A High-Efficiency, Small-Size GaN Doherty Amplifier for LTE Micro ...

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power is about 25W; gain is approximately 16dB; the maximum saturated efficiency is approximately 75%. Fig. 2 T1G6001528-Q3 package T1G6001528-Q3 package is show as Fig.2, the device dimension except input / output lead is 5mm x 6mm. The performance of this small device comes from its high power density. The small form factor of the transistor is a key factor that enables development of a smaller-scale Doherty amplifier. The T1G6001528-Q3 provided this performance: Vd=28V, Idq=100mA, at pulse waveform PW=50uS, duty=10%, its load-pull data at 2.65GHz is measured as following in Fig. 3. The reported Doherty amplifier is designed based on this loadpull data. Fig. 3 T1G6001528-Q3 load-pull data III. DOHERTY AMPLIFIER CONFIGURATION A symmetric Doherty amplifier is a very popular RF high power, high efficiency amplifier configuration for contemporary wireless base stations. The amplifier demonstrated in this paper is designed using two T1G6001528-Q3 discrete packaged HEMTs, and the whole Doherty amplifier size is 30mm x 70mm as shown as Fig. 4. The small size is very necessary for micro-cell base stations or active antenna systems base stations where space is at a minimum. RF in Bias Vgs≈-‐3.7V Idq=100mA Bias Vgs=-‐5V Carrier amplifier T1G6001528-‐Q3 Peakingamplifier T1G6001528-‐Q3 28V Vds 28V Vds Fig. 4 30mm x 70mm Doherty Amplifier Board RF out The amplifier PCB material is Taconic RF35B, with a thickness (H) of 16.6mm, and a dielectric constant (εr) of 3.66. There is a 3dB hybrid at the input area, which is used to split the input signal into a carrier amplifier (the up path) and a peaking amplifier (the down path). The carrier amplifier is biased in class AB at Idq=100mA; the peaking amplifier is biased in class C mode. Since the carrier amplifier and peaking amplifier operate in different modes, their output impedances are not identical, so their output matching circuitry is slightly different. When designing a Doherty amplifier, the ideal situation is to design load impedance at Zopt equal to the maximum Psat point, and design load impedance at 2*Zopt equal to the maximum efficiency point. But because the T1G6001528-Q3 is a wideband general purpose device, it is not specifically designed for 2.65 GHz Doherty purposes; its maximum efficiency is not at a 2:1 VSWR circle of the maximum saturated power. When we designed this Doherty amplifier, we had to compromise the load impedance at Zopt and 2 * Zopt. This means the load impedance at Zopt is not at the maximum saturated power point and the load impedance at 2 * Zopt is not at the maximum efficiency point. IV. DOHERTY AMPLIFIER PERFORMANCE The Doherty amplifier utilizing the T1G6001528-Q3 (as documented in Fig 3) has been measured with several kinds of signal waveforms to characterize its performance for base station applications. Fig.5 shows its AM/AM and AM/PM curve, which is a critical parameter for DPD correction performance. In a typical normal LDMOS device Doherty amplifier, phase always drops when input power increases, which will reduce DPD correction performance. But in this amplifier utilizing the T1G6001528-Q3, phase variation of input power is totally different, which will be a benefit for DPD correction.
Fig.6 shows its PAR and efficiency variation with output power, which is measured at 2.65GHz, the WCDMA signal waveform with PAR @ 0. 01% CCDF = 10.2dB. Fig. 7 shows its high power performance over the standard LTE frequency range of approximately 2.62GHz to 2.69GHz, which is measured with a WCDMA signal waveform, the data is tested at 38dBm average output power, the saturated power is calculated using an average output power plus PAR. It can be seen from Fig.7 that in a standard LTE frequency range (2620 MHz~2690 MHz), when average output power at 38dBm, its efficiency is higher than 55%. In contemporary base station designs, the RF power amplifier output need varies when the number of phones calling users within a given geographic area varies. This requirement is called “traffic control”. In order to let base stations always operate at high efficiency, generally the variation of output power is implemented through adjusting RF power amplifier working voltages. So that the base station Gain(dB) 17 16 15 14 13 12 11 10 Output_PAR(dB) 15 13 11 9 7 5 2.65GHz AM/AM & AM/PM Gain Phase 13 15 17 19 21 23 25 27 29 31 33 Pin( dBm) Fig. 5 AM/AM and AM/PM T1G6001528 Doherty Output PAR and Efficiency vs. Pout 1C-‐WCDMA , 10.2dB PAR at 0.01%CCDF 30 31 32 33 34 35 36 37 38 39 Pout (dBm ) 130 128 126 124 122 120 118 116 Phase(° ) AM/AM AM/PM Fig. 6 PAR and efficiency variation with output power 60 56 52 48 44 40 36 32 28 24 20 Efficiency ( % ) Psat ( dBm) & Eff ( % ) Pout (dBm) 60 58 56 54 52 50 48 46 44 42 40 39.5 39 38.5 38 37.5 37 36.5 36 35.5 35 WCDMA Performance 1C WCDMA , 10.2dB PAR at 0.01%CCDF Eff Psat Psat Eff Gain 40 10 2620 2630 2640 2650 2660 2670 2680 2690 Frequency ( MHz ) Gain Fig. 7 Psat , Eff, Gain variation over frequency range T1G6001528 Doherty Eff and Pout @7.5dB OBO vs Drain Voltage 1C-‐WCDMA , 10.2dB PAR at 0.01%CCDF 24 26 28 Drain Voltage (V) 30 32 Fig. 8 Eff and Pout variation with drain voltage Always operates at the highest efficiency, base station RF power amplifiers are required to provide stable efficiency when working voltages change. Fig. 8 shows the drain voltage range of 24V ~ 32V, with the same WCDMA waveform, and the same PAR (7.5dB), the efficiency and average output power varies with the voltage. To satisfy the requirements of high-efficiency base station operations, most RF power amplifiers need to operate with digital pre-distortion (DPD) to obtain linear performance, which is represent with ACPR in RF power amplifiers. So for today’s base station RF power amplifiers, the performance of DPD correction is very important. In order to verify this Doherty amplifier’s DPD correction effect , the system utilized in this paper was tested with a Netlogic standard DPD system, in a two carrier LTE (20 MHz signal bandwidth) environment, with PAR @ 0.01% CCDF = 8dB. The measured ACPR performance at 2.65GHz is shown Fig 9, at 20 19 18 17 16 15 14 13 12 11 60 59 58 57 56 55 54 53 52 51 50 Gain ( dB ) Efficiency (%)
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A High-Efficiency, Small-Size GaN Doherty Amplifier for LTE Micro ...

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