Design of a GSM/DCS dual-band RF front-end module using a single power amplifier

In modern wireless communication systems, RF power amplifiers are key components for wireless transmission of RF signals. As the number of mobile communication users increases, a single frequency resource is far from meeting the needs of users for calls, which requires mobile communication operators to open up new frequency bands to expand user capacity, so multi-frequency mobile phones are widely used. Multi-frequency mobile phones refer to mobile phones that can use different frequency bands for transmission in the same mobile communication network standard. Since different frequency bands are used for transmission, RF power amplifiers of different frequency bands are also needed in mobile phones to achieve this.

At present, the GSM system is the most widely used mobile communication standard in the world. The RF front-end architecture used in the GSM system mainly includes solutions combining GSM/DCS dual-band power amplifier modules and single-pole four-throw (SP4T) RF switch modules. Among them, the GSM/DCS dual-band power amplifier module mostly uses single-band RF power amplifier tube cores of the GSM and DCS frequency bands and the corresponding input and output matching networks and CMOS controllers to package into one chip module to achieve dual-band operation. The SP4T RF switch module mostly integrates the GSM/DCS dual-band filter with the SP4T switch tube core.

This paper proposes a novel RF power amplifier circuit structure, using one RF power amplifier to achieve GSM/DCS dual-band power amplification function. RDA6218 adopts this structure. The RF power amplifier die is reduced from two to one. At the same time, the RF power amplifier and output matching network of this structure are integrated with the CMOS controller and RF switch into a chip module to form a GSM/DCS dual-band RF front-end module, as shown in Figure 1.

Figure 1 Schematic diagram of the GSM/DCS dual-band RF front-end module.

Single chip amplifier circuit

The RF power amplifier circuit in this design adopts a three-stage amplification circuit. As shown in Figure 2, the first stage of the RF power amplifier circuit is divided into two independent input ends, corresponding to the GSM and DCS power amplification frequency bands respectively. Then the second and third stage amplifier circuits are shared. At the output end, an output matching network that can be applied to both the GSM and DCS frequency bands is implemented. Since the second and third stages are circuit amplifier stages shared by the GSM and DCS frequency bands, the requirements of both the GSM and DCS frequency bands need to be taken into account when designing this two-stage circuit.

Figure 2. Schematic diagram of dual-band power amplifier circuit.

The third stage in this circuit is designed as a power amplifier stage. In order to achieve 35dBm and 33dBm power outputs in the GSM band and DCS band respectively under normal battery voltage, the power output impedances of the GSM band and DCS band are designed to be 2Ω and 3Ω respectively. Since the output power of the GSM band is greater than the output power of the DCS band, the maximum output power of the third stage power tube Q3 is designed to reach 35dBm.

The second stage in this circuit is the power driving stage. Because it needs to cover both GSM and DCS frequency bands at the same time, the frequency range is very wide. Therefore, the second stage amplifier circuit is designed to adopt a negative feedback structure to widen the operating frequency from the GSM frequency band to the DCS frequency band. At the same time, the second and third stage inter-stage matching networks are also designed as broadband matching networks. In this design circuit, the overall gain of the second and third stages is designed to be 25dB, and the frequency range covers the GSM and DCS frequency bands. The simulation results are shown in Figure 3.

Fig. 3 Second and third stage gain simulation results.

Since the gain of the high frequency band (DCS) is slightly lower in the second and third stages, when designing the first stage amplifier circuit, the first stage gain of the DCS band is about 3dB higher than that of the GSM band. At the same time, a filter network is added to the RF input of the DCS band, as shown in Figure 2. This filter network has a band-stop effect on the GSM band signal and a band-pass effect on the DCS band signal. Adding this filter network can effectively improve the cross isolation. The simulation schematic diagram and simulation results of the filter network are shown in Figures 4 and 5 respectively. The simulation results of the total gain of the GSM band and DCS band of this design circuit are shown in Figures 6 and 7.

Figure 4 DCS band input filter network simulation schematic.

Figure 5 DCS frequency band input filter network simulation results

Figure 6 GSM band total gain simulation results

Figure 7 DCS band total gain simulation results

High Isolation RF Switch

In the GSM/DCS dual-band RF front-end module designed in this paper, the output end of the GSM/DCS dual-band RF power amplifier die is connected to the same node with the GSM output matching network and the DCS output matching network respectively. The DCS operating frequency band ranges from 1710MHz to 1910MHz, covering the second harmonic frequency range (1760MHz to 1830MHz) of the GSM band (880MHz to 915MHz). Therefore, when the GSM band is transmitted, the second harmonic of the GSM band RF signal can leak through the common node to the DCS output matching network, and then be transmitted to the antenna.

Although the DCS end of the RF switch is in the off state when the GSM band transmission is enabled, the isolation is only about 20dB when the ordinary RF switch is in the off state. Therefore, when the second harmonic signal of the GSM band is strong, there is still a certain amount of RF signal power coupled to the antenna through the DCS end of the RF switch, so that when the GSM band is transmitted, the second harmonic signal of the GSM band output by the antenna end is high, exceeding the system index requirements. In order to meet the communication system's requirement that the harmonic component is below -30dBm, the DCS end of the RF switch is designed as a high isolation structure. When the GSM end of the RF switch is enabled, the isolation from the DCS end to the antenna end is as high as 80dB, so that the second harmonic of the GSM band signal cannot be transmitted to the antenna through the DCS end of the RF switch, thereby greatly reducing the RF interference between the two bands.

Conclusion

This paper proposes a novel structure that uses an RF power amplifier to realize the GSM/DCS dual-band power amplification function. At the same time, this structure integrates the RF power amplifier and output matching network of RDA Microelectronics Co., Ltd. with the CMOS controller and RF switch into a chip module to form a GSM/DCS dual-band RF front-end module, in which the RF switch adopts a high isolation switch design so that the harmonics meet the requirements of the communication system. The GSM/DCS dual-band RF front-end module designed in this paper has an output power of 33dBm at the antenna end of the module in the GSM transmission mode, an efficiency of 38%, and harmonic suppression below -33dBm; in the DCS transmission mode, the output power of the antenna end of the module is 30dBm, the efficiency is 30%, and the harmonic suppression is below -33dBm.