5.4 Broadband SMPA Behaviour Over the Frequency
6.2.4 Experimental Results
6.2.4.1 Optimum Bias Points
Fig. 6.26.a and Fig. 6.26.b show the measured output power and PAE of the manufactured 100 W PA as a function of gate and drain bias voltage for three different frequencies (the two edge frequencies and the centre frequency) and for 40 dBm input power (saturation level), respectively.
The drain voltage was 28 V, as recommended in [56], during the gate bias voltage sweeping.
From Fig. 6.26.a, it is evident that the performances do not degrade by varying the gate voltage Vgg. For the targeted band, the output power has a maximum variation of less than 0.5 dB. The variation for PAE is around 3 %. Looking at the PAE vs. the gate voltage, the best bias point that gives a tradeoff between the PAE and the output power over the bandwidth is−2.9 V (Idq =650 mA).
Using this bias point, a sweep for drain supply voltage was performed. Fig. 6.26.b shows
Pout[dBm]
80 90 100 110
Freq. [GHz]
1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3
Pout_ideal
Pout_realised
(a)
PAE[%]
60 70 80 90
60 70 80 90
Freq. [GHz]
1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3
PAEideal
PAErealised
(b)
Figure 6.23:Ideal (hollow symbol) and realies from ideal lumped OMN (filled symbol) for a) output power and b) PAE. The input power was 40 dBm,Vdd= 28 V andId=400 mA.
114 6. L and S Band Broadband PA
Pout[dBm]
60 80 100 120
PAE[%]
60 70 80 90
Freq. [GHz]
1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3
Figure 6.24: Simulated output power and PAE performances for different case temperature (i.e., 25◦C, 50◦C and 85◦C). The input power was 40 dBm, Vdd=28 V andId=400 mA.
the PAE and the output power behaviour vs. the drain supply voltageVdd. It is seen that the PAE varies by less than±3 % for every frequency point, with a drain voltage variation of 10 V.
Maintaining high PAE over a wide range of drain bias voltages makes the designed amplifier suitable for supply modulation techniques such as envelope elimination and restoration (EER) or envelop tracking (ET). After a suitable gate voltage was found, the suitable drain supply voltage can be found from Fig. 6.26.b to be 28 V for all frequency points. Hence, all subsequent measurements’ bias point will beVdd=28 V andIdq=650 mA (Vgg=−2.9 V).
6.2.4.2 Small Signal Behaviour
The small signal gain is measured by stimulating the amplifier with very small signals where the output is measured with a spectrum analyser. The measurement (symbol) and simulated (solid) results are shown in Fig. 6.27. It can be seen clearly that both measured and simulated results are in a close fit together. The maximum small signal gain is 16.4 dB at 1.65 GHz, and the minimum small signal gain is 12.4 dB at 2.25 GHz. This makes a flatness of±2 dB of small signal gain.
Input Output
Figure 6.25: Photo of the fabricated broad-band harmonically tuned power amplifier using 120W GaN HEMT from Cree Inc.
6.2 100 W L-Band Power Amplifier 115
Pout[dBm]
48 50 52 54 56
PAE[%]
50 55 60 65 70
Gate Voltage, Vgg [V]
-3.5 -3.0 -2.5 -2.0
@Freq=1.65 GHz
@Freq=1.95 GHz
@Freq=2.25 GHz
Vgg=−2.9 V Vdd=28 V Pin=40 dBm
(a)
Pout[dBm]
46 48 50 52 54
PAE[%]
50 55 60 65 70
Drain Voltage, Vdd [V]
20 24 28 32 36
@Freq=1.65 GHz
@Freq=1.95 GHz
@Freq=2.25 GHz Vdd=28 V
Vgg=−2.9 V Pin=40 dBm
(b)
Figure 6.26:Measured output power and PAE performances of the designed PA for different frequency vs.(a) gate bias voltage (b) drain supply voltage.
6.2.4.3 Large Signal Behaviour
Fig. 6.28 shows a comparison between simulation (solid) and measurements (symbol) for output power, gain, drain efficiency and power added efficiency at the centre frequency of the targeted band, i.e., 1.95 GHz. From the measurement, the maximum PAE of 64 % with power gain of 10 dB is achieved at output power of 49.8 dBm, where the drain efficiencyηis 72 %. The peak drain efficiency is 75 %, and the peak output power is 50 dBm. However, it is worth mentioning that the measurements fit the simulations over all the frequency points.
Fig. 6.29.a and 6.29.b show the measured output power, power gain (Gp), PAE and drain efficiency as a function of frequency for different input power levels, respectively. The meas-urements show that the 3 dB bandwidth is larger than the designed one and covers 1.55 GHz to 2.35 GHz. With the same targeted centre frequency, i.e., 1.95 GHz, the relative bandwidth is higher than 41 %. The maximum output power was 50 dBm over the bandwidth with a power gain of 10 dB.
The worst case PAE of 60 % was achieved in combination with 100 W of output power across the bandwidth from 1.55 GHz to 2.25 GHz. Best value for PAE is 70 % as was measured.
Gss[dB]
12 13 14 15 16 17
Freq [GHz]
1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3
Gss_sim Gss_meas
Figure 6.27:Simulated (solid) and measured (symbol) Small signal gain. The input power was−20 dBm,Vdd=28 V andId=650 mA.
116 6. L and S Band Broadband PA
Pout[dBm],Gain[dB]
0 10 20 30 40 50
Efficiency[%]
0 20 40 60 80 100
Pin [dBm]
20 25 30 35 40 45
Figure 6.28:Simulated (solid) and measured (symbol) performances at 28 V drain supply voltage andId=650 mA for CW signal at frequency of 1.95 GHz.
In order to improve the output power of the amplifier, post-tuning was implemented. Reducing the width of some microstrip lines was performed until a maximum power at the centre frequency is achieved. As a result of this, the output power reached more than 100 W, Fig. 6.30. The efficiency in this case is degraded by approximately 10 % from the maximum PAE achieved in Fig 6.29.b.
6.2.4.4 Linearity Measurement
A WCDMA signal with 8.5 dB peak to average power ratio (PAPR) centered at 2.15 GHz and 5 MHz bandwidth was used as a stimulating signal for the designed power amplifier. Fig. 6.31 shows the measured upper and lower ACLR for different average output power Pout_avg. A memory polynomial DPD was used to improve linearity and ACLR. An ACLR of more than 40 dBc was achieved with memory polynomial DPD with 38.5 dBm average output power. At this point, the average drain efficiency was 25 %. The implemented memory polynomial model
Pout[dBm]
15 25 35 45 55
Gain[dB]
0 10 20 30 40
Freq. [GHz]
1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3
Pin
(a)
PAE[%]
0 20 40 60 80
η[%]
0 20 40 60 80
Freq. [GHz]
1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3
Pin
(b)
Figure 6.29:performance over the frequency for different input power i.e.; 22, 27, 32, 37 and 40 dBm for (a) output power (black) and power gain (blue) , (b) drain efficiency (green) and power added efficiency (brown). The supply voltage isVdd=28 V andId=650 mA.
6.2 100 W L-Band Power Amplifier 117
Pout[dBm]
0 40 80 120
Freq [GHz]
1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3
Pout_sat Pout_3dB Pout_1dB
Figure 6.30:Performance over the frequency for the post-tuned power amplifier with different output power levels i.e.; Pout_sat, Pout_3dBand Pout_1dB. The supply voltage isVdd=28 V andId=650 mA.
follows the Hyunchul and Kenney model [58]:
y(n) =
m
∑
M=0 p
∑
P=1
c2P−1,M|x(n−M)|2(P−1)x(n−M) (6.9) wherex(n)is the input complex base band signal,y(n)is the output complex base band signal, cp,qare complex valued parameters, m is the memory depth and p is the order of the polynomial.
The polynomial depth used in this DPD was p=6 and the memory depthm=2.
ACLR[dBc]
-50 -45 -40 -35 -30 -25
Pout_avg [dBm]
30 35 40 45
Lower ACLR w/o DPD Upper ACLR w/o DPD Lower ACLR w DPD Upper ACLR w DPD
Figure 6.31:Measured upper (red) and lower (black) ACLR without memory DPD (hollow symbol) and with memory DPD (filled symbol) vs. average output power at 2.15 GHz operating frequency. The supply voltage isVdd=28 V and Id=650 mA.
118 6. L and S Band Broadband PA
7 | Conclusion
This work presents different techniques to maintain a highly efficient operation over wide bandwidth. It was shown that the designed amplifiers are among the state of the art in the last few years, Table 1.1. Comprehensive studies of different modes of operation are discussed to choose the best suitable operation for targeted band and applications. After the introductory chapter, where the motivation and the state of the art comparisons are discussed, three main parts are presented to give a better overview.
7.1 Thesis Outcome
The first part is the theory work which is discussed in the first two chapters. Chapter two gives the most important figure of merits of power amplifier design and measurements regarding to this work. The stability analysis is discussed where the suitable suppression techniques are presented.
Chapter three starts with the key design of any power amplifier, i.e., load line theory and load-pull techniques. Class-A PA design equations were derived from the load line theory. Later in the chapter, design equations for all classical classes were derived. General closed form equations for the efficiency and output power were given and analysed. Furthermore, highly efficient classes were presented in details starting from switch mode power amplifier through harmonic tuning classes, i.e., Class-J PA, and finally to the mixed mode of operation like Class-E/F PA. In the end of the chapter, physical limitations from a small signal FET model on the PA performances, in terms of output power, gain and efficiency, were analysed and discussed. It was shown that the square of the quality factors of the output parasitic capacitors are the limiting factors of these performances. However, the input transconductance degrades the performances also because of the presence of the feedback capacitor (Cgd).
In the fourth Chapter, two single band power amplifiers working with high efficiency are demonstrated. The first was designed for UMTS applications, i.e., 2.14 GHz using Class-D−1 power amplifiers. The resonator utilizes the fact that the microstrip DC-feed has small inductance.
The capacitors of the resonator were implemented using wide microstrip stub, and a gap capacitor
120 7. Conclusion obtained between the ends of these stubs. The input/output-BALUNs were designed from 3-dB hybrid 90◦ couplers with oneλ/4 implemented at one side of the balanced terminal. This PA showed outstanding results at 50W output power withηand PAE equal to 62.7% and 60.3%, respectively.
The second amplifier in this chapter is working on 2.45 GHz for ISM applications. The designed amplifier is working on a Class-F−1 power amplifier. This class requires different termination for different harmonic. Hence, multi resonator output matching network is designed to provide the correct termination for each harmonic. The novel technique for this OMN allows separate harmonic control with an integrated matching for the fundamental frequency. During the measurements, the maximum performance was observed at 2.35 GHz. An output power of 46 dBm at this frequency was measured with 10 dB power gain. A maximum drain efficiency of 60.8 % (PAE=55.7 %) was observed at the same frequency. Drain bias voltage was swept to determine the ability of this amplifier in the envelope elimination and restoration and/or envelope tracking EER/ET applications. Over 14V of Vddrange down from Vdd= 48 V, the drain efficiency remained within 4 % of the maximum efficiency level, and the output power within 2.3 dB of the 1 dB compression point.
Chapter five presented the first two broadband power amplifies working in high efficiency operation. These two amplifiers are working below 1 GHz frequency. The chapter started with the theoretical limits on designing a broadband PA using Bode-Fano theorem. Furthermore, matching network design technique, which is implemented for all the designed broadband PAs in this work, is shown. This technique depends on the understanding of the matching elements movement on the smitch chart.
A broadband Class-E power amplifier (PA) was presented in this chapter for a frequency range that covers 250 - 400 MHz using 45 W GaN HEMT from Eudyna. A novel and easy method of designing broadband Class-E PA was presented. It was shown that a good matching network with certain considerations followed with a bandpass filter gave a good broadband highly efficient PA.
All the considerations for this design were discussed and presented. Optimum load impedances were extracted using the load/source-pull simulation, and it was observed that it has a constant phase over the entire band and a constant negative slope for the magnitude. The drain efficiency between 63.7 % and 80.3 % and maximum output power with output power from 45.9 dBm to 47.8 dBm, gain flatness: 2.3 dB at the 1 dB compression point, power added efficiency in the range of 62.5 % - 78.5 %. This amplifier showed a good PA reliability over a wide range of impedance mismatch and drain supply voltage. It is because of the filter which eliminates any influences on the performances from any harmonic impedance mismatch.
A second broadband amplifier working on a higher frequency band was, furthermore, dis-cussed in this chapter. It used the same matching network disdis-cussed as the preceding PA. The impedances in this amplifier also have a constant phase over the entire band and a definite negative slope for the magnitude. Maximum drain efficiency of 87.8 % and peak output power of 46.9 dBm
7.2 Future Work 121 was measured. A 49 W highest output power was achieved with a flatness of 1.7 dB over the entire band (600 MHz - 1000 MHz).
This PA has minimum drain efficiency which was 66 %. This fluctuation of the measured drain efficiency requires more clarification. An analysis based on time domain waveforms was presented. This analysis presented the extraction method of the time domain waveforms. The analysis was based on two different characteristics at the die reference plane; the time domain drain current/voltage and the load impedances. A different operational mode over the entire frequency was the result of this analysis. The PA started from Class-E/F2PA (i.e.; at 600 MHz) passing by optimal Class-E PA (i.e.; at 800 MHz) and ending with weak Class-E PA (i.e.; at 1000 MHz). Overall, the broadband PA operation is maintained within the Class-E PA mode of
operation that it is designed for (i.e.; either weakly or strongly).
In chapter six, two power amplifiers working on L and S band were presented. The first one presented a highly efficient broadband power amplifier with harmonic tuning for a 10 W GaN HEMT transistor. Studying the harmonic impedances effect on the output power and the efficiency makes it easy to realize the broadband matching network that gives adequate performance. Drain efficiency and PAE between 71 % and 84 % and between 58.5 % and 78.0 %, respectively, was achieved across the bandwidth between 1.5-2.7 GHz (60 % BW at 2.125 GHz center frequency) at 3 dB compression point, Pout is 40 dBm±1.5 across this band. An ACPR of -36 dBc with 35 dBm Pout_avg and 35 %ηavgwas found by sweeping the drain supply voltage and the bias point for a typical WCDMA signal with 8.51 dB PAR centered at 2.15 GHz.
The second amplifier presented in this chapter is a broadband power amplifier centered at 1.95 GHz with relative bandwidth of 41 %. Harmonic termination is designed using 3-poles output matching network. The realized output matching network is using step impedances. This network fits the optimum load impedance obtained from the load-pull data as well as an initial lumped element output matching network very well. The PA is fabricated using a 120 W GaN HEMT device from Cree Inc. Worst case PAE of 60 % was achieved in combination with 100W of output power across the bandwidth from 1.55 GHz to 2.25 GHz. PAE best values up to 70 % was measured. Using an UMTS input signal, more than 40 dBc ACLR can be achieved with memory DPD for an average output power up to 38.5 dBm.
7.2 Future Work
These broadband power amplifiers give a good starting point for multi-standard applications.
However, most of these standards require high average drain efficiency with good linearity performance. Hence, different techniques can be used to investigate the PA for these standards.
These can include (but not restrictively):
• Harmonic injection concept which aims for an improvement of the efficiency of RF power amplifiers showed good results as shown in [Paper N]. For that purpose a triplexer is
122 7. Conclusion designed, which allows combining the fundamental signal at 2.6 GHz with the respective second and third harmonic. This component is used to study the effect of the second harmonic injection on the efficiency of a 25 W GaN HEMT device. It is shown that, choosing the correct injection phase and power level, the efficiency of the device can be improved by up to 6 % compared with the classic Class-AB operation without harmonic injection.
• Broadband Doherty PA (DPA) is one of the optimistic solutions in terms of efficiency and linearity. However, a broadband impedance transformer is the bottle neck for the DPA.
• Source and load modulation using a variable lumped elements is a unique method to increase the efficiency over a wide range of the input power. However, because it changes the source impedance it can keep the linearity as high as possible during high efficiency operation. This technique, ideally, can give high efficiency over 20 dB output power back off.
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