Design of Balanced Amplifier for Protection Against Load Mismatch in Plasma Applications

This article proposes a new design method for a balanced amplifier using a reactance elimination technique and a complex conjugation property to provide the output voltage balance and the voltage standing wave minimization of individual power amplifier (PA) components in plasma systems. The reactance elimination scheme is applied to both outputs of individual PAs for their output impedance conditions, being real and lower than the characteristic impedance of the RF system. For the complex conjugation property between both through and coupling coefficients, transformer-based quadrature couplers are adopted. The presented balanced amplifier consists of two sets of PAs, transformer-based quadrature couplers, and reactance cancellation components. The designed balanced amplifier was verified by applying it to a vacuum ultraviolet (VUV) plasma lamp system. In measurement, the proposed balanced amplifier provided the output voltage difference and the maximum output voltage reduced by 20.78 and 2.22 dB, respectively, compared with the conventional structure. Furthermore, in this work, a simple oscilloscope-based impedance measurement method for the PA output in a balanced amplifier—comprising two sets of PAs and transformer-based quadrature couplers—is presented, using the relative magnitude and phase of output voltages for the coupler connected to both dummy and actual loads. The introduced measurement technique is compared with reflection coefficient results obtained using vector network analyzer (VNA)-based methods for small-signal inputs of −20 and −10 dBm.