The MAXREFDES39# power amplifier (PA) bias reference design uses the MAX11300, also called a programmable mixed-signal I/O (PIXI™), to bias and monitor a power amplifier for an RF base station application. In this case, an Infineon PTMA210152M-20W is used but any Power Amplifier can be used. The MAX11300 provides a highly integrated solution for the RF power amplifier biasing infrastructure. The design utilizes all of the MAX11300’s analog peripherals, while only requiring one MAX11300 device to both bias and monitor a 2-stage RF power amp. All the features of this reference design are listed below.
Figure 1. The MAXREFDES39# system block diagram.
The USB interface to the microcontroller and serial interface to the MAX11300 are digitally isolated from the main circuit through the MAX14850. The microcontroller section is powered through USB power, whereas the rest of the circuit is powered by a separate 36V DC supply. Figure 1 also shows power control and monitoring for RF PA.
The design uses the highly configurable input/output capability of the MAX11300 to bias an Infineon PTAM210152M; 15W, two-stage, LDMOS power amp. The MAX11300 internal design as used in this PA bias reference design is shown in Figure 2. It consists of six single-ended ADCs, four DACs, two switches, and two GPOs.
Figure 2. Internal schematic of MAX11300 for PA biasing.
In Figure 2, three of the ADCs are configured for 0 to 2.5V input range for current-sense measurement and the other three are configured for 0 to 10V input range for supply voltage detection. Two of the DACs are configured to regulate MAX17503 DC-DC converter for optimal power output for the PA. The other two DACs are used to bias stage 1 and stage 2 of the PA. GPI-controlled analog switches can be used for fast switching of DAC outputs for TDD application. If fast switching is not required, these can be left connected internally or through an isolated GPIO of microcontroller (see schematic for more details).
Input power is supplied through a 36V, 150W DC power supply. MAX17503 DC-DC converters are used to regulate 15V to 28V for PA. Their main characteristics are 4.5 to 60V input range and up to 2.5A output. As mentioned above, DAC outputs regulate the FB input of these converters, as shown in Figure 3.
Figure 3. DC-DC converter circuit.
MAX44285 current-sense amplifiers (CSAs) are used to measure bias current of stage 1 and stage 2 of the PA. The CSA output range is designed to vary from 0 to 2.5V for 0 to 1A current with a 0.05Ω sensor resistor, and a gain of 50. Therefore, the ADC range of 0 to 2.5V is selected for all ADCs used to monitor current.
To measure PA’s temperature, an external diode is attached to the PA using thermal paste and connected to the MAX11300 EXT Temp 1 pins. Another diode is attached to the PA board and connected to the MAX11300 EXT Temp 2 pins, as shown in Figure 4.
Figure 4. External temperature sensor connection scheme.
Steps to test the MAXREFDES39# hardware:
Figure 5. Screenshot of expected ADC values during initial startup of the PA bias reference design.
Figure 6. Screenshot of DAC values in the PA bias GUI.
Figure 7. Connections of the MAXREFDES39# board with the PTMA210152M EV kit.
The software provides all required features including
The Configuration tab provides the option to load the MAX11300 configuration file generated by the MAX11300 configuration software. Once the file is loaded, users can view whole configuration for the device, as shown below in Figure 8. If the configuration matches Figure 8, it can be written to the MAX11300 device. This configuration can also be used with other 2-stage power amplifiers instead of PTMA210152M.
Figure 8. Screenshot of the MAXREFDES39# software - Configuration Tab.
The Control / Monitor tab provides features to set the voltages of all DAC configured ports and read the data of all ADC configured ports. Instantaneous output of ADC ports as well as averaged output can be read. The software also calculates the standard deviation of the averaged data as shown in Figure 5 and in Figure 9.
Figure 9. Screenshot of MAXREFDES39# software – Control / Monitor tab.
The Calibration tab (Figure 10) is used to calibrate the ADC and DAC gain and offset errors. The values of gain and offset error compensation are set on this tab and the effect can be seen on the ADC and DAC configured ports when data is read or written to the MAX11300 device through the Control / Monitor tab. As an example, if gain is set to 0.5 for a DAC configured on port 0 and output is set to 10V, then actual output will be 5V.
Figure 10. Screenshot of MAXREFDES39# software – Calibration tab.
The power amplifier is biased by using the MAXREDES39# board with a 2.1GHz RF input connected to the PTMA210152M EV kit. The output of the EV kit is connected to a spectrum analyzer through an RF attenuator. The power amplifier is biased through the DAC configured ports, port 6 and port 14. The spectrum analyzer result is shown in Figure 11.
Figure 11. Spectrum of power amp output when biased with the MAX11300.
The output in Figure 11 does not show any spikes added by the MAX11300 within 100kHz span, whereas the DAC update rate is 25kHz.