A Linear, High Efficiency, HBT, CDMA Power Amplifier

A Linear, High Efficiency, HBT, CDMA Power Amplifier

RF Micro Devices Inc.
Greensboro, NC

As code-division multiple access (CDMA) systems begin to see implementation in the US and other countries, many manufacturers are rolling out their CDMA cellular handsets. The final power amplifier for these handsets is critical to the competitive positioning of the product. This power amplifier must operate at full power with the CDMA waveform without generating adjacent-channel interference, operate in analog Advanced Mobile Phone Service (AMPS) or Extended Total Access Communications System mode while in systems that do not support CDMA, have high efficiency to maximize talk time, and maintain efficiency and linearity at lower power levels.

Traditionally, CDMA power amplifiers have been based on hybrid modules, which are large and expensive. In contrast, the model RF2108 integrated power amplifier provides full performance from a single 16-lead SOIC plastic package. This package may be assembled using standard surface-mount techniques and enables a small footprint to be maintained for the RF output interface.

Using heterojunction bipolar transistor (HBT) semiconductor technology, the RF2108 unit allows full high efficiency, linear performance from a single positive 4.8 V DC power supply. The part operates from 4 to 4.8 V DC to accommodate battery voltage degradation. Power down is accomplished with 0 V on a single control pin, consuming less than 10 mA in standby mode. During full-power operation, the power-added (or total) efficiency can be as high as 53 percent while meeting CDMA adjacent- and alternate-channel interference requirements simultaneously. When driven with a CW signal (as in FM AMPS mode), the output power at 4.8 V increases to 31.5 dBm at 57 percent efficiency. Depending upon the implementation of the power automatic level control (ALC) loop, an on-board control may be used for AMPS mode to provide greater than 30 dB of power control with a fixed input level. More commonly, the input level may be varied as is done in CDMA mode.

HBT Power Amplifier Advantages

The HBT power amplifier drives several key features of the phone operation and design. Some advantages are beneficial to the end customer, such as talk time and overall phone size. Other advantages are related to the ease of designing and manufacturing the phone, such as single-voltage supply, on-board power down, device linearity and on-board power control.

The current consumption of the transmitter is dominated nearly always by the power amplifier. For battery-operated applications, the power-added (or total) efficiency is extremely important. Linear applications such as CDMA typically have required class A biased power amplifiers, which have efficiencies of approximately 25 to 35 percent. The model RF2108 provides efficiency of greater than 50 percent while meeting CDMA adjacent-channel power rejection (ACPR) requirements. Also important for CDMA systems is the quiescent bias current. Since in the CDMA system the average power levels are kept low, the current consumption at an average +10 dBm output is an important figure of merit. The RF2108 is biased deep into class AB, so the current consumption at +10 dBm output is 40 mA. This low current consumption extends the lifetime of the battery under nominal operating conditions in the phone. Also, in an analog mode, the RF2108 provides up to 31.5 dBm output with 57 percent efficiency, without a matching circuit change.

CDMA cellular phones are entering a consumer market that is relatively mature. The consumer demands small handsets, which determine the PCB area available for the RF components. Most existing CDMA linear power amplifiers use hybrid modules, which are difficult to implement in the required area. Thus, the SOIC-packaged, integrated amplifier approach is extremely beneficial. The RF2108 replaces a module implementation with a single 16-lead SOIC device.

HBT is a unique technology, allowing performance better than GaAs MESFETs, yet also allowing biasing similar to silicon bipolar transistors from a single positive voltage. This performance eliminates one of the primary disadvantages of GaAs MESFETs, that is, the requirement for a negative voltage. For negative voltage to be implemented with sufficient current to drive a power MESFET gate, some kind of switching regulator or charge pump must be used, which can be expensive and cumbersome. If the charge pump is implemented on chip, excessive low frequency noise, additional current and additional external components minimize the benefit. HBT provides an elegant solution to the high efficiency linear power amplifier. With no need for additional components, the unit provides an overall smaller, more efficient and lower cost solution.

The RF2108 HBT power amplifier provides a single pin for power down. This function powers down the amplifier with 0 V on the control pin and provides full power with 3.6 V on the control. In power-down mode, less than 10 mA of total current is consumed, allowing long standby times for the phone.

To utilize a GaAs MESFET power amplifier, a switch generally must be inserted into the unit's bias supply line for shutdown. This switch must be capable of supporting high currents and tends to be expensive as a result. A MOSFET switch costs approximately 50¢ to 75¢, which is a substantial portion of the overall power amplifier cost. The loss through the switch also reduces the voltage available on the drain of the MESFET power amplifier, thus requiring more current to achieve the same output power.

Using the same pin that is used for power down, the gain can be controlled over 30 dB with a 2 to 3.6 V control range. Since linearity is not maintained during this power control, this function is useful in AMPS mode only. For CDMA mode, the input power to the power amplifier will be varied. If this function is not required, the pin may be tied strictly to a power-down control.

A key parameter related to the power amplifier in AMPS mode is the noise power output in a 30 kHz bandwidth. This noise power output defines the required rejection in the receive band (869 to 894 MHz) for the duplexer, since for a full-duplex system the transmitter will tend to self-jam the receiver. The noise transmitted by the power amplifier is related to its noise figure and gain in the receive band. The RF2108 operates with better than -90 dBm/30 kHz in the receive band.

HBT Technology

The model RF2108 is one of a family of power amplifiers based upon HBT technology for both linear and constant-envelope applications. This technology, provided by TRW, was developed originally for military and space applications. Based upon a GaAs/AlGaAs heterostructure, the power and efficiency performance is the highest of any commercially available integrated solution. The RF2108's single-tone power and efficiency vs. input power along with the two-tone intermodulation distortion (IMD) products vs. output power are shown in Figures 1 and 2 , respectively. Because it is a bipolar structure, the amplifier can operate from one positive voltage supply without adding components. This feature is extremely important in a battery-operated system such as a cellular phone.

Fig. 1: Power output and efficiency vs. input power.

Fig. 2: Two-tone IMD vs. output power.

The critical geometries in an HBT transistor are vertical structures, not lateral. The emitter, base and collector are stacked vertically by semiconductor layer growth using molecular beam epitaxy, an accurate and repeatable growth process. Since each layer is placed over the entire wafer at once, no photolithography is required for this process. Thus, mask alignment and optical resolution are not an issue. Also, the wafers can be prepared and stockpiled, eliminating this step from the critical path of product manufacturing.

Once the layers are completed, the lithography begins. Since all the critical geometries are already defined, the minimum feature size is currently 2 mm, making these devices much more manufacturable than the 0.5 to 1 mm gate geometries typically required by GaAs MESFETs.

The TRW HBT process is the most reliable commercially available HBT process in the world. As a military subcontractor, TRW has qualified the process for many of its military programs. Additionally, as a space equipment manufacturer, the HBT process has been qualified for class S space applications. This level of ruggedness is absolutely needed for spacecraft (since it is somewhat difficult to repair a failed component in space) but is also demanded by the commercial marketplace today. The HBT process and products are being tested diligently to determine their ruggedness and failure rates. The mean time between failures is 4×10⁷ hours at a 125°C junction temperature and 5×10⁵ hours at 185°C. Over 1.35 million device hours have been tested on packaged power amplifiers under full RF stress conditions at a 250¡C junction temperature without a failure to date. Additional information is available on the reliability of HBTs and may be obtained with the application information package on the RF2108.

Theory of Operation and Application

The block diagram for the model RF2108 is shown in Figure 3 . The part is a two-stage device with 28 dB linear gain. The drive required for full-power CDMA operation is approximately 0 dBm. To saturate the output fully, approximately +4 dBm CW input is required. Bias control is provided through a single pin interface, and the final stage ground is achieved through the large pins on both sides of the package. First-stage ground is brought out through a separate ground pin for isolation from the output. These grounds should be connected directly with vias to the PCB ground plane. The output is brought out through the four output pins, with two pins providing bias and harmonic termination and the other two pins feeding the RF output.

Fig. 3: The power amplifier's block diagram.

The amplifier operates in class AB bias mode. The final stage is deep AB, meaning the quiescent current is extremely low (approximately 40 mA). As the RF drive is increased, the final stage self-biases, causing the bias point to shift up and, at 27 dBm average output power, draw approximately 240 mA. The bias level changes according to the drive level without distorting the RF signal through a proprietary biasing technique (patent pending). The optimum load for the output stage (the load at the output collector) is approximately 10 ohms. The load is created by the series inductance formed by the output bond wires, leads and microstrip, and a shunt capacitor external to the amplifier. With this match, a 50 ohms terminal impedance is achieved. The input is matched to 50 ohms with only a blocking capacitor required.

The input is DC coupled; thus, a blocking cap must be inserted in series with the input. Also, the first-stage bias may be adjusted by a resistive pull-up with high value resistors on this pin to V_pc . However, for nominal operation, no external adjustment is necessary as internal resistors set the bias point optimally.

V_cc2 provides supply voltage to the first stage as well as some control over the operating band. Essentially, the bias is fed to this pin through a short microstrip section. A bypass capacitor sets the inductance seen by the amplifier. Therefore, placement of the bypass cap can affect the frequency of the gain peak. This supply should be bypassed individually before being combined with V_cc for the output stage to prevent feedback and oscillations.

The RF_out pins provide the output power. Bias for the final stage is fed to pins 14 and 15, which also provide the second-harmonic trap. Pins 10 and 11 provide the output line, which is matched for optimum operation at either the minimum battery voltage or nominal voltage.

The RF2108 unit operates over a 4 to 4.8 V range. If the full power is desired at minimum voltage, then the load can be optimized at that point. The specified efficiency, linearity and power are also attainable. However, as the voltage is increased the output power will increase. Thus, in a phone design, the ALC loop will back down the power to the desired level. This power level will occur at a less-than-optimum efficiency since the load is optimized for 4 V. This condition is true of any power amplifier. However, it is important to note that the RF2108 can be set up to provide the specified power at 4 V, if desired. The HBT breakdown voltage is greater than 20 V, so nominally at 4.8 V there should be no issue with overvoltage. However, under extreme conditions, which can occur in a cellular handset environment, the supply voltage can be as high as 8.5 V. These conditions may correspond to operation in a battery charger, especially with the battery removed, which unloads the supply circuit. To add to this worst-case scenario, the RF drive may be at full power during transmit and the output SWR could be extremely high corresponding to a broken or removed antenna. Under all of these conditions, the peak RF voltages could well exceed two times the supply voltage, forcing the device into breakdown. The RF2108 includes overvoltage-protection diodes at the output, which begin clipping the waveform peaks at approximately 13 V. These diodes protect the device's output from breaking down under these worst-case conditions and provide a rugged, robust component for the system.

High current conditions are also potentially dangerous to any RF device. High currents lead to high channel temperatures and may force early failures. The RF2108 includes a proprietary bias circuit to temperature compensate the RF transistors, thus limiting the current through the bias network and protecting the devices from damage. The same mechanism works to compensate the currents due to ambient temperature variations, and the unit is remarkably consistent over the full -30° to +85°C commercial temperature range.

Performance

Since the CDMA utilizes quadrature phase-shift keying signaling, the adjacent-channel power (ACP) measurements are the ultimate test for linearity. Table 1 lists the RF2108's key performance characteristics. The RF2108's ACP performance with 27 dBm output power and 44 percent efficiency is shown in Figure 4 . The 885 kHz ACPR is -46.8 dBc in a 30 kHz sideband relative to the 1.23 MHz center band. Figure 5 shows the ACP performance with 29 dBm output power and 53 percent efficiency. The 885 kHz ACPR is -40.4 dBc in a 30 kHz sideband relative to the 1.23 MHz center band. Figure 6 shows the ACPR performance and efficiency over the entire usable power range.

Fig. 4: ACP at 27 dBm output power and 44 percent efficiency

Fig. 5: ACP at 29 dBm output power and 53 percent efficiency.

Fig. 6: ACPR and efficiency vs. input power.

A common problem associated with high efficiency cellular power amplifiers is stability, especially into a duplexer. Since duplexers can be designed with various out-of-band characteristics, a robust cellular power amplifier must operate without oscillation into ceramic, helical and surface acoustic wave duplexers. The RF2108 has been tested into various types of duplexers without oscillating.

Conclusion

The RF2108 HBT, CDMA cellular dual-mode power amplifier provides the best overall performance of any CDMA power amplifier on the commercial market. Operating from a single positive supply, linear efficiencies of 50 percent and power levels of 29 dBm are achievable from a single 16-lead SOIC surface-mount package from 4 to 4.8 V. In AMPS mode, the part provides 31.5 dBm power with 57 percent efficiency. Power down and power control are integrated on chip without requiring additional components. The new power amplifier can be used to simplify cellular phone design and to improve operation, as well as to reduce overall cost significantly.

RF Micro Devices Inc.,
Greensboro, NC (910) 855-8085.