A high efficiency broadband harmonic-controlled power amplifier (PA) features a new high cut-off bandstop network. The network comprises many parallel open-circuit microstrip lines (POCMLs) and many pairs of sectorial microwave lines. Each POCML is a quarter wavelength for different frequency points. This network improves efficiency and bandwidth by suppressing the out-of-band impedance of the output matching circuit. A high efficiency PA with more than an octave bandwidth is designed and fabricated using a 10 W GaN HEMT device. Measurements show that the PA achieves an output power (P_out) of 39.68 to 42.56 dBm, a drain efficiency (DE) of 58.3 to 79 percent and a gain of 10.68 to 13.56 dB from 0.5 to 2.5 GHz.

With the rapid development of 5G communication systems, the demand for broadband PAs capable of supporting high speed transmission is increasing.^1-13 As a core component of wireless communication systems, the high efficiency and broadband PA plays an extremely important role.

The use of the traditional continuous Class-F PA is an important approach to achieving high efficiency over a wide bandwidth. Still, its second-harmonic impedance does not have a real part, which requires controlling the second harmonic at the left edge of the Smith chart.^14-17 When the PA’s bandwidth exceeds one octave, the higher harmonic in the low band range coincides with the fundamental in the high band range, which contradicts the use of the traditional continuous Class-F PA.^18-21

To address this problem, some researchers have further expanded the PA impedance solution space to provide theoretical support for the design of broadband, high efficiency PAs. PA modules are usually connected to a filter to suppress out-of-band signals, but this may affect the performance and size of the communication system. Therefore, it is more advantageous to combine the matching circuit of the harmonic and the fundamental.

To control the harmonic, a novel bandstop network is proposed here. This network comprises several sectorial and open-circuit microstrip lines. More specifically, it uses pairs of sectorial microstrip lines corresponding to a single frequency band and POCMLs corresponding to a quarter wavelength for different frequency points. The network also has a narrow transition that provides a fast roll-off from the passband to the stopband. This controls the PA’s harmonic frequencies and the fundamental, broadening the bandwidth and improving efficiency.

To verify the approach, a highly efficient broadband extended continuous Class-F (ECCF) PA is designed and fabricated with a Cree CGH40010F 10 W GaN HEMT device. Measurements demonstrate a 58.3 to 79 percent DE from 0.5 to 2.5 GHz with a P_out of 39.68 to 42.56 dBm.

THEORETICAL ANALYSIS

Extended Continuous Class-F PA

The standard Class-F PA requires open-circuit terminations at odd harmonic frequencies and short-circuit terminations at even harmonic frequencies. The square-wave voltage waveform does not overlap with the half-rectified current waveform, and 100 percent DE can theoretically be achieved. In practice, it is generally controlled to the third harmonic. To extend the impedance design space, two factors (γ and δ) are introduced for the drain voltage as shown in Equation 1:²²

To maintain a positive voltage, γ is swept over the range between -1 and 1, while δ should be confined to between 0 and 1. The drain-voltage waveform is affected by two factors. Figures 1a–d show the variation of voltage waveforms with different values of δ. Smaller δ results in less overlap of drain voltage and current, corresponding to a higher DE. The expressions for extending the continuous impedance design space of the PA are given by Equations 2 through 4:

The fundamental and harmonic impedances obtained through Equations 2 through 4 are shown in Figure 2. The fundamental and harmonic impedances present an area distribution in the Smith chart, and some parts of the fundamental and the harmonic impedance overlap. These overlapping areas provide a theoretical basis for achieving high efficiency PAs with bandwidths greater than one octave. The expression of DE for the ECCF PA is given in Equation 5:

PA efficiency gradually decreases as δ increases (see Figure 3). For an efficiency greater than 60 percent, δ must be greater than 0.7.

Bandstop Network

The bandstop network consists of several pairs of sectorial and open-circuited microstrip lines (see Figure 4). The sectorial microstrip lines are symmetrical in pairs corresponding to a frequency band, while the open-circuited microstrip lines are asymmetrical corresponding to a quarter wavelength for different frequency points.

By inserting the sectorial microstrip line, the bandwidth of the matching circuit can be extended.²³ The sectorial microstrip line input impedance (Z’_sec) is given by Equations 6 through 10:

where α, r₁ and r₂ represent the angle, the inner radius and outer radius of the sectorial microstrip line, respectively; φ₂ is a phase constant; h is the thickness of the dielectric substrate; ε_r is the equivalent dielectric constant; λ₀ is the wavelength of free space; and J_i(x) and N_i(x) are Bessel functions of the first and second type.

In practice, it is difficult to keep the overlapped frequency band for odd and even harmonics as open and short circuits, respectively. Therefore, all odd and even harmonics are treated uniformly as harmonics. The impedance expression is given in Equation 11:

where θ=βl is the phase angle, β is the transmission coefficient, l is the electrical length of the transmission line, Z₀ is the characteristic impedance and Z_L is the load impedance. When θ is an odd multiple of π⁄4 and Z_L is infinite, Z_IN is a short circuit, i.e., Z_IN = 0. Based on this, the electrical length l is an odd multiple of a quarter wavelength. The impedance of the load (Z_L) is infinite (open circuit) and the input impedance (Z_in) is 0 (short circuit).

The bandstop network is composed of several quarter-wavelength open-circuit microstrip lines corresponding to different frequency points and several pairs of symmetrical sectorial microstrip lines, as shown in Figure 4. The sectorial lines are open-circuited for specific frequency sub-bands. This network has 2x open-circuit microstrip lines and (x-1) pairs of sectorial microstrip lines. In this work, x = 2, i.e., the bandstop network comprises four POCMLs and a pair of sectorial lines in the design of the output-matching circuit.

The target band for the bandstop network, from f_st to f_sp, is divided into six parts by f₁, f₂, f_sub, f₃ and f₄. The phase angles θ₁, θ₂, θ₃ and θ₄ are mapped to frequency points f₁, f₂, f₃ and f₄. They are determined using Equations 12 and 13.

where λ_m are the harmonic wavelengths corresponding to m open-circuited microstrip lines.

The input port impedance of the bandstop filter is set to 20 Ω to aid in matching the bandstop network to the transistor’s drain output terminal. A pair of sectorial microstrip lines corresponds to the center-frequency sub-band of the bandstop network, and each of the four open-circuited microstrip lines corresponds to a different frequency point at the ends of the bandstop network.

By adjusting the order of the open-circuit microstrip lines, the resonant points of the bandstop network are obtained (see Figure 3). The 4 and 5 GHz resonant points come from TL₁ and TL₃, as designed. The other resonant points show slight deviations in frequency. The sectorial microwave line resonance at 6 GHz and the TL₂ and TL₄ resonances at 8 and 7 GHz, respectively, are shifted due to mutual coupling between the microstrip lines. Compared with conventional filters, this filtering network exhibits a rapid roll-off from the passband to the stopband, a characteristic well-suited to the design of harmonic-control PAs.

ECCF PA Design

To verify the bandstop network, a broadband high efficiency ECCF PA is designed. The PCB is fabricated on a 30-mil Rogers 4350B substrate with an equivalent dielectric constant of 3.66. The transistor is a Cree CGH40010F GaN HEMT. A schematic of the PA is shown in Figure 5. The gate and drain bias voltages are -3 and 28 V, respectively. The transistor’s package parasitic equivalent network is also shown.²⁴ It comprises three series inductors and three capacitors in parallel to ground.

The working band of the PA is 0.5 to 2.5 GHz. The second harmonic (1 to 1.66 GHz) for the low frequency portion of the band (0.5 to 0.83 GHz) is located in the working band and the third harmonic (1.5 to 2.5 GHz) is in the working band, as well. The low frequency portion of the band’s harmonics are located in the working band.

This means that the fundamental impedance in the high frequency portion of the band and the harmonic impedance of the low frequency portion of the band overlap. However, there is some overlap between the fundamental and the harmonic impedance in the impedance design space of the ECCF PA. This can meet the design requirement of a wideband, high efficiency PA.

If the fundamental, second harmonic and third harmonic cannot simultaneously meet the requirements of the matching circuit, consider the fundamental impedance first, then the second harmonic impedance and finally the third harmonic impedance. Since second-harmonic impedance matching has a greater impact on efficiency than third-harmonic impedance matching, it should be given priority when the two conflict.

Figure 5 shows the output-matching circuit comprising the harmonic and fundamental matching circuits. The harmonic-matching circuit primarily controls the harmonic impedance and the partial-fundamental impedance. The fundamental matching circuit is used to match the fundamental impedance.

The output matching impedance at the package plane and the I-gen plane is shown in Figure 2b. It illustrates the impedance trajectories versus frequency at the I-gen plane and the package plane. The fundamental impedance at the I-gen plane is positioned in the slightly right-of-center area of the Smith chart, while the majority of the harmonic impedance plot is close to the edge of the Smith chart. The impedance of the fundamental and harmonic is transformed from the package plane to the intrinsic current generator plane through the parasitic equivalent network in Figure 5.

There are six harmonic impedance points at the I-gen plane that deviate from the ECCF design space (circled in green in Figure 2b), but their number is less than 10 percent. The fundamental and most harmonic impedances at the I-gen plane are located within the ECCF PA design space (the effect of the third harmonic on PA efficiency is generally around 1 to 2 percent, small enough to be considered negligible), as shown in Figure 2b. Therefore, this PA conforms to the ECCF mode of operation.

To meet the overall design requirements of the output-matching circuit, a slight optimization is applied to the bandstop network. With the output matching circuit completed, the input matching circuit is designed with a gradual transition for better performance. The PA module’s size is 50 by 77 mm² (see Figure 6), including copper pedestals, PCB and SMA connectors.

CIRCUIT SIMULATION AND MEASUREMENT

The simulated |S₂₁| of the original bandstop network is shown in Figure 3. It shows that the resonant frequencies at 4 and 5 GHz are as designed, while those at 6 and 7 GHz are slightly shifted to the left and right, respectively. The resonant frequency point at 8 GHz shows considerable deviation but does not affect out-of-band suppression. Out-of-band suppression from 4 to 9 GHz exceeds 30 dB.

The output power of the second and third harmonics reaches the PA output terminal, representing the power leaked through the proposed networks. Since efficiency is calculated based only on the power within the fundamental operating band, output power of these harmonics constitutes an energy loss, negatively impacting the overall efficiency.

Simulated harmonic output power of the designed PA module is shown in Figure 7. Second harmonic power from 0.5 to 1.1 GHz is higher than that from 1.2 to 2.5 GHz. This is because impedance matching of the second harmonic in the low frequency band falls within the high frequency working band, so the second harmonic cannot be suppressed. The third harmonic power from 0.5 to 1.4 GHz is higher than that from 1.5 to 2.5 GHz because the working band from 1.5 to 2.5 GHz cannot be suppressed. Second harmonic power is greater than third harmonic power, which also verifies that the influence of the second harmonic on efficiency is greater than that of the third harmonic.

The prototype PA is measured using a continuous-wave signal from 0.5 to 2.5 GHz at room temperature (see Figure 8). A 29 dBm drive level is used for all measurements. DE is between 58.3 and 79 percent. Output power ranges from 39.68 to 42.56 dBm, with an average of 41.12 dBm. Fractional bandwidth is 133 percent. Gain ranges from 10.68 to 13.56 dB.

PA performance in terms of output power and efficiency over a wide bandwidth verifies that the filtering matching network, characterized by low passband loss, high out-of-band suppression and a narrow transition band, is effective for PA matching. The performance of this PA is compared with other similar amplifiers in Table 1. Compared with previous work,^25,27,29 it achieves a wider frequency bandwidth. Compared with previous work,^26,28,30 it achieves a higher maximum power and efficiency. Compared with previous work,³¹ it provides higher gain. It shows that this bandstop network structure contributes to better overall PA performance by simultaneously optimizing output power, efficiency and bandwidth.

CONCLUSION

A high efficiency broadband harmonic-controlled PA features a new bandstop network comprising multiple POCMLs and pairs of sectorial microwave lines. Measured results yield a maximum output power of 42.56 dBm and maximum DE of 79 percent from 0.5 to 2.5 GHz. This provides a novel design approach for a broadband PA with high efficiency.

ACKNOWLEDGMENTS

This work was supported by a Project of Hangzhou Dianzi University (Grant KYS045624199), Zhejiang Provincial Science and Technology Plan (Grant:2024C01076) and a Project of the Ministry of Science and Technology (Grant D20011).

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