Discrete and integrated control of power amplifiers in base station applications

In wireless base stations, the power amplifier (PA) determines the performance of the signal chain in terms of power consumption, linearity, efficiency, and cost. By monitoring and controlling the performance of the PA in the base station, the PA output can be maximized while achieving optimal linearity and efficiency. This article will discuss PA monitoring and control solutions using discrete components and introduce integrated solutions.

Analog Devices offers a range of components suitable for this application, including multi-channel digital-to-analog converters (DACs), analog-to-digital converters (ADCs), temperature sensors, and current sensors, as well as single-chip integrated solutions that can be used in base stations to monitor and control various types of analog signals. Discrete sensors and data converters offer the greatest performance and configuration flexibility, while integrated solutions offer the advantages of lower cost, smaller size, and higher reliability.

Optimizing the power efficiency of base stations is also a major consideration for telecommunications companies for environmental reasons. Significant efforts are being made to reduce the total energy consumption of base stations to reduce their environmental impact. The main daily operating cost of a base station is electricity, and the power amplifier can consume more than half of the power required by the base station, so optimizing the power efficiency of the power amplifier can improve operating performance and increase environmental and financial benefits.

Using Discrete Components for Amplifier Control

Figure 1 shows a basic power stage using an LDMOS transistor. The inherent trade-offs between linearity, efficiency, and gain determine the optimum bias state for the PA transistor. By controlling the drain bias current to maintain a constant value over temperature and time, the overall performance of the PA can be greatly improved while ensuring that the PA operates within the regulated output power range. One method of controlling the gate bias current is to use a resistor divider to fix the gate voltage to an optimum value determined during an evaluation phase.

Figure 1 Simplified control system

Unfortunately, while this fixed gate voltage approach is very cost effective, its major drawback is that it cannot be calibrated for changes in the environment, manufacturing tolerances, or supply voltage. The two main factors that affect the PA drain bias current are changes in the PA high voltage supply line and changes in the die temperature.

A better approach is to dynamically control the gate voltage of the power amplifier by measuring the drain current using a digital control algorithm, converting the drain current to a digital value through an ADC, and using a high-resolution DAC or a lower-resolution digital potentiometer to set the desired bias. This user-adjustable control system can maintain the power amplifier in the desired bias state to achieve optimal performance regardless of changes in voltage, temperature, and other environmental parameters.

A key element in this control method is to use a high-side sense resistor and the AD8211 current sense amplifier to accurately measure the current provided to the LDMOS transistor via the high-voltage power line. The AD8211 has a common-mode input range of up to +65 V and provides a fixed gain of 20 V/V. The full-scale current reading is set by an external sense resistor. The output voltage of the current sense amplifier can be provided to the ADC through a multiplexer to generate a digital value for monitoring and control. It must be noted here that the output voltage of the current sensor needs to be as close to the full-scale input range of the ADC as possible. Constant monitoring of the high-voltage power line allows the gate voltage of the power amplifier to be readjusted to maintain an optimal bias state even when a surge voltage appears on the high-voltage power line.

The source-drain current IDS of an LDMOS transistor is a function of the gate-source voltage Vgs, which contains two temperature-dependent terms, namely the effective electron mobility µ and the threshold voltage Vth

The threshold voltage Vth and effective electron mobility µ decrease with increasing temperature. Therefore, changes in temperature will cause changes in output power. Using one or more ADT75 12-bit temperature sensors to measure the ambient temperature and the power amplifier chip temperature, the temperature changes on the circuit board can be monitored. The ADT75 is a complete temperature monitoring system in an 8-pin MSOP package with ±1°C accuracy over a temperature range of 0°C to 70°C.

The output voltage, drain current and other data of the temperature sensor are input into the ADC through a multiplexer, and the temperature measurement results can be converted into digital quantities for monitoring. Depending on the system configuration, it may be necessary to use several temperature sensors on the circuit board. For example, if multiple power amplifiers are used or several pre-drivers are required in the front end, using a temperature sensor for each amplifier can provide better control capabilities for the system. To monitor current sensors and temperature sensors, ADI's AD7992, AD7994 and AD7998 multi-channel 12-bit ADCs can be used to convert analog measurement results into digital quantities.

Using control logic or a microcontroller, the digital values ​​of the current and temperature sensors can be continuously monitored. While monitoring the sensor readings and processing the digital outputs, dynamically controlling the gate voltage of the power amplifier using a digital potentiometer or DAC can maintain an optimal bias state. The amount of control required for the gate voltage will determine the resolution of the DAC. Telecommunications companies often use multiple power amplifiers in base station designs, as shown in Figure 2, which provides more flexibility when selecting the power amplifier for each RF carrier device, and each power amplifier can be optimized for a specific modulation scheme. Connecting power amplifiers in parallel can also improve linearity and overall efficiency. In this case, the power amplifier may require the use of multiple gain stages in cascade, including the use of variable gain amplifiers (VGAs) and pre-drivers to meet gain and efficiency requirements. Multichannel DACs can complete the various level setting and gain control requirements in these functional blocks.

Figure 2 Typical high power amplifier signal chain

In order to achieve precise control of the gate voltage of the power amplifier, ADI's 12-bit DAC AD5622, AD5627 and AD5625 can provide single, dual and quad outputs respectively. The internal buffers of these devices have excellent source and sink current capabilities, and external buffers are not required in most applications. At the same time, they have the advantages of low power consumption, monotonicity and fast stabilization time, which are suitable for precise level setting applications.

In applications where accuracy is not the primary concern and 8-bit resolution is acceptable, digital potentiometers are a more cost-effective choice. These digitally adjustable variable resistors perform the same electronic adjustment function as mechanical potentiometers or variable resistors, but with higher resolution, the reliability of solid-state technology, and excellent temperature performance. Nonvolatile and one-time programmable (OTP) digital potentiometers are ideal in time-division duplex (TDD) RF applications; during TDD receive, the PA is turned off and during transmit, the PA is turned on by a fixed gate voltage. This preprogrammed start-up voltage reduces turn-on delay and improves efficiency when turning on the PA transistor to enter the transmit state. The ability to turn off the PA transistor during receive prevents transmit circuit noise from corrupting the received signal and improves the overall efficiency of the PA. There are many digital potentiometers to choose from, depending on the number of channels, interface type, resolution, and requirements for nonvolatile memory. For example, the AD5172 from Analog Devices is a 256-position, one-time programmable, dual-channel I2C® potentiometer that is ideal for level-setting applications in RF amplifiers.

In order to monitor and control the gain and achieve optimal linearity and efficiency, it is necessary to accurately measure the power level of complex RF signals at the output of the power amplifier. ADI's AD8362TruPower™ rms power detector provides a 65 dB dynamic range over a frequency range of 50 Hz to 3.8 GHz, enabling accurate measurement of the rms power level of typical RF signals in W-CDMA, EDGE, and UMTS cellular base stations.

In Figure 3, the output of the power detector, VOUT, is connected to the gain control of the power amplifier to adjust the gain of the power amplifier. The output voltage of the power amplifier drives the antenna; the directional coupler samples the output voltage of the power amplifier in that direction, attenuates it appropriately, and applies it to the power detector. The output of the power detector, the RMS measurement of the transmit output signal, is compared with the value VSET programmed by the DAC, and the power amplifier gain is adjusted to make the difference zero. In this way, VSET can accurately set the power gain. The output of the ADC, the digital measurement of VOUT, is fed into a larger feedback loop, which can track the transmit power output measured by the AD8362 to determine the value of VSET and the gain requirement determined by the system.

Figure 3 Power detection

This gain control method can be used in conjunction with variable gain amplifiers (VGAs) and variable voltage amplifiers (VVAs) in the previous stages of the signal path. To measure both transmit and receive power, the Analog Devices AD8364 dual power detector can measure two composite input signals simultaneously. In systems that use a VGA or predriver and require only one power detector, the gain of one of the two devices is fixed and VOUT is fed to the control input of the other device.

If the feedback loop determines that the current on the power supply line is too high, a command is issued to the DAC to reduce the gate voltage or shut down the part. However, in some applications, if a voltage spike or an uncontrolled large current appears on the high-voltage power supply line, the digital control loop cannot detect the high-side current, convert the signal into a digital quantity, and process the digital quantity with the external control logic circuit fast enough to protect the device from damage.

In the analog method, an Analog Devices ADCMP371 comparator and an RF switch are used to control the RF signal input to the PA, as shown in Figure 4. The output voltage of the current sense amplifier can be directly compared with the fixed voltage set by the DAC. When the voltage generated at the output of the current sensor is higher than the set voltage, the comparator can control a control pin on the RF switch to flip its level and immediately cut off the RF signal at the PA gate to prevent the PA from being damaged. This direct control method bypasses digital processing, so it is faster and can provide better calibration.

Figure 4. Control loop protection using analog comparators

In summary, a typical PA monitoring and control architecture using discrete components is shown in Figure 5. Only the PA itself is monitored and controlled, but the same principles can be applied to any amplifier in the signal chain. A master controller controls all discrete components and operates on the same I2C data bus.

Figure 5: Power amplifier monitoring and control using discrete components

Depending on the requirements of the signal chain, many amplifiers may be needed in the pre-driver and final stages to increase the total power gain of the signal in front of the antenna. However, these additional power gain stages have an adverse effect on the overall efficiency of the power amplifier. To minimize the impact, the driver must be monitored and controlled to optimize performance. For example, as shown in Figure 2, the user needs multiple discrete components to monitor temperature, power, voltage levels of the VGA, two pre-drivers, and the gain of the two final power amplifiers.

Integrated monitoring and control

To solve this derivative problem, Analog Devices has developed the AD7294, an integrated monitoring and control solution. The AD7294 integrates all the functions and features required for general-purpose monitoring and control of current, voltage, and temperature into a single chip.

Figure 6: Integrated solution for monitoring and controlling the power amplifier stage

The AD7294 integrates a 9-channel 12-bit ADC and a 4-channel DAC with 10 mA sink/source current capability. It is manufactured using a 0.6 µm DMOS process, which enables the current sensor to measure common-mode levels up to 59.4 V. The internal ADC provides two dedicated current sensing channels, two channels for detecting external temperature, one channel for detecting the internal temperature of the chip, and four non-dedicated ADC input channels for general-purpose monitoring.

The advantage of this ADC channel is that it has a hysteresis register and an upper and lower limit register (also available in the AD7992/AD7994/AD7998). The user can pre-program the upper and lower limits of the ADC channel; an alarm flag is generated when the monitored signal crosses these limits. The hysteresis register provides the user with the ability to determine the reset point of the alarm flag when a limit crossing event occurs. The hysteresis register prevents noisy temperature sensor or current sensor readings from continuously triggering the alarm flag.

Analog-to-digital conversion operations can be initiated in two different ways. Command mode enables the user to convert a single channel to a sequence of multiple channels as needed. Cycle mode can automatically convert based on a pre-programmed sequence of multiple channels. This cycle mode is ideal for system monitoring applications, especially for continuous monitoring of signals such as signal power and current detection. This cycle mode only issues an alarm when a pre-programmed upper or lower limit is crossed.

In this solution, two bidirectional high-side current sense amplifiers are also provided (Figure 7). When the drain current of the power amplifier flows through the sampling resistor, the small differential input voltage generated will be amplified. The integrated current sense amplifier can reject common-mode voltages up to 59.4 V and can provide an amplified analog signal to one of the multiplexed ADC channels. Both current sense amplifiers have a fixed gain of 12.5 and use an internal 2.5 V output offset reference source.

Figure 7 AD7294 high-side current sense amplifier

For each amplifier, an analog comparator is provided for fault detection above a 1.2x full-scale voltage threshold.

Four 12-bit DACs provide digitally controlled voltages (1.2 mV resolution) for controlling bias currents in power transistors. They can also be used to provide control voltages for variable gain amplifiers. The heart of the DAC is a thin-film, 12-bit inherently monotonic serial DAC that uses a 2.5 V reference with a 5 V output range. The output buffer of this DAC is capable of driving high voltage output stages. The output range of the DAC is controlled by the offset input and is from 0 V to 15 V. This provides the end user with 12-bit control over a 5 V range while also providing the flexibility to use bias voltages up to 15 V, as power amplifier transistors often use larger control gate voltages. In addition, the four DACs’ ability to sink and source currents up to 10 mA eliminates the need for external driver buffers.

in conclusion

Power amplifier suppliers are using a variety of gain stages and control techniques to design more complex power amplifier front-end signal chains. The existing multi-channel ADC and DAC and analog RF component product lines are ideal for solving different system partitioning and architecture problems, allowing designers to implement cost-effective distributed control solutions. However, as an alternative, monolithic solutions such as the AD7294 have significant advantages in terms of circuit board area, system reliability and cost. From the perspective of user-customized design, both dedicated discrete functional blocks and integrated system functional blocks can provide system designers with an unprecedented wide range of design options.