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.

Topics
Electrical properties and parameters, Amplifiers, Triodes
1.
J. W. Gray in Vacuum Tube Amplifiers, edited by G. E. Valley, Jr., and H. Wallman (McGraw‐Hill Book Company, Inc., New York, 1948), Vol. 18 of MIT. Rad. Lab. Series, pp. 409–497.
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C. M.
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4.
If high gain and exceptional stability are needed, the use of either a Telefunken 12AX7 or a Western Electric Company 420A/5755 dual triode as the differential amplifier should be considered. A suitable circuit employing the 5755 is given by J. Millman and H. Taub, in Pulse and Digital Circuits (McGraw‐Hill Book Company, Inc., New York, 1956), p. 483.
5.
Field Modification Report 60A, Tektronix, Inc., Portland, Oregon (February 5, 1957). The Telefunken 12AX7 is likewise superior to the “premium” 5751 in dc amplifier service.
6.
Manufactured by Elcor, Inc., P. O. Box 354, McLean, Virginia, and called the Isoply.
7.
Normally no current is taken from the battery. However, should the positive voltage supply faill, a current (limited to about 0.1 ma by the 1‐megohm resistor) will flow.
8.
Reference 4, p. 22.
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G.
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© 1958 The American Institute of Physics.
1958
The American Institute of Physics
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