AWR Application Notes

AWR Software for the Design of a High-Efficiency Broadband GaN HEMT Doherty Amplifier for Cellular Transmitters 3 www.cadence.com/go/awr Figure 2a shows the simplified circuit schematic of a single-ended 80W GaN HEMT power amplifier operating in a Class-AB mode with external input and output matching circuits to operate over a frequency bandwidth from 1.7-2.7GHz. Here, the input and output matching circuits implemented on RO4350 substrate (material from Rogers Corporation) represent the two-stepped microstrip-line transformer, each with different characteristic impedance ratio and different electrical lengths of the microstrip-line sections, providing the conjugate matching with the device input and equivalent output impedance at the fundamental frequency.As a result, an output power P1dB of more than 48dBm with a power gain of more than 12dB and drain efficiency of more than 52 percent was measured across the required frequency range from 1.8-2.7GHz, as shown in Figure 2b. Previously, drain efficiencies greater than 60 percent were achieved between 1.9-2.9GHz with a 45W GaN HEMT CGH40045F device using a simplified real-frequency technique to determine the optimum impedances and element values for highest efficiencies across the frequency range. 8 Figure 2: Single-ended Class-AB power amplifier with conjugate matching Broadband Two-Stage Doherty Amplifier In order to maximize operational bandwidth, it was important to minimize the loaded Q factor. (A loaded Q of unity has limitless bandwidth). However, in Doherty operations where a λ/4 transformer is necessary, the value of the loaded Q begins at a figure of 2. Therefore, it is possible to achieve wideband operation by balancing the amount of loaded Q necessary for Doherty operation and operational bandwidth. The classical two-stage Doherty amplifier has limited bandwidth capability in a low-power region since it is necessary to provide an impedance transformation from 25-100ohms when the peaking amplifier is turned off, thus resulting in a loaded quality factor Q L = = 1.73 at 3dB output-power reduction level, which is sufficiently high for broadband operation. However, at high-power levels, due to broadband impedance output matching of the carrier and peaking amplifiers and using a broadband output quarter-wave transformer, it is possible to maximize the frequency bandwidth. Figure 3a shows the circuit diagram of a conventional two-stage Doherty amplifier implemented on a 20mil RO4350 substrate and based on two 80W GaN HEMT power transistors with internal input matching in metal-ceramic flange packages. The input and output matching circuits are fully based on microstrip lines of different electrical lengths and characteristic impedances composing the two-stepped structures. An input splitter represents a broadband coupled-line coupler from Anaren, model X3C17A1-03WS, which provides maximum phase balance of ±5 degrees and amplitude balance of ±0.5dB across the frequency range of 690-2700MHz. Figure 3b shows the measured power gain and drain efficiency of such a GaN HEMT Doherty amplifier across the entire frequency bandwidth for five in-band frequencies. In this case, a power gain of more than 9dB was achieved across the entire frequency range of 1.8-2.7GHz. At the same time, the drain efficiencies of about 60 percent at saturation power P 3dB (except high-bandwidth frequencies) and between 40 and 50 percent at 6dB backoff output powers were measured. In view of the bandwidth limitations of the conventional structure, the Doherty effect is not as strong across the bandwidth, with more effect at lower bandwidth frequencies. Figure 3: Circuit diagram and performance of two-stage Doherty amplifier a) b) a) b)

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