Power Amplifier Tackles WiMAX

April 16, 2009
This first-in-a-series of pallet power amplifiers employs GaN transistors to achieve the best efficiency and linearity trade-offs at WiMAX frequencies from 2500 to 2700 MHz.

Digital modulation formats place great demands on a communications system's transmit power amplifier. Systems such as WiMAX and LTE employ signals with high peak-to-average (PAR) power ratios, requiring power amplifiers with low error-vector magnitude (EVM) and good linearity. Fortunately, Micro- Precision Technologies, Inc. has leveraged gallium nitride (GaN) device technology into a line of high-performance power amplifiers for such demanding applications. The first model in the firm's new line is the MPT2527PA30, with 30-W output power from 2500 to 2700 MHz.

Model MPT2527PA30 (See figure) is self-contained, designed to be dropped into a WiMAX, MMDS, or BRS system operating from 2500 to 2700 MHz and provide maximum efficiency and linearity with high PAR signals. The amplifier achieves about 27 dB gain with 30-percent typical efficiency with WiMAX or LTE signals. It measures just 2.00 x 3.48 x 0.65 in. and exhibits 10-dB typical input return loss and 15-dB typical output return loss.

The amplifier operates with or without digital predistortion (DPD) to provide maximum efficiency or linearity, respectively. The model MPT2527PA30 is designed to provide high performance in both cases, when operating in linear fashion without DPD or when running with additional digital-signalprocessing (DSP) circuitry when using DPD to boost efficiency.

The MPT2527PA30 amplifier employs GaN transistors, inherently low memory devices that perform well when used in conjunction with DPD. Due in part to their relatively high input and output impedances, they support the design of conventional Class AB amplifiers, such as the MPT2527PA30, with efficiencies similar to Doherty silicon LDMOS amplifier designs, but with much greater bandwidths.

Model MPT2527PA30 is a twostage GaN pallet amplifier with about 30 W output power when boosting WiMAX or LTE signals. It features a single-ended Class AB input stage and quadrature-balanced Class AB output stage. With a simple nonmemory-based DPD algorithm, the EVM is easily held to less than 2.5 percent at this power output, with efficiency of around 25 percent. This efficiency is for the entire amplifier including the input and output stages, all bias generation, and other functions such as temperature compensation and bias sequencing. It requires no external bypassing, filtering, temperature compensation, or DC blocking capacitors. It is assembled on a copper carrier and designed for a microstrip interface, although it can be fitted with SMA connectors.

Other than the RF input and output ports, the only connectors are for +28 to +32 VDC power and a logic-level enable signal (+5 VDC) to pinch off the power transistors during the receive interval of time-division-duplex (TDD) systems. When using the amplifier for frequency-division-duplex (FDD) or other applications, the enable line is tied high for continuous operation.

The MPT2527PA30 is the first in a line of highly efficient pallet amplifiers designed to be a complete RF amplifier solution for various wireless applications. It is RoHS compliant and lead free. Users need only provide a heatsink capable of dissipating 100 W at a surface temperature of +85C or less as protection for the active devices. Micro-precision Technologies, Inc., 10 Manor Parkway, Salem, NH 03079; (603) 893-7600, FAX: (603) 893-9110, Internet: www.micropt.com.

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