Broadband Harmonic Control

A radial microstrip line, commonly found in mixers and filters, is used in the active bias circuit. The radial microstrip stub input reactance is given by the equations

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where Ji(x) and Ni(x) are i-order Bessel functions of the first and second classes, α is the angle of the radial microstrip line, εre is the equivalent dielectric constant and λ0 is the free space wavelength. r and R are the inner and outer radii of the radial microstrip line, respectively, and h and w are the dielectric substrate thickness and microstrip width, respectively. The relationship between frequency, impedance, radius and angle of the radial microstrip line is shown in Figure 3.5-8 The open microstrip line is the equivalent of a capacitor.

Figure 3

Figure 3 Radial microstrip stub Xin vs. frequency and angle (a) and Xin vs. frequency and radius (b).

In the broadband harmonic control network topology shown in Figure 2, the third harmonic impedance can be obtained from

Figure 4

Figure 4 Drain voltage and current waveforms simulated with ADS.

Figure 5

Figure 5 Simulated second and third harmonic impedances with broadband harmonic matching and gate-source parasitic compensation.

Figure 6

Figure 6 Fabricated PA.

Figure 7

Figure 7 Measured vs. simulated output power, drain efficiency and gain vs. frequency.

Figure 8

Figure 8 Measured drain efficiency and gain vs. input power at 1.8, 2.1 and 2.4 GHz.

Figure 9

Figure 9 Second and third harmonic suppression vs. frequency.

where Z7 and Z8 are the characteristic impedances of microstrip lines TL7 and TL8. The dimensions l7 and l8 are the lengths of the microstrip lines, respectively. The lengths are determined by Equation 10 so the third harmonic is open circuited. At 2f0, the microstrip lines TL5 and TL6 are stepped to match the second harmonic impedance to 0. Radial stub 1 plays a role expanding the bandwidth.

Simulations of the drain voltage and current waveforms (see Figure 4) show the voltage and current do not overlap in the crests and troughs, which enhances efficiency. By compensating the gate-source parasitic effect and using the broadband harmonic matching circuit, the second and third harmonic impedances of the PA are maintained in the low and high impedance regions, respectively, as shown in Figure 5.

FABRICATION AND MEASUREMENT

The GaN HEMT used in this design is Wolfspeed’s CGH40025F. The broadband PA is fabricated on a Rogers 4350B substrate, which has a dielectric constant of 3.66 and a thickness of 0.762 mm (see Figure 6). The gate bias is −3 V, operating the device class B. To obtain higher power, the drain voltage is set to 32 V instead of the recommended 28 V. The amplifier is operated CW.

The measured output power, drain efficiency and gain are shown in Figure 7 and compared with the simulated performance. The measured output power is between 43.4 and 45.6 dBm between 1.5 and 2.6 GHz, with the drain efficiency between 65 and 76.9 percent. The gain is greater than 10 dB. The maximum measured output power is 45.6 dBm at 1.5 GHz, and the minimum is 43.4 dBm at 2.6 GHz. The maximum measured drain efficiency is 76.9 percent at 1.8 GHz.

Measured drain efficiency and gain versus output power at 1.8, 2.1 and 2.4 GHz, respectively, is plotted in Figure 8. These frequencies are chosen to represent the entire frequency range, with 1.8 and 2.4 GHz the lower and higher frequencies, 2.1 GHz the center. As the input power increases, the drain efficiency gradually increases; when the input power reaches a certain level, the gain begins to drop rapidly. The decrease in gain indicates a linear loss and shows that high efficiency and high linearity are difficult to obtain simultaneously. The two parameters must be weighed in the PA design.

Figure 9 shows measured second and third harmonic distortion levels relative to the fundamental. Suppression of the second and third harmonics are 15.6 to 26.1 and 19.4 to 40.5 dBc, respectively.

For comparison, recent broadband PA results are shown in Table 1. The design described here demonstrates greater output power and drain efficiency with equivalent gain over a similar operating band.

CONCLUSION

This article discusses two innovative improvements in wideband PA design: a novel gate-source parasitic compensation circuit reduces the influence of harmonics caused by GaN HEMT gate-source parasitics. At the same time, a broadband harmonic control network increases PA bandwidth. Overall performance results demonstrate an advance in the state of the art.

ACKNOWLEDGMENT

This work is supported by Key Project of Zhejiang Provincial Natural Science Foundation of China (No. LZ16F010001), Zhejiang Provincial Public Technology Research Project (No. 2016C31070) and National Natural Science Foundation of China (No. 61306100).

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