Improved Direct-Coupled High-Bandwidth Voltage Amplifier for B-H Characterization of Magnetic Materials

B-H characteristics of magnetic materials are an essential input to the design of electrical machines and magnetics in power converters. Wide frequency range operation of machines and converters increasingly requires experimental characterization of the magnetic core material at low as well as high frequencies. Therefore, power amplifiers having wide bandwidth, high voltage and current gains, along with low output offset are needed for characterization. Linear closed-loop multistage amplifiers are often used. Direct coupling of amplifier stages is desirable to characterize the magnetic materials at very low frequencies. The middle stage of such linear multi-stage amplifiers provides the voltage gain. A conventional common-emitter based middle voltage gain stage is reviewed first. A folded cascode amplifier with buffered output is proposed as an alternative. This is capable of direct-coupling with the input (differential), and output (current amplification) stages. The proposed amplifier is shown experimentally to yield a high gain-bandwidth product. Experimental results of the proposed voltage amplifier and the closed loop power amplifier are presented.