Microwave Journal
www.microwavejournal.com/articles/23383-second-harmonic-tuning-for-a-broadband-high-efficiency-gan-power-amplifier

Second Harmonic Tuning for a Broadband High Efficiency GaN Power Amplifier

November 15, 2014

Figure 1

Figure 1 Harmonically-tuned power amplifiers: traditional design (a) wideband design (b).

A new method for designing a harmonically tuned wideband power amplifier (PA) achieves high efficiency over a wide bandwidth. The operating bandwidth is divided into two bands in which the second harmonic output impedances of each are controlled for higher power added efficiency (PAE) and output power. To validate this approach, a broadband GaN power amplifier is designed to operate in the 2.1 to 2.7 GHz range. The frequency band is divided into two bands, 2.1 to 2.4 GHz and 2.4 to 2.7 GHz with their second harmonic output impedances each controlled to improve PAE and output power. The measured power output is greater than 40 dBm and the efficiency is better than 60 percent between 2.1 and 2.7 GHz. The power gain is better than 12 dB and the flatness is less than 1.5 dB in the design band.

In modern cellular base stations, output power, efficiency and linearity are the key requirements in the design of the PA. But with the growing number of wireless services, there is an increasing demand for wideband communication systems. The narrow frequency band power amplifier cannot meet future demands for massive data transmission; so bandwidth is another important design specification.

Although very high efficiencies have been recently reported for Class-F and Class-F-1PAs,1-3 the use of harmonic traps in the load network makes the bandwidth of these amplifiers rather narrow. Some wideband switched power amplifiers have been designed, including wideband Class E PAs,4-6 but are limited by load conditions. Harmonic tuning is good for high efficiency,7-10 but is not suitable for broadband operation. If the operating band is segmented, however, we show that harmonic tuning can provide broadband performance.

HARMONICALLY TUNED PA DESIGN

Generally, the second and the third harmonic output impedances are important for efficiency and output power. Impedances at higher harmonics have less effect on efficiency and increase the complexity of the circuit. Thus, in order to simplify the circuit and improve efficiency, only the second harmonically tuned GaN power amplifier is considered.

Figure 1a shows a typical second harmonically tuned circuit. The length of the TL2 transmission line is λ/4 at the fundamental center frequency (ƒ0), open for odd harmonics, but shorted for even harmonics. The optimal reflection coefficient for the second harmonic (Γ2ƒ0) is obtained with the TL2 and TL1 transmission lines. Over a broad bandwidth, however, the λ/4 transmission line is not a perfect short at the second harmonic, and in turn, the reflection coefficients of second harmonic frequencies for wideband operation are not optimum. In order to optimize second harmonic output impedances for wideband performance, the operating band is divided into two frequency bands (low band and high band) and the harmonic impedances are separately controlled for each band.

Figure 2

Figure 2 Efficiency and output power versus second harmonic load reflection coefficient ( f = 2.4 GHz, VDD = 28 V, VGS = –2.7 V).

Figure 3

Figure 3 Optimal second harmonic reflection coefficient.

In the new circuit, shown in Figure 1b, the second harmonic reflection coefficients (Γ2ƒ1, Γ2ƒ2) are determined by TL1, TL2, TL3 and TL4. TL1 and TL2 are used to tune the low band, while TL1, TL3 and TL4 are used to tune the high band. The length of TL1 is λ/4 at the low band center frequency (ƒ1) and length of TL4 is λ/4 at the high band center frequency (ƒ2).

Figure 4

Figure 4 Matching scheme of the harmonically-tuned power amplifier.

Figure 5

Figure 5 Simulated drain voltage and current waveforms (VDD = 28 V).

To determine the effect of the harmonic terminations at the output, the second harmonic reflection coefficients are swept during load-pull simulations while the fundamental source and load impedances are kept constant. Figure 2 shows the efficiency and output power of the device versus phase variation of the second harmonic reflection coefficient at different reflection coefficient magnitudes ranging from 0.3 to 0.9, with an interval of 0.2. The change of efficiency and output power with phase is apparent for a fixed magnitude of reflection coefficient. In particular, when the phase = 220°, PAE and Pout are significantly lower. In the process of designing the power amplifier, these phases should be avoided.

In general, the optimal second harmonic reflection coefficient is

Γsecond,opt=
magnitude.exp(j.phase)                         (1)

in which the magnitude is close to 1 on the edge of the Smith Chart (see Figure 3). The second harmonic input reflection coefficient, which also has an impact on efficiency and output power can be adjusted with the input λ/4 line of the DC choke.

The design band is 2.1 to 2.7 GHz, which includes a number of communications standards like the W-CDMA downlink bands of 2.11 to 2.17 GHz, and 2.6 GHz as defined by TD-LTE. A 10 W GaN HEMT (CGH40010F) from Cree Inc., is selected as the PA device. Input and output fundamental impedances and output second harmonic impedances are obtained from load and source-pull measurements. Given that the length of TL2 is λ/4 at 2.2 GHz, TL1’s electrical length is adjusted to provide suitable second harmonic output impedances over the operating band of 2.1 to 2.4 GHz. Given that the length of TL4 is λ/4 at 2.6 GHz, TL3’s electrical length is adjusted to provide suitable second harmonic output impedances over the operating band of 2.4 to 2.7 GHz. Finally, input and output wideband matching is performed at the fundamental. The complete matching scheme is shown in Figure 4.

Figure 5 shows the simulated drain voltage as a function of time for different frequencies. With optimized fundamental and harmonic impedances, the voltage waveform is similar to the ideal voltage waveform when the maximum PAE is achieved at an output power of 10 W. The bottom of the voltage waveform is flat and there is little overlap of current and voltage between 2.1 and 2.7 GHz, so the power dissipation is small.

Figure 6

Figure 6 Photograph of the assembled harmonically tuned power amplifier.

EXPERIMENTAL CHARACTERIZATION

The broadband second harmonically tuned PA is fabricated on a dielectric substrate (RO4350, εr=3.66, h=0.508 mm) as shown in Figure 6. With a continuous wave (CW) input signal and biased with VDD=28 V and VGS=-2.7 V, the PA has a small-signal gain higher than 14 dB in the design frequency band, with a return loss better than 10 dB. Figure 7 shows good agreement between simulation and measurements.

Figure 7

Figure 7 Simulated and measured |S11|, |S21| versus frequency.

Figure 8

Figure 8 Simulated and measured output power (Pout), PAE and gain versus frequency.

Measured output power, gain and PAE are shown in Figure 8. An output power of at least 40 dBm and efficiency of greater than 60 percent are achieved across the operating band. The efficiency between 2.0 to 2.2 GHz and 2.55 to 2.7 GHz is better than at 2.4 GHz due to benefits of second harmonic tuning in the low and high bands. The gain is better than 12 dB in the 2.1 GHz and 2.7 GHz range, with the gain flatness less than 1.5 dB. Good agreement is achieved between the simulations and experimental results.

CONCLUSION

A new method for the design of broadband harmonically tuned power amplifiers is described and demonstrated. The operating frequency band is divided into two for the control of second harmonic impedances to obtain high PAE and output power across the entire band. Performance of a wideband, high efficiency PA built using this design approach agrees closely with simulation.

References

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