A 500-600 MHz GaN power amplifier with RC-LC stability network

    Corresponding author: Xinyu Ma, maxinyu1993@126.com
  • School of Microelectronics, Xidian University, Xi'an 710071, China

Key words: P-bandGaNstability networkpower amplifierhigh efficiency

Abstract: A 500-600 MHz high-efficiency, high-power GaN power amplifier is designed and realized on the basis of the push-pull structure. The RC-LC stability network is proposed and applied to the power amplifier circuit for the first time. The RC-LC stability network can significantly reduce the high gain out the band, which eliminates the instability of the power amplifier circuit. The developed power amplifier exhibits 58.5 dBm (700 W) output power with a 17 dB gain and 85% PAE at 500-600 MHz, 300 μs, 20% duty cycle. It has the highest PAE in P-band among the products at home and abroad.

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1.   Introduction
  • With the rapid development of the modern radio systems, communications equipment and radar performance have achieved unprecedented progress. As one of the most important and largest power consumption modules, the power amplifier directly affects the performance of a radio system[1, 2]. Power devices based on Si, GaAs or other materials are limited by gain, bandwidth, thermal stability and output power density. When the required power is reached, the size of the power amplifier is relatively large. The power amplifier cannot achieve high efficiency with the use of GaAs devices[3, 4]. It is gradually unable to meet the requirements of current communications and radar equipment for the high power and high efficiency. So the power amplifier is still under constant development[5]. For example, the power amplifier MMRF1016H in this band of NXP, working in the pulse (100 $\mu $s, 10% duty cycle) state, the output power is 600 W and the drain efficiency is about 60%.

    In recent years, the third generation of semiconductor materials, such as the GaN, SiC, which is the representative of the wide band gap semiconductor materials that can withstand higher electric field, has higher electron mobility and high thermal conduction efficiency. So it can work under high voltage conditions, with smaller parasitic parameters, achieve high power and wide bandwidth in a smaller volume[6]. When the GaN material is used in the production of a power amplifier, it is necessary to meet the requirements about gain, efficiency, power, flatness and others, and the most important thing is stability. GaN HEMT has a higher power density, so it is more prone to instability in low frequency[7, 8].

    To eliminate the instability of GaN power amplifier at low frequency, the RC-LC stability network structure is proposed, by connecting the stability network in front of the input-matching network. The RC-LC network forms a more perfect band-pass filter than the traditional RC structure. The inductor in the stability network adds a first-order filter and the stability circuit leads to a mismatch out of the band. Which greatly reduces the gain in low-frequency band and significantly increases the stability of the circuits.

2.   RC-LC stability network

    2.1.   RC-LC structure

  • In order to improve the stability of low-frequency, high-power power amplifier, commonly used methods are the following. For example, putting some consuming components such as resistors in series or parallel in front of the input circuit; putting RC network parallell to the ground or adding negative feedback. These methods stabilize the amplifier, but meanwhile, the consuming components will reduce the gain of the power amplifier, which affects the efficiency of the power amplifier[9].

    The RC-LC stability network is proposed in this paper, as shown in Fig. 1. The concept of the filter is applied to the design of the power amplifier, which significantly improves the stability of the power amplifier without affecting the gain and efficiency.

  • 2.2.   Simulation of RC-LC structure

  • The $S_{11}$ curve is obtained by simulating the circuit in Fig. 1 and traditional RC stability circuit, as is shown in Fig. 2. It can be seen that this stability network is well-behaved between 500-600 MHz and does not affect the standing wave of the power amplifier circuit.

    The $S_{\mathrm{21}}$ curve is compared between the RC-LC structure that is proposed in this paper and traditional RC stability structure, as is shown in Fig. 3. It can be seen that the RC-LC stability network increases the insertion loss out of band, compared with the traditional RC stability structure, which means the RC-LC is more effective in improving stability. The stability of the power amplifier is measured by $K$ (StabFact1).

    The power amplifier circuit is absolutely stable when $K>1$, otherwise the input and output circuits should be adjusted or the stability network be added to avoid instability.

    Putting the RC-LC stability network into the power amplifier circuit, simulation software is used to simulate the gain and stability of the power amplifier circuit, and the simulation results are compared with the power amplifiers with the traditional RC stability network. The results are shown in Figs. 4 and 5. It can be seen that the gain in low frequency is significantly inhibited, and the stability of the power amplifier circuit is significantly improved with the using of RC-LC stability network, eliminating the instability of GaN power amplifier caused by the high gain in low frequency.

3.   Power amplifier circuit design

    3.1.   Circuit inside tube

  • When designing a power amplifier, the total gate width of power chips have a very significant impact on output power and efficiency of the power amplifier[10]. Limited to the output capacity of signal unit cell, parallel connections are used to increase the total gate width, thereby obtaining the corresponding output power considering the impedance of high-power GaN power amplifier is low[11]. In order to reduce the difficulty of matching circuit design, the L-C-L matching method is used in the tube to improve the device's input impedance[12]. The equivalent circuit is shown in Fig. 6.

  • 3.2.   Matching circuit design

  • In the designing of the input and output matching circuits of the power amplifier, the input circuit mainly affects stability, insertion loss and gain flatness; the output circuit mainly affects output power and efficiency[13, 14]. First, the input and output circuits are built and simulated by ADS software, and then the amplifier is tested and debugged. Eventually the required power amplifier is obtained.

    The traditional push-pull power amplifier circuit is optimized by adding the RC-LC stability network. The schematic of the push-pull circuit is shown in Fig. 7.

    The signal produces a 180 degree phase difference through the balun structure at the input circuit, and respectively can be amplified by the power dies, then gets the same phase and is synthesized by the balun structure at the output circuit[15].

    Balun structure can increase bandwidth, gain and is helpful to amplify power and harmonic suppression in signal transmission[16]. The push-pull structure with RC-LC stability network achieves flat power gain, better matching characteristics and high stability. Therefore, the push-pull balanced structure with a RC-LC stability network can ensure good stability and increase the amplifier's bandwidth and power gain[17, 18].

4.   Circuit simulation and testing

    4.1.   Circuit simulation

  • The model of the GaN HEMTs that we used in this simulation is developed by The 13th Research Institute, CETC. Electromagnetic field simulation is used in the simulation of power die, tube, bonding wire, single layer capacitor and other passive components to improve the accuracy of simulation[19]. The simulation of the schematic does not take into account the coupling among the devices, so there will be a certain gap compared with the actual circuit. So the simulation of the circuit layout is necessary to improve the accuracy.

    After debugging and optimization, the schematic of the circuit is completely replaced by the layout. After simulation with ADS software, results are as follows: $S_{11}$ is less than $-15$ dB, the output power is greater than 59 dBm (800 W), the PAE is greater than 80%, as shown in Fig. 8. Meanwhile, the flatness is less than 0.3 dB and the gain is 17.5 dB with the power amplifier working in 500-600 MHz, showing an excellent performance.

  • 4.2.   Power amplifier testing

  • After ADS simulation, the circuit is produced. The total size of this power amplifier circuit is 110 $\times$ 60 mm$^{\mathrm{2}}$, as shown in Fig. 9. An RF circuit testing system (network analyzer, signal source, power supply, microwave power sensor, attenuator) is used to test this RF power amplifier, as shown in Figs. 10 and 11. In order to achieve a good impedance matching with the testing system, RF connector is welded to a 4 mm long 50 $\Omega $ microstrip line. Setting the power amplifier bias condition: $V_{\mathrm{d}} =48$ V, $V_{\mathrm{g}} = -2.3$ V, input signal is 300 $\mu $s, 20% duty, $P_{\mathrm{in}}=41.5$ dBm. The test results between 500-600 MHz, are as follows: Gain > 17 dB, $P_{\mathrm{out}} > 58.5$ dBm (700 W) with a maximum $P_{\mathrm{out}}$ of 58.8 dBm (760 W), PAE > 85% with a maximum PAE of 87%, as shown in Fig. 12.

5.   Conclusion
  • A method is proposed to improve the stability of a high-power, low-frequency power amplifier, that is, put the RC-LC stability network into the input circuit of the RF power amplifier. The RC-LC stability network reduces high gain out the band without affecting the bandwidth and the gain inside the band, which significantly improves the stability of the power amplifier circuit. The developed power amplifier exhibits 58.5 dBm (700 W) power output with a 17 dB gain and 85% PAE at 500-600 MHz, 300 $\mu $s, 20% duty cycle. It has the highest PAE in P-band among the products at home and abroad.

Figure (12)  Reference (19) Relative (20)

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