Low-noise, high-bandwidth amplification of prebreakdown current pulses in liquid dielectrics

Results of prebreakdown current measurements made with a low-noise, high-bandwidth preamplifier (0.4-μA minimum detectable current and 58-MHz bandwidth) are summarized. These waveforms demonstrate that low-noise, high-bandwidth preamplifiers are needed in order to observe the shape of prebreakdown current pulses, including the earliest pulses which are extremely small and fast. These and other factors influencing the design of an improved preamplifier are considered. Data collected indicate that at least a 50-MHz bandwidth is desirable to see the higher frequency components of the current pulses and that the noise level should be as low as possible so very small currents will be detectable. A major constraint on the design is the need to protect the preamplifier from a complete breakdown of the test sample while still maintaining bandwidth and noise performance. Another important consideration is the tradeoff between bandwidth and noise level. In general an increased bandwidth implies higher noise levels and, hence, higher minimum detectable currents. In light of these considerations, the relative merits of current-sensitive (transimpedance) preamplifiers and voltage-sensitive preamplifiers (where current develops a voltage across the input impedance) are examined, and the possibility of greater current sensitivity is considered