Control of gamma in C.R.T. Displays using amplifiers with exponential negative feedback

It is shown that networks can be produced to correct accurately the effects of the non-linear transfer characteristics of a cathode-ray tube. The gamma correction required is related to the undesirable non-linearity in mathematical terms and the basic gamma equation evolved. A simple exponential equation can be converted into a gamma equation by the introduction of a linear function and the appropriate constants. A network with the required gamma characteristic is equivalent to an amplifying stage incorporating exponential negative feedback. Circuits employing semiconductors are presented and discussed and the operational equations and performances are found to compare favourably with the theoretical gamma equation, and to support the theory. Networks employing negative feedback intended for operation at video frequencies call for special design to counteract the effects of gain and phase shift variation at those frequencies and a gamma stage suitable for television applications is presented. Stabilization of the operating point of the gamma stages with respect to temperature and power supply variations is described. Testing a gamma stage involves measurement of phase shift, frequency response and faithfulness of gamma as well as checks for the presence of waveform distortion. Although the treatment deals in particular with the linearization of c.r.t. displays, the techniques are applicable to accurate logarithmic amplifiers, photographic-film contrast correction and in colour television camera tube applications.