A 900 MHz, Single-supply GaAs Power Amplifier IC
The current trend toward making cellular telephones smaller, lighter and more efficient continues to place high demands on component manufacturers to follow suit. Global System for Mobile communications (GSM), the largest segment of the cellular marketplace, appears to be leading that charge. The model AWT0908 GaAs IC power amplifier answers this challenge by providing the first single-supply, 4.8 V monolithic GaAs IC power amplifier for use in GSM cellular telephone applications. The new power amplifier is designed to operate in the 880 to 915 MHz frequency range and provides GSM telephone manufacturers with a more compact and efficient solution to their power amplifier requirements.
The AWT0908 amplifier comprises five basic elements: three amplification stages, a bias circuit and a DC/DC converter. Figure 1 shows the power amplifier’s functional block diagram. The converter transforms a positive supply voltage into a negative voltage for
biasing the gate of each amplifier stage. The on-chip bias network provides a stable voltage source over temperature, eliminating the need for additional temperature-compensation circuitry.
The AWT0908 IC uses three stages, permitting it to operate with an input power of 5 dBm. The amplifier’s output power and efficiency are determined by a single external capacitor matching the amplifier output to its load. Typical efficiency at 35 dBm power output is 50 percent. Figure 2 shows the power amplifier’s output power and power-added efficiency (PAE) at 902 MHz as a function of the IC’s power control voltage VREF at room temperature for a +5 dBm pulsed input at a 577 ms pulse width and 12.5 percent duty cycle.
The IC contains a pulse control pin that determines the output power level during the pulse, a feature that simplifies the design of the power and pulse control loop. The amplifier produces very little noise in the GSM receive band (–90 dBm in a 30 kHz band). The unit is available in a 28-pin, thermally enhanced SSOP package that reduces thermal resistance to less than 25°C/W and increases the part’s reliability. Figure 3 shows the IC’s outline.
Typical Operating Characteristics
In addition to the AWT0908 power amplifier’s 880 to 915 MHz operating frequency range, 35 dBm typical output power and 50 percent typical PAE, the device’s operating characteristics include second- and third-harmonic levels of –38 and –28 dBc, respectively. Input return loss is 12 dB and isolation from output to input is –40 dBm. DC/DC converter current is 5.5 mA and pulse control current is 2 mA maximum. In terms of stability, all spurious outputs relative to the desired signal resulting from an output SWR of 6 rotated through all phase angles are –80 dBc or less for outputs equal to or less than +35 dBm at a +4.8 V supply. The IC’s specified operating temperature range is –20° to +70°C.
The AWT0908 amplifier’s maximum ratings include a +12 dBm maximum RF input level, +5 V maximum to 0 V minimum pulse control input and +7.5 maximum to 0 V minimum rating on most other active pins. (Voltage should not be applied to pin 10, the DC/DC converter’s output, and pins 11 and 12.)
This new GaAs MMIC power amplifier combines high efficiency and high output power with low cost and compact size, making the device an ideal solution for today’s GSM telephone headsets. The single power supply and pulse control simplify power control circuitry and the built-in pulse shaping adds to the unit’s versatility. The AWT0908 power amplifier IC is available for $3.75 each in 100,000-piece quantities. Additional information is available at the company’s Web site at http://www.anadigic.