A STUDY ON BROADBAND GAN PHEMT POWER AMPLIFIER USING NON-FOSTER MATCHING

Abstract

In this thesis, a study on broadband GaN pHEMT power amplifier (PA) using non-Foster matching is presented. A watt-level PA with multi-octave bandwidth is required for broadband applications such as jamming system for electronic warfare (EW). To guarantee the high power operation, GaN device is suitable due to its high power density and high voltage operation. Therefore, in this thesis, the high PAs are fabricated through a GaN device. For broadband operation, a new wideband PA structure with high gain and high efficiency is proposed. The new broadband PA using non-Foster circuit (NFC) is referred to as a negative impedance matched power amplifier (NMPA). The bandwidth limitation from high-Q interstage matching is overcome through the use of negative capacitor, which is realized with a negative impedance converter (NIC) using the cross-coupled GaN FETs. However, since the negative impedance transducer also has a frequency limit, the following design strategy has been established. For high power operation over the entire bandwidth, the natural interstage matching is optimized for the upper sub-frequency band and the lower sub-frequency band is compensated by the negative capacitance (NC) presented by NFC. For this strategy, detailed analysis is performed to understand the frequency limitation of NIC approach, which shows that high-frequency limit comes from the self-resonance and the low-frequency limit from the power handling capability. Besides, to overcome the frequency and power limits of NFC, a cascaded stage negative impedance converter (CSNIC) structure is proposed with improved positive loop gain. In addition, the cause of the NIC loss at the high frequency is also analyzed and solved using CSNIC. Two NMPAs with NIC and a NMPA with CSNIC are fabricated with commercial 0.25-μm GaN pHEMT process. The implemented PA with 2× combining shows the output powers of 35.7-37.5 dBm with the power added efficiencies (PAEs) of 13-21% from 6 to 18 GHz. The 4× combining PA achieves over 5 W output power from 7 to 17 GHz. The NMPA with CSNIC shows the output powers of 7.6-10.4 W with the PAEs of 16-23% from 7 to 18 GHz. At frequencies, where NFC is optimized for interstage matching, the power improvement by 1.2 dBm and PAE improvement by 5.7% have been achieved. The NFC boosts the efficiencies and power below 12 GHz to achieve broadband performance without using any lossy matching or negative feedback. This work also demonstrates that the CSNIC overcomes the frequency and power capability limit of the conventional NIC. To our knowledge, this is the first demonstration of NIC-based broadband amplifiers with Watt-level output power. The NMPA can provide a new perspective in designing the broadband PAs.