**Design examples of GaN HEMT power amplifiers**

170 Wireless Communications and Networks – Recent Advances

Fig. 11. Power sweep simulation without and with the description of the knee-walkout

small companies and educational institutions with low budget.

the external drain port (Schmelzer and Long, 2007).

As discussed before, large signal model is required in order to describe nonlinearities of the power device. However, device modeling is a complex task which requires extensive experience of modeling engineers and special modeling software, so that power amplifier design engineers are mostly forced to rely on the large signal model provided by device's manufacturer. Due to progress in RF measurement techniques, a measurement system has been developed which allows measurement of the so-called X-parameters (Betts et al, 2011). Unlike with S-parameters, not only small signal behavior of the device can be described, but also nonlinearities arising under large signal conditions. In general, the input signal power is swept and the output response at the fundamental as well as at higher harmonics is measured. The measured information is then concluded into the Xparameter set which can be directly used in the circuit simulation software as the device's behavioral model. This kind of device modeling is very convenient and can be combined with source and load tuners to obtain load dependence of the X-parameters. In addition, the extracted behavioral model is accurate, robust and does not require large computational resource. However, the behavioral model cannot provide insights into physical properties of the device and the measurement setup is relatively expensive for

Packaged transistors comprise also parasitic components of the package and bond wires. These typical parasitic inductance and capacitance can compromise the performance of the amplifier circuit especially at high frequencies. For example, for class F amplifiers where short or open circuit must be provided at the drain node of the transistor at harmonic frequencies in order to shape the output current and voltage waveforms for high efficiency. Optimization for efficiency can be done best, if the package model of the transistor is known. The current and voltage waveforms which are optimized for minimum overlap should be presented at the internal drain node of the device inside the package and not at

compared to measured values.

**Large signal behavioral model** 

**Package modeling** 

As examples, two GaN power amplifiers are presented. The first one is a 2.45 GHz GaN HEMT class AB power amplifier (Monprasert et al, 2009). This power amplifier is intended for the use in a WLAN system. The power transistor used in this amplifier is NPTS00004 GaN HEMT from Nitronex Corporation. The performance of the 2.45 GHz power amplifier is shown in Table 2. The drain supply voltage was varied with Vdsq=20V and 28V. For the drain supply voltage of 28V, the output power is not as high as in the case with Vdsq=20V since the drain current was increased as the device started to be saturated. The DC power exceeded the limit of 7 Watts given in the datasheet and the device was damaged. Fig. 12 shows a photograph of the fabricated class AB amplifier.


Table 2. Measured performance of 2.45 GHz GaN HEMT class AB power amplifier.

Fig. 12. Fabricated 2.45 GHz GaN HEMT class AB power amplifier.

Another design example is the VHF class E power amplifier (Khansalee et al, 2010). Using the same GaN power device Nitronex NPTB00004, a class E power amplifier for the operating frequency from 140 MHz to 170 MHz has been designed and fabricated. The values of load network L, C, L0 and C0 (see Fig. 13.) were determined using equations in the work published by Gebrennikov (Gebrennikov, 2002).

Fig. 13. Schematic of class E power amplifier with parallel circuit.

The optimal load impedance was determined using load pull simulation in Advance Design System (ADS). Simulated drain voltage and current waveforms show that class E operation is achieved (see Fig. 14.).

Fig. 14. Simulated drain current and voltage waveforms of the class E amplifier.

The fabricated class E power amplifier delivers maximum output 33.9 dBm, peak Power-Added Efficiency (PAE) of 72.5% and power gain of 16.4 dB at the center frequency of 155 MHz. Fig. 15. shows output power, efficiency and gain over the required operating frequency from 140 MHz to 170 MHz. A photograph of the fabricated GaN class E amplifier is depicted in Fig. 16.

Fig. 15. Simulation and measurement results of power gain, output power, and PAE over the frequency 140 MHz to 170 MHz at input power of 18 dBm with the drain supply voltage of 24 V and gate supply voltage of -1.4 V over frequency.

Fig. 16. Fabricated class E GaN VHF power amplifier.

172 Wireless Communications and Networks – Recent Advances

The optimal load impedance was determined using load pull simulation in Advance Design System (ADS). Simulated drain voltage and current waveforms show that class E operation

> **Drain Voltage Drain Current**

Fig. 14. Simulated drain current and voltage waveforms of the class E amplifier.

The fabricated class E power amplifier delivers maximum output 33.9 dBm, peak Power-Added Efficiency (PAE) of 72.5% and power gain of 16.4 dB at the center frequency of 155 MHz. Fig. 15. shows output power, efficiency and gain over the required operating frequency from 140 MHz to 170 MHz. A photograph of the fabricated GaN class E amplifier

time, nsec

Time (ns.)

34 36 38 40 42 44 46

800


Power-Added Efficieny (%)

Power-Added Efficienc

y (%)

I\_ds1.i, mA

Drain Current (mA)

Fig. 15. Simulation and measurement results of power gain, output power, and PAE over the frequency 140 MHz to 170 MHz at input power of 18 dBm with the drain supply voltage

**130 135 140 145 150 155 160 165 170 175**

Frequency (MHz)

Frequency (MHz)

 Output Power (simulation) Output Power (measurement) Power Gain (simulation) Power Gain (measurement) PAE (simulation) PAE (measurement)

of 24 V and gate supply voltage of -1.4 V over frequency.

Output Power (dBm), Power Gain(dB)

put Power (dBm), Power Gain (dB)

Out

is achieved (see Fig. 14.).

Vds, V

Drain Voltage (V)

80


is depicted in Fig. 16.
