Wireless Systems Design

Gain Amplifier Eyes 30-To-300-MHz Range

By Incorporating An ADC Driver, This Digitally Controlled Variable Gain Amplifier Has Primed Itself For Base-Station Use.

The rollout of 2.5 and 3G networks is quickly gaining momentum. North America, for example, is witnessing the ongoing build-up of GSM/GPRS cellular network systems. In addition, one major U.S. carrier is in the process of deploying that region's first commercial W-CDMA network.

These cellular-network rollouts bode well for Maxim Integrated Products. Its latest offering—the MAX2055 digitally controlled variable-gain amplifier—targets a variety of related base-station applications (See Figure). Specifically, it focuses on the applications that require high linearity and low noise for good bit-error-rate performance. The chip can be utilized with base-station receivers in the intermediate-frequency (IF) range of 30 to 300 MHz. This range targets most of the major wireless carrier networks, such as GSM, DCS/PCS, EDGE, iDEN, PHS, cdma2000, and W-CDMA.

The MAX2500 is not only limited to base-station applications, however. It is an equally good fit for broadband systems, automatic test equipment, and terrestrial links. In fact, another suitable application for this chip is the Personal Access System (PAS) market. In the early 1990s, PAS was designed to extend the reach of fixed phone services. Although it is better known in Asia, this technology allows customers to move around city streets or office blocks.

To meet the needs of this range of applications, the MAX2055 had to pack in a variety of features. For instance, one of the chip's more notable aspects is its differential output. This output has been designed to drive high-performance, high-speed analog-to-digital converters (ADCs). The device also boasts a tightly integrated, digitally controlled attenuator and a high-linearity, single-ended-to-differential output amplifier.

Thanks to the presence of these features, the need for an external transformer is eliminated. For other reasons, however, a transformer may sometimes be required. In those scenarios, the MAX2055 chip promises to improve the even-order distortion performance of the transformer-coupled circuit. This capability, in turn, effectively eliminates the commonly used anti-aliasing filter just before the ADC.

To perform impedance matching, the MAX2055 can be used to directly drive a transformer. It has a fixed 50-ohm matched output impedance. That impedance remains stable over the entire operating frequency range.

Although it has multiple options, the primary application for the MAX2055 gain amplifier is as a driver for analog-to-digital converters. For this purpose, its gain can be adjusted either dynamically or as a one-time channel-gain setting. In addition, the amplifier integrates a digital attenuator with a 23-dB selectable attenuation range and a single-ended-to-differential output amplifier. Five logic lines (B0 to B4) control the attenuator. Following common practice, external DC blocking capacitors are needed for both the input and output lines. When the control bits are set to 0 dB, the attenuator maintains a low insertion loss of about 2 dB.

The device's high gain and linearity features are achieved through the use of negative feedback in the single-ended-input-to-differential-output amplifier. The optimized operating frequency for this amplifier ranges from 30 to 300 MHz.

The chip also boasts a high-output third-order intercept point (OIP3). This intercept point represents a theoretical point on the RF input versus intermediate-frequency (IF) output curve. At this theoretical point, the desired input signal and third-order products become equal in amplitude. This equalization occurs as the RF input is increased.

The bias current is chosen to optimize the amplifier's IP3. The third-order intercept point performance for the MAX2055 is 40 dBm over the −3-dB-to-+20-dB gain range. When used to drive analog-to-digital converters, the device's OIP3 reduces in-band intermodulation noise to below the level found on most 12-to-16-b ADCs.

One of the main benefits of the MAX2055 chip is its noise level. It is significantly below the level of most high-performance 14-b ADCs. This low noise level is the result of the on-board fixed gain amplifier, which delivers a low 6-dB noise figure.

The device also stands out for its tight integration, which serves to minimize the size footprint. Its overall board-space utilization is 50% less than the space used by comparable discrete solutions. The device is packaged in an exposed-paddle (EP) -design, thermally enhanced, 20-pin thin-shrink small-outline package (TSSOP). It operates over the −40° to +85°C temperature range. The package's exposed paddle provides a low thermal-resistance path to the board die.

An evaluation kit is available now for use as a board-layout reference. Non-proprietary Gerber files are available upon request. The MAX2055 is priced at $4.95 in quantities of 1000.

Maxim Integrated Products, 120 San Gabriel Dr., Sunnyvale, CA 94086; (408) 737-7600; FAX: (408) 737-7194, www.maxim-ic.com.

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