This work presents a gallium nitride (GaN) high electron mobility transistor (HEMT)based cascaded multistage power amplifier (MPA) in class-AB for L-band radar applications. The purpose of this endeavour is to develop an MPA using GaN HEMT devices to achieve optimised parameters such as high gain, high power, better efficiency, and linearity in a compact size. In an MPA design with multiple stages, oscillations are common owing to unwanted high gain at the lower frequency range. To overcome this issue, we introduced interstage harmonic termination networks as a novel approach to suppress high gain at low frequencies, which are prone to oscillations. The proposed cascaded MPA provides the maximum radio-frequency output power of 89 W and a power gain of 52 dB with an associated power-added efficiency of 51%. Second and third harmonic levels are −32.5 and −37 dBc, respectively. Two-tone measurements are performed with a frequency separation of 10 MHz, and an intermodulation level of less than −33 dBc is achieved.
| INTRODUCTIONPower amplifiers (PAs) are widely used in wireless communication and radar applications. Microwave frequencies (UHF, Lband, S-band etc.) have a dominant role in commercial and defence applications. Modern communication standards such as GSM, CDMA, W-CDMA, and WiMAX with digital modulation schemes, such as QAM, QPSK, DQPSK, OFDM, are characterised by varying signal envelopes with a high peak-toaverage power ratio [1,2]. To fulfil the requirements of communication standards, PA design includes the frequency of operation, output power, bandwidth, efficiency, gain, linearity, and cost. However, it is challenging to achieve optimum values of all parameters simultaneously [3,4]. Therefore, trade-offs must be made, and a subset of the desired parameters can be satisfied depending on the specific needs of the application. Some traditional trade-offs are gain versus bandwidth, operating frequency versus output power, and linearity versus efficiency [4]. In wireless communication, PA has the main role of amplifying the desired modulated signal before transmission [5]. Thus, multistage PAs (MPAs) are designed to accomplish the need for a link with high gain and high output power. In MPA design, a critical concern is the possibility of oscillations caused by instabilities including very high gain, positive feedback, resonant tanks, poor isolation between DC and radiofrequency (RF) paths, and parametric oscillations of nonlinear capacitances. These instabilities are particularly worse at lower frequencies owing to high gain, and detection to avoid such oscillations is always tedious [5,6].Gallium nitride (GaN), a wideband gap semiconductor, has emerged as a potential candidate to fulfil the requirements of current and future communication systems [7,8]. GaN high electron mobility transistor (HEMT)-based PAs provide a better trade-off with high output power with a compact size, owing to their high output power densities, compared with travelling wave tube amplifiers [9]. Gurdal et al. demonstrated GaN-based...
