Wireless Systems Design

Amplifiers Drive Broad Applications

These SOT-89-packaged, general-purpose gain blocks leverage reliable InGaP HBT technology for high gain with moderate output power at frequencies through 3.6 GHz.

General-purpose amplifiers (GPAs) can be thought of as the "RF glue" in wireless systems. Although they're simple in design, these amplifiers must boost RF signals to acceptable levels. They also must overcome the losses that are accrued by a system's many passive components, such as filters and connectors. In addition, GPAs must perform reliably and with low power consumption. While numerous GPA suppliers are currently available, few offer the lineage represented by Freescale Semiconductor (formerly Motorola Semiconductor). Although Freescale has long been associated with discrete power transistors, it has now entered the GPA market with a trio of InGaP heterojunction-bipolar-transistor (HBT) GPAs.

The GPAs include the MMG3001NT1, MMG3002NT1, and MMG3003NT1. Each model covers 40 MHz to 3.6 GHz and delivers as much as 20 dB gain. For the MMG3001NT1, MMG3002NT1, and MMG3003NT1 amplifiers, respectively, the output-power levels at 1-dB compression are +18.5, +21, and +24 dBm (FIG. 1). The third-order intercept points range from the following: +32 dBm for the MMG3001NT1, +37.5 dBm for the MMG3002NT1, and +40.5 dBm for the MMG3003NT1 (FIG. 2). All three devices are designed to operate in Class A small-signal applications from a single DC supply. Each device is internally matched to 50(omega). The GPAs are mounted in a SOT-89, surface-mount plastic package with lead-free leads (SEE TABLE).

Compared to competitive devices with similar power dissipation, these GPAs are designed for extremely low thermal resistance. During operation, their lower junction temperatures ensure high reliability over a long operating life. Calculations of mean time to failure (MTTF) for the MMG3003NT1 gain block, for example, factor in a collector supply voltage of +6.2 VDC and a supply current of 180 mA. These calculations reveal MTTFs in excess of 10,000 years—even when running at junction temperatures in excess of +130°C (FIG. 3).

These devices must provide very competitive performance, as this market already has entrenched competitors and established performance criteria for specific applications. The broad frequency coverage of these GPAs allows them to be used in a wide range of wireless applications. Their typical uses include predrivers or driver amplifiers, buffer amplifiers in mixer applications, and laser-modulator predrivers or drivers with signal boosters in the emerging fiber-to-the-premises (FTTP) market.

Many broad applications fall into the spectrum that is covered by these devices: wireless phones and infrastructure; broadband wireless access; Personal Handiphone Systems (PHSs); CATV, VHF, and UHF radios; RFID tags; RF remote controls; telemetry systems; meter readers; avionics and radar systems; and various industrial products. As the product line expands into additional power levels and gain options, it will essentially encompass every major wireless application including WiMAX.

Although the company is a relative latecomer to the GPA merchant market, it draws upon a strong heritage in high-power, silicon-bipolar and LDMOS devices. The firm actually ran GaAs processes internally for several decades. But the devices from those processes were captive products, which were used only by internal customers.

As the market for GPAs changed over the last several years, the opportunity to leverage this GaAs capability presented itself. The growth of cellular-based wireless applications coincided with the emergence of IEEE 802.11a/b/g wireless-local-area-network (WLAN) systems, network cards, and peripheral devices. As a result, wireless-systems integrators began to rely on simple GPAs for level matching within their multifunction designs. Going forward, the WiMAX broadband-access method promises to significantly add to the GPA market.

As Freescale recognized this potential, its customers encouraged the company to provide an alternative to current vendors. As a result, Freescale entered the merchant GPA market—initially with InGaP HBTs and later with GaAs HFETs. InGaP HBTs have inherently high gain. They operate from a single power source and are small compared to HFETs. In contrast, HFETs have lower noise figures. They also are more rugged and have higher linearity as measured by the output third-order intercept point (OIP3). Never-theless, the linearity of the initial HBT products is still quite high. Due to the higher and higher operational data rates of current and proposed wireless (and optical) systems, the optimization of this parameter was a primary design concern.

Generally, Freescale is better known for its silicon devices than its GaAs technology. As Motorola Semiconductor, however, the company was the first semiconductor manufacturer to implement industry standards for cost-effective plastic packaging for high-power RF devices (including GaAs ICs). It also was the first manufacturer to add electrostatic-discharge (ESD) protection to such devices. Today, Freescale Semiconductor remains the only supplier of plastic-packaged RF amplifiers that operate reliably at a junction temperature of +200°C while delivering as much as 100 W of RF output power. This accomplishment has made plastic-packaged RF amplifiers and devices practical alternatives to more expensive, traditional, metal-ceramic-packaged devices.

The new GPA line marks the first GaAs-based commercial product line in the company's long history. Future models in the series will have different combinations of output power, frequency coverage, and other parameters. Prices for the MMG3001NT1, MMG3002NT1, and MMG3003NT1 are around $0.88 in quantities of 8000 with delivery in 12 weeks.

Freescale Semiconductor
2100 East Elliot Rd., Tempe, AZ 85284; (800) 521-6274, www.freescale.com.

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