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Single-carrier Power Amplifiers for GSM Applications
AML Communications Inc.
As the sophistication of GSM has evolved, the demands for ever-increasing data rates have imposed new design challenges for high power, single-channel amplifiers. In order for a power amplifier to meet the stringent requirements of the GSM 05.05 specification, it must embody design techniques that support constant gain while conforming to the demands of the GSM dynamic power profile.
Prior to the most recent revisions to the GSM air interface, class C amplifiers provided a cost-effective solution to the single-channel power amplifier requirements. With the imposition of the dynamic power profile, the gain change that occurs as a function of drive power has led to major design complications, prompting a new examination of the available device technologies and circuit techniques. Research and development efforts have resulted in the introduction of a series of high power amplifiers that employ laterally diffused metal oxide semiconductor (LDMOS) devices. These products provide GSM 05.05-compliant solutions at output powers of 50, 125 and 250 W when operated in the 925 to 960 MHz GSM band. A rack-mounted shelf designed to accommodate multiple amplifiers to support cell-site sectorization is also available.
Class AB power semiconductor device operation retains much of the desired DC-to-RF efficiency, which is enhanced by the use of LDMOS devices over traditional bipolar device technology. The GSM power control profile defines the amplitude limits of the output signal as a function of time. The power profile shown in Figure 1 illustrates the GSM 05.05 standard for normal duration bursts. This amplitude vs. time envelope is difficult for most amplifiers that display a nonlinear amplitude transfer characteristic (such as class C and AB amplifiers) to achieve. These amplifiers typically require a trigger signal to indicate an upcoming pulse and data to indicate the power level so that appropriate gain correction may be implemented. This gain control circuitry must be critically damped to avoid undesirable overshoot and sufficiently accurate to meet the amplitude requirements of the specification. Ordinary analog gain control circuits usually yield unacceptable overshoot or undershoot characteristics.
Implementation of a high power amplifier circuit using LDMOS devices has produced an amplifier that meets the required amplitude vs. time requirements without the need for complex gain control circuitry. In addition, the resulting amplifier is capable of operating within wide input voltage and temperature ranges of 21 to 28 V DC and 0° to +50°C, respectively, while maintaining GSM 05.05-required performance. Figure 2 shows the amplifier's typical output at 125 W.
LDMOS Device Advantages
In the past, LDMOS devices were chosen for the power amplifiers due to a number of distinct advantages. The devices exhibit higher gain and efficiency over other high power transistors, thus lowering system cost due to fewer stages and less required power consumption. The LDMOS devices tend to be easier to match and require simpler power control circuitry. In addition, they offer improved thermal properties because LDMOS devices do not require a beryllium oxide isolation layer in the package. Finally, LDMOS intermodulation distortion performance is better than standard bipolar transistors and the devices operate from a single voltage supply, simplifying power supply designs.
The E-GSM series single-channel power amplifiers are designed to operate over the 925 to 960 MHz frequency range and are available in 50, 125 and 250 W models (models E-GSM 900-50, 900-125 and 900-250, respectively) with corresponding nominal gains of 32, 36 and 39 dB. Gain flatness is specified at ±0.5 dB (max). Figure 3 shows the typical gain of the 50 W amplifier at 25°C. The amplifiers are specifically designed to meet or exceed the GSM 05.05 requirements, including the output intermodulation requirements for GSM base station equipment. The units also feature input and output SWRs of 1.5 (max) and the output is isolator protected, enabling operation into an infinite load SWR without damage. Input and output connectors are SMA-F and N-F, respectively. Figure 4 shows the model E-GSM 900-50 amplifier module.
The E-GSM 900-50, 900-125 and 900-250 power amplifiers operate from 21 to 28 V DC and draw 6, 17 and 34 A (max), respectively. All units handle +15 dBm ±1 dB (max) input power and operate over a 0° to +50°C temperature range. Both the 50 and 125 W amplifiers are supplied in a 7.3" ¥ 2.5" ¥ 16.5" housing; the 250 W package measures 7.3" ¥ 5.0" ¥ 16.5". The mounting chassis is a 19" rack-mount configuration measuring 8.75" tall that can house up to six 50 or 125 W units, three 250 W units or any combination within that range.
The amplifiers feature fault alarms for gain, overcurrent, overtemperature (with overtemperature shutdown) and SWR. A summary fault indication warns if any alarm is active. A remote enable/disable control, front-panel enable/disable reset and on/off circuit breaker are also included. In addition, the units feature an analog output proportional to output power level. The cooling fan modules in each amplifier are field replaceable for easy maintenance.
Highly efficient 50, 125 and 250 W single-carrier power amplifiers are available that provide a mix-and-match capability for GSM base station applications and offer flexible, cost-effective deployment. The units feature LDMOS technology for high performance and reliability.
AML Communications Inc.,
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