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The apparatus described is intended for measurements of tunnel junctions. By using voltage-controlled current feedback the sample bias (AC and DC) is kept independent of sample resistance. The separation of the current feedback into a DC channel and a narrow bandwidth AC channel (modulation frequency) makes calibration easy. By further separating the AC feedback output into its in-phase and out-of-phase components the parallel capacity of the sample can be determined. The use of potential sensing lines allows measurement on four-probe samples.
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Altwein et al. (1973) studied this question.
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