Bootstrapped differential amplifier with reduced common-mode effects

Abstract

The appearance of a common-mode voltage at both grids of a conventional twin-triode differential amplifier produces, by cathode follower action, an equal swing of the common cathode point. The resulting change in the operating point of the tubes causes undesired shifts of both the differential gain and the balance of the amplifier. In the present work, the change of voltage at the common cathode point is entirely transferred ("bootstrapped") to the plate supply, with the result that a common-mode voltage does not affect the operating points of the differential amplifier tubes. By this means, a common-mode rejection ratio of >104:1 is readily achieved without tube selection or adjustments. With tube selection and some adjustment, >105:1 may be obtained. A common-mode signal of ±35 v changes the differential gain of the circuit by 0.5% or less. A bootstrapped differential VTVM circuit which deflects a 1 ma 1.5K-ohm pen recorder to full scale with ∼0.2 v dc input is described in detail. © 1958 The American Institute of Physics.