The use of GaN in the design of cellular power amplifiers have been steadily increasing since it was first used in 4G LTE remote radio heads (RRH) in 2014 due to its higher efficiency, power density, and wider bandwidth capability. These benefits are now being exploited down in the sub-6 GHz or 5G’s FR-1 band where GaN on SiC has been replacing LDMOS.
Wolfspeed leads market
Since creating the industry’s first GaN on SiC HEMT over two decades ago, Wolfspeed has enabled and driven the market by offering GaN on SiC products that effectively replace LDMOS parts in applications such as those shown in Figure 1. As the only vertically integrated GaN and Silicon Carbide manufacturer, the company leads competition by continually investing in the development of products and processes that meet market needs with a wide variety of frequency bands in the DC – 40 GHz range and with 28 V, 40 V, and 50 V bias voltages.
5G gains are best realized with Wolfspeed’s GaN on SiC components that allow RF designers to achieve application requirements of higher linearization, greater power density and improved thermal conductivity.
5G’s tough requirements
Even as 5G offers a boost in bandwidth, its higher frequencies are more prone to signal attenuation. Since higher bandwidth spells a lower signal-to-noise ratio (SNR), increasing bandwidth does not lead to a linear increase in capacity. One way to overcome this problem is to boost signal strength, which means increasing transmitter power, the number of antennas, and the number of cells, as is the case for 5G deployments.
Operators, however, want all of this in the smallest base station form factor, at the lowest cost, and with the least power consumption possible. The radio system designer is therefore faced with both the technology pressures from the 5G standard requirements as well as the commercial imperatives of the operator they must satisfy.
Power amplifier challenges
The main PA design areas affected by 5G radio requirements include:
- Requirement for higher power and smaller packages
- Improvement in linearized amplifier efficiency in Doherty architecture
- Instantaneous bandwidth (IBW) requirements up to 280 MHz and trending higher toward >400 MHz
- Amplifier’s fractional BW for new bands increased to:
- n41: 194 MHz (7.5%)
- n78: 500 MHz (14%)
- n79: 600 MHz (12/%)
- n77: 900 MHz (24%)
- Stricter spectrum emission mask (SEM) requirements
- Synergetic operation with digital pre-distortion (DPD) system
5G uses the 256-QAM modulation scheme and its multicarrier signals have a very high Peak to Average Power Ratio (PAPR). If an amplifier is designed to operate efficiently and linearly at peak levels, it typically has low efficiency at average power levels. Moreover, high PAPR signals tend to operate in the amplifier’s compression region. The resulting signal clipping due to dynamic range limitations and other nonlinearities cause distortion and interference that is often measured as error vector magnitude (EVM). 5G’s 256-QAM designs require a low EVM of <3.5%, and new designs will increase design challenges with 1,024-QAM.
To address the challenging requirements described above, designers have several options of which the following most frequently considered are best delivered by GaN on SiC technology.
- The design can employ the Doherty amplifier configuration that comprises two amplifier circuits within the overall amplifier to accommodate different signal levels. This increases both efficiency and linearity because designers are not required to significantly increase power backoff.
- The Doherty amplifier can be combined with digital pre-distortion (DPD) to linearize the PA. This lets PAs operate in the nonlinear region for part of the signal with distortions due to signal compression accounted for earlier. The result is higher output power, greater power efficiency, and high linearity.
- Another technique is to maintain constant gain by modulating the drain supply. With RF GaN PAs, significant linearity improvement can be achieved with this method. LDMOS gain, however, does not change adequately with drain bias to derive appreciable benefit from this technique.1
- Amplitude modulation to phase modulation (AM-PM) distortion can be lowered by using GaN HEMTs, which have lower input capacitance, Cgs, compared with LDMOS.2
- GaN HEMT’s lower Cgs combined with lower output capacitance, Cds, and higher input and output resistances, allows for simpler and lower loss circuits and wide bandwidth matching networks.
- GaN’s high frequency performance is leveraged to achieve wider IBW and fractional BW required by 5G.
New Wolfspeed HEMTs alleviate PA design pains
The advantages of GaN on SiC for cellular transmitter amplifiers are realized in four new of high-power Doherty transistors from Wolfspeed. Targeting frequencies above 2.3 GHz up to 4 GHz, this latest generation of 48 V products enable the high-performance amplifiers needed for 5G and give designers the flexibility they need to design cost-effective, smaller, and green cellular radios. All four products come in a thermally enhanced package with earless flange.
GTRB246608FC: The 2,300 – 2,400 MHz HEMT offers 49.3 dBm POUT(avg), 57.8 dBm Psat, 52% efficiency, and 15 dB gain with a wide IBW of 100 MHz.
GTRB266908FC: The 500 W (P3dB), 2,515 – 2,675 MHz RF GaN on SiC HEMT for multi-standard cellular PAs offers 549 W of POUT and 69.2% efficiency at P3dB. It has a POUT(avg) of 50.1 dBm, Psat 57.8 dBm, efficiency 48%, gain 15 dB, and a higher 194 MHz IBW.
GTRB384608FC: For a 440 W POUT @ P3dB, this part allows designs in the 3,300 – 3,800 MHz frequency range, delivering 47.5 dBm POUT(avg), 56.1 dBm Psat, 42% efficiency, 13 dB gain, and 200 MHz IBW.
GTRB424908FC: Operating in a high 3,700 – 4,000 MHz band, the GTRB424908FCisa 450 W (P3dB) GaN HEMT specified with POUT(avg) of 47.5 dBm, Psat of 56.1 dBm, efficiency 40%, gain 13 dB, and a huge IBW of 280 MHz.
Wolfspeed offers an extensive portfolio of GaN on SiC transistors for telecommunication systems supporting all global standards and frequency bands. Inquire today about the new products and many more solutions for your 5G PA design.
- Cripps, et al., The benefit of GaN characteristics over LDMOS for linearity improvement using drain modulation in power amplifier system (https://ieeexplore.ieee.org/document/5773334)
- Sáez, et al., LDMOS versus GaN RF Power Amplifier Comparison Based on the Computing Complexity Needed to Linearize the Output. Electronics (https://www.researchgate.net/publication/336970919_LDMOS_versus_GaN_RF_Power_Amplifier_Comparison_Based_on_the_Computing_Complexity_Needed_to_Linearize_the_Output)