Video amplifiers for video display units

Higher performance from VDUs clearly requires more performance from the deflection circuitry and the video channel which drives the CRT cathodes to energise picture elements (pixels). The particular interest of this paper are the special needs of the video amplifiers used in a VDU. These amplifiers take the 0.7 Vp-p output of the video DACs situated within the PC, amplify and condition the signal to around 4 Vp-p. This is then fed to a high voltage amplifier that further amplifies the signal in the range 40 to 60 Vp-p to drive the CRT cathodes. In this context the term video amplifier is widened to include the system features necessary for the VDU. In addition to amplification and wide bandwidth, or more appropriately fast rise and fall times, video amplifiers for VDUs include contrast controls, a.c. gain adjust controls and d.c. restoration circuits which are also used for brightness control