Common-Base Bypassing: and How It Affects CB Frequency Response

by Dennis L Feucht

The common-base (CB) BJT configuration is the second stage of cascode amplifiers. It is useful as a voltage translator to the high side of an amplifier with a large voltage range. As a stage of amplification it contributes to the frequency response of the overall amplifier. Within feedback loops, its effect on the loop gain is of interest. CB (or CG for FETs) usually has a base (or gate) circuit that is bypassed to ground by a capacitor to keep the base voltage constant. This C_B is usually added when the external circuit resistance at the base node, R_B, is appreciable. This TechNote addresses the question of how the R_B, C_B combination at the base node affects the frequency response.

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