Today's market for mobile telephone systems puts tough requirements on the RF power transistors that are used in basestations. In the past, RF power amplifiers were designed to amplify single-carrier signals that transmitted the speech data of one user. AMPS and its European equivalent NMT used single-carrier power amplifiers. To increase capacity, basestation manufacturers simply used a combiner to merge multiple single-carrier amplifiers just before the antenna.
AMPS and NMT used modulation schemes (FM) that created RF signals with a constant envelope that did not put any linearity requirements on the power amplifiers. But basestation manufacturers who needed to serve the increasing capacity needs of the service providers were forced into other modulation schemes in which one RF carrier supports more than one user. For example, GSM, TDMA (IS-54/136) and CDMA have one RF carrier that supports multiple users. With the introduction of those modulation techniques, RF power amplifiers suddenly had to meet increased linearity requirements. One GSM carrier supports eight users, for instance, using a TDM method.
The RF power amplifier has to meet certain gain expansion/compression requirements. In the case of TDMA and CDMA, the RF carrier does not have a constant envelope anymore. An RF power amplifier amplifying a non-constant envelope signal introduces non-linearities because of amplitude and phase-modulation distortion introduced by the semiconductors used in those power amplifiers. However, because of the increasing popularity of wireless communication, amplitude and phase-modulation techniques still can't keep up with capacity limitations. Basestation manufacturers had to consider the use of multicarrier power amplifiers. Those devices amplify multiple carriers supporting multiple users per carrier.
While feedforward and predistortion techniques can improve amplifier linearity to meet the tougher system requirements, a great burden is put on the RF power transistors used in those amplifiers. First of all, the type of power transistor being used determines the linear performance of an amplifier. Power-amplifier designers can choose between bipolar junction transistor (BJT), lateral diffused MOS (LDMOS) and GaAs FETs.
All have advantages and disadvantages, but LDMOS has numerous benefits when it comes to linear basestation amplifiers. Its excellent linear behavior in Class AB operation makes it possible to build a linear amplifier with high gain and good efficiency at reasonable cost. On top of this, LDMOS devices have high peak-power capability, which makes them suitable for systems with high crest factors like CDMA and W-CDMA multicarrier amplifiers. While current drift with elevated temperatures is still a concern, new generations of the LDMOS technology will soon minimize that issue. The three major RF semiconductor suppliers-Philips Semiconductors, Motorola and Ericsson Components-have introduced a full range of LDMOS transistors to cover any application in the 800-MHz to 2.2-GHz cellular bands to complement their bipolar and vertical DMOS technologies.
When designing the RF power amplifiers for basestation applications, multiple parameters must be considered. Design parameters, set by the service providers, can include more functionality, higher integration and reliability. They should also include size reduction, to enhance efficiency and minimize costs.
Service providers are not only looking for transmission of speech signals. They are also driven by subscribers to provide data communication as well as support for such elements as the Internet and e-mail access via the combination of a handheld computer and cell phone. Video-data transmission is also a consideration. Those features require higher data rates, which can only be achieved by signals that use more bandwidth in the frequency spectrum in combination with intelligent signal-compression techniques.
When they design power amplifiers, manufacturers are also forced to increase the level of integration, which is mainly driven by the requirement to create small amplifier and basestation concepts. Service providers face community resistance to large basestation towers, and in densely populated areas there is simply no room for them. Unfortunately, the capacity requirements in such areas force service providers to use multiple smaller basestation or micro/picocells. In some cases, service providers have resorted to renting apartments in high-rise buildings just to erect small basestations.
To tackle size reduction, the three main suppliers have introduced integrated hybrid power-amplifier concepts. Nonlinear and linear power-amplifier modules are available in power levels up to about 20 to 30 W. The modules are most often used in the driver or predriver stages of an amplifier lineup and often have a 50-ohms input and output impedance.
Semiconductor suppliers have become an integral part of the design process for basestation amplifiers. When a module's output power levels are not sufficient, one can use discrete RF power transistors to boost them. Today, almost every RF power transistor that is used in the cellular frequency bands between 800 MHz and 2.2 GHz includes some form of integration to improve performance parameters such as power gain and efficiency. An integrated matching at both the input and output of a discrete transistor will make the device easier to use for RF designers. At the same time, it will increase power gain and efficiency because of the reduction of circuit losses and other effects inside the transistor.
Field replacement is expensive for service providers, so power amplifiers have to be reliable. Among the various components used in basestation RF power amplifiers, RF power transistors are the components that fail the most. The reliability of the RF power transistor is therefore a key factor. LDMOS transistors show no thermal runaway, which makes them generally more rugged than bipolar transistors. A smart die layout and the right packaging concepts will keep the operating junction temperatures below 150 degrees C . Layout and packaging, in combination with gold top metallization and gold bonding wires (a long-proven concept for RF power semiconductors), are considered critical to ensuring long-term reliability of the device and thus of the RF power amplifier and basestation.
Size reduction is achieved with hybrid amplifier modules and integration of matching components in discrete RF power transistors. That also serves to decrease complexity and design time, thus allowing RF designers to come up with cheaper amplifier concepts.
LDMOS technology is seen as having a bright future in amplifiers for linear basestations.