What happens behind the scenes of RF base stations? (Part 2)


In my previous blog post, I talked about power amplifiers (PAs) and their use in wireless base stations. The main takeaway was how quiescent current, or the DC current through the PA, plays a large role in overall system performance and efficiency. This quiescent current, IDSQ, is chosen to optimize power dissipation and signal (radio frequency [RF]) gain.

Now that you understand this concept, let’s say you immediately grab a PA and head out to your nearest lab to bias the gate for a specific IDSQ current value. After some load-line analysis, you choose a q-point and use this information to bias the gate of the PA. Success! Everything seems to operate exactly as planned. To congratulate yourself, you decide to grab a quick snack. After your break you head back to the lab, and to your surprise discover that the IDSQ value has changed.

How did this happen? Well, it turns out that PAs display nonlinearities during device operation, mostly dependent on temperature. While you were away, the PA was slowly heating up, resulting in a different IDSQ value. This is a problem because changes in IDSQ impact system performance, as variations create signal distortion.

To mathematically explain this concept of temperature dependency, Figure 1 simplifies the PA into a metal-oxide semiconductor field-effect transistor (MOSFET) model.

The figure above displays the characteristics of a transistor while in saturation. From Equation 1, you can see that the two parameters contributing to the nonlinear behavior of the device across temperature are the mobility of the carriers, µ, and the threshold voltage, Vth, of the device. This simplified model demonstrates how temperature affects PA performance – in reality, RF PAs range from laterally diffused metal-oxide semiconductors (LDMOS), to Gallium nitride (GaN), to Gallium arsenide (GaAs) technologies. Although these technologies demonstrate different device behavior, they all exhibit similar temperature dependencies due to their respective change in threshold voltage, Vth, and mobility, µ, across temperature. The smallest change in these parameters will affect performance because it will change the current across the PA, resulting in unpredictable output power.

To combat the effects of nonlinearities, most solutions include a monitoring scheme that keeps track of either the current or temperature across the PA. Let’s review what they are.

Current sensing

This method includes an analog-to-digital converter (ADC) and a current-sense amplifier to monitor IDSQ by measuring the differential voltage across the sense resistor, RSENSE, as illustrated in Figure 2. A microcontroller (MCU) uses an integration algorithm to digitally adjust digital-to-analog (DAC) voltage as IDSQ changes across temperature. As the measured IDSQ changes from the intended value, the MCU updates the DAC voltage, bringing IDSQ closer to the desired value.

 

 

Temperature sensing

To implement this method, the PA is characterized across temperature to obtain a look-up table (LUT) containing temperature versus gate voltage (VGS) data for a given IDSQ. Figure 3 displays example characterization curves.

 

During operation, an MCU refers back to the LUT stored in memory. A temperature sensor measures the temperature across the PA, which is used to interpolate the gate voltage required for the specified current. Once this value is calculated, the MCU digitally updates the DAC, resulting in an updated bias voltage. Figure 4 illustrates this system.

 

As you may have noticed, several components are required to achieve a robust solution that effectively optimizes PA performance. These components include an ADC; current-sense amplifier; DAC; temperature sensor; precision reference block; and last but not least, an interfacing MCU.

To reduce component count, many new devices are being designed with integrated ADCs, DACs, precision references and temperature-sensing capabilities. Among these devices are TI’s analog monitor and control (AMC) products. These AMC products will be explained in great detail in the next blog post, but for those of you who like to study beforehand, check out these products: the AMC7812, AMC7836 and AMC7834.