Microwave Journal
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Design of a Broadband Harmonically-Tuned Power Amplifier with Gate-Source Parasitic Compensation

November 10, 2019

A 1.5 to 2.6 GHz broadband power amplifier (PA) based on a GaN HEMT is designed to achieve high-power and efficiency. The transistor’s gate-source parasitic effect is reduced with a novel parasitic compensation circuit at the input. To achieve a broadband harmonic match and expand its bandwidth at the fundamental, radial microstrip theory is employed in a harmonic control network. The output power is between 43.4 and 45.6 dBm, and its drain efficiency is 65 to 76.9 percent from 1.5 to 2.6 GHz. Gain is above 10 dB. Second and third harmonic suppression levels are −15.6 to −26.1 and −19.4 to −40.5 dBc, respectively. The measured results are consistent with the simulation.

With the development of wireless communication technology, the need for higher data rates and correspondingly wider bandwidths is growing rapidly. At the RF transceiver, this puts a greater demand on the PA. As it consumes the most prime power, PA efficiency has a large impact on the operating budget of a communications system.

Traditional PAs, i.e., classes A, AB and B, are inefficient. To save energy and increase signal coverage, communication system PAs usually employ high efficiency and high output modes, such as classes D,1 E,2 F3-5 and inverse class F.6 They also leverage the excellent performance of third-generation semiconductor transistor technology, such as GaN HEMT.7-8

CLASS F PA ANALYSIS AND DESIGN

Since the PA operates in a large signal state, harmonics are inevitably generated, reducing output power and efficiency. Class F PAs, however, achieve good performance through harmonic control. The voltage and current waveforms at the transistor drain are shaped with a harmonic control network so the voltage waveform is a square wave, and the current waveform is a half sine wave. The waveforms of the drain voltage and current are expressed as

Figure 1

Figure 1 GaN HEMT model.

According to theory,5,9 even harmonics are matched to 0 Ω, odd harmonics to infinity and the fundamental to 50 Ω. Due to deficiencies of the class F PA in practical designs, this work describes two improvements: one, because the gate-source parasitics of the GaN HEMT transistor degrade the output power and efficiency of the PA, a gate-source parasitic compensation circuit is employed. And two, because the traditional class F PA uses a high Q output impedance matching transformer, limiting its bandwidth, this work uses low Q radial microstrip lines to control harmonics.

Gate-Source Parasitic Compensation

The GaN HEMT used in this article is an active nonlinear device, with harmonics caused by its parasitics. Gate-to-source parasitics cause the input of the transistor to deviate from a pure sinusoidal wave, reducing PA performance. The transistor model, shown in Figure 1, includes the parasitic as well as intrinsic elements. Gate-source parasitic parameters include the gate-source capacitance, Cgs, gate parasitic inductance, Lg, and parasitic resistance, Rg. Cgs is given by the expression

where ε is the dielectric constant of the GaN material, and d is the equivalent depletion depth. The gate parasitic inductance and parasitic resistance are found from

where m is the grid index, u0 is the permeability in a vacuum and ρ is the conductivity of the gate metal.

The gate-source parasitic parameters are calculated from Equations 3 and 4 and are used in the design of the input circuit (see Figure 2). Microstrip lines TL2, TL3 and TL4 compensate for the influence of Cgs at the input, where the use of stepped-impedance matching increases the bandwidth. Microstrip line TL1 offsets the effect of parasitic inductance Lg and resistance Rg. The gate-source compensation suppresses the input harmonics, which increases output power and efficiency.

Figure 2

Figure 2 PA matching circuit design.



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

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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