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

B-H characteristics of magnetic core materials are an essential input to the design and performance evaluation of electrical machines, electromagnetic bearings, and, magnetic components in power converters. The magnetic cores in different applications experience different types of excitations including sinusoidal, non-sinusoidal, alternating with a dc offset, pulse-width modulated (PWM), etc. This paper reports a cost-effective direct-coupled multistage power amplifier that can provide the above types of excitations. A variant of cascode amplifier is utilized for voltage gain stage to achieve high bandwidth and low DC offset. It is challenging to directly couple the output of this stage to the input of the succeeding current-gain stage due to reasons including divergent bias current requirements of the two stages. An intermediate stage is proposed between the voltage and current gain stages which effectively decouples the DC bias circuits of the two stages and passes the signal from DC to high frequencies, without any attenuation. The developed multistage amplifier is tested for its bias current stability, frequency response, linearity, and heat sink temperature rise. The capabilities of the developed amplifier are demonstrated through characterization of M36 and ferrite cores, which are popular in electromechanical and power conversion applications respectively. M36 core is tested with excitations including sinusoids upto 1 kHz, sinusoids with DC bias as in electromagnetic bearings, and, sinusoidal PWM that are typical in motor drives. The ferrite core is excited with quasi-square wave excitations and square wave excitations with and without DC bias, as encountered in power electronic converters.