# A Broadband Analog Predistortion Linearizer Based on GaAs MMIC for Ka-Band TWTAs
###### Abstract  
In this article, a Ka-band broadband analog predistortion (APD) microwave monolithic integrated circuit (MMIC) with independent tunability based on a 0.15 \(\upmu\)m GaAs pHEMT process is proposed, which can be cascaded in front of traveling wave tube amplifiers (TWTAs) to improve their linearity. The influence of different diode sizes on the parameters of Schottky diodes is analyzed and used to design the gain and phase nonlinear branches. The broadband APD MMIC is realized based on a dual-branch vector synthesis design and nonlinear frequency adjust module (NFAM). The independent tunability and broadband characteristics of the APD MMIC are verified by simulated and measured results with an error of less than 5%. Furthermore, a Ka-band 60 W TWTA is linearized by the APD MMIC, and the gain and phase compressions are reduced from 8 dB and 50\({}^{\circ}\) to within 3 dB and 12\({}^{\circ}\), respectively. The third-order intermodulation (C/IM3) is greater than 28 dBc and noise power ratio (NPR) is greater than 15.7 dBc at 3 dB output power backoff (OPBO) over the operating band of 25.1-27.5 GHz, indicating that the APD MMIC can improve the nonlinearity of TWTA effectively under broadband signals. analog predistortion (APD); broadband; pHEMT Schottky diode; microwave monolithic integrated circuit (MMIC); traveling wave tube (TWT); independently tunable linearizer (ITL) 2023 121 122  
power amplifiers is compensated more accurately and integration with other microwave monolithic integrated circuits (MMICs) is facilitated. Circuit structures adopting dual-branch vector synthesis designs are often used in APD circuits because of their flexible nonlinear adjustability and broadband characteristics. In [11], a dual nonlinear branch vector synthesis technique was adopted to improve APD accuracy but the circuit's details are not provided. A dual-branch vector synthesis APD circuit structure with independently adjustable Ka-band gain and phase expansion was proposed in [12]; however, the use of PIN diodes is a barrier to the realization of linearizer based on standard commercial GaAs MMIC structures. A power-dependent impedance matching network was used to achieve shape-tuning in the PD linearizer of [13]. However, the technology is not suitable for broadband applications due to its reflective predistortion circuit structure. In [14; 15; 16], broadband dual-branch vector synthesis predistortion circuits were proposed. The circuits were all implemented using a single-layer ceramic substrate with a large area and low integration potential. In [14], multiple device types were adopted, such as phase shifters, fixed delay lines, and adjustable attenuators. In [17; 18], a linearization technique was introduced where the signal sampled from the output is processed and combined with the input signal to excite the amplifier, which needs to be co-designed along with the power amplifier. In [19; 20; 21; 22; 23], the predistortion function was realized using parallel cold-mode HEMTs or diodes, which allowed for only a single function and had poor adjustability. In such cases, the predistortion circuit must be designed together with the amplifier to form a linearized solid-state power amplifier, which is not suitable for the linearization of TWTAs. In this article, a broadband APD MMIC with an area of 1.6 mm \(\times\) 2.0 mm for TWTAs is presented based on a dual-branch vector synthesis design. The APD MMIC has a small size and flexible nonlinear adjustability, it can be designed independently from the TWTA and easily integrated with other MMICs. In Section 2 of this article, the APD topology and diode model parameters' variation caused by different diode sizes in the 0.15 \(\upmu\)m GaAs pHEMT process are analyzed. In Section 3, a nonlinear cell circuit based on diodes is proposed and two nonlinear branches are used to form an APD circuit in which the gain and phase expansion can be independently adjusted.