.

Figure 3. DS1847 dual, temperature-controlled variable resistor controls the gate of an LDMOS amplifier
Figure 3. DS1847 dual, temperature-controlled variable resistor controls the gate of an LDMOS amplifier



Credit(s)



Share this article:
Share via emailShare via LinkedInPrint this page

Further reading:

X-band radar
RF Design Editor's Choice Telecoms, Datacoms, Wireless, IoT
X-band radar systems, particularly those leveraging beamforming ICs (BFICs), advanced gallium nitride (GaN) and gallium arsenide (GaAs) components, are leading the way in providing the high-performance radar capabilities required for modern defence and surveillance.

Read more...
Reference board for cardio monitoring
Altron Arrow Telecoms, Datacoms, Wireless, IoT
The STDES-ESP01 reference board from STMicroelectronics demonstrates the capability of the ST1VAFE6AX and ST1VAFE3BX biosensors to detect ECG and SCG signals.

Read more...
LTE Cat 1 bis communication
iCorp Technologies Telecoms, Datacoms, Wireless, IoT
The EG810M series is a series of LTE Cat 1 bis wireless communication modules specially designed by Quectel for M2M and IoT applications.

Read more...
Quad-channel 16-bit converter
RFiber Solutions Telecoms, Datacoms, Wireless, IoT
The ARF0468 from Advance RF is a quad-channel mixed-signal processing chip, with each channel comprising three major functional modules: ADC/DDC/DDS.

Read more...
Tactical navigation system
Etion Create Telecoms, Datacoms, Wireless, IoT
Etion Create’s CheetahNAV Compact is a versatile tactical navigation system designed for security services, emergency services, and light all-terrain vehicles (ATVs) using offline navigation maps.

Read more...
Smart module for multi-media devices
iCorp Technologies Telecoms, Datacoms, Wireless, IoT
Powered by a Qualcomm processor, Quectel’s new SC200V is designed to deliver exceptional performance across system capabilities, multimedia functions, and network connectivity.

Read more...
Remote provisioning firmware added to SIMCom modules
Otto Wireless Solutions Telecoms, Datacoms, Wireless, IoT
SIMCom recently announced that its range of Cat 1 bis IoT modules are now being prepared with the firmware necessary to support SGP.32 functionality.

Read more...
GNSS antenna redefining what’s possible
RF Design Telecoms, Datacoms, Wireless, IoT
u-blox has achieved what was once thought impossible with the launch of the DAN-F10N, the industry’s smallest and most reliable L1, L5 dual-band GNSS antenna module.

Read more...
Innovative satellite navigation receiver
Altron Arrow Telecoms, Datacoms, Wireless, IoT
STMicroelectronics has released an innovative satellite navigation receiver to democratise precise positioning for automotive and industrial applications.

Read more...
u-blox expands NORA-B2 BLE modules
RF Design Telecoms, Datacoms, Wireless, IoT
The new nRF54L chipset-based wireless modules reduce current consumption and double processing capacity, catering to diverse mass market segments.

Read more...