Video Amplifier

a wide-band tube or semiconductor amplifier used in television, radar, oscillographs, and other equipment to amplify video signals before passing them on to a cathode-ray tube. To preserve the shape of the video signals, a video amplifier must amplify them uniformly (with no more than 1-3 decibels’ variation) over a wide frequency pass band (from 10-30 Hz up to 4-6 MHz) without appreciable phase distortion. Circuits of one- and two-stage video amplifiers, with a low-value resistor and various combinations of inductance coils, capacitors, and resistors in the load circuit of the gain stage, are the most commonly used. These combinations are selected in such a way as to increase the gain in the low and high-frequency regions, thus producing uniform gain and reducing phase distortions over a very wide frequency band. The circuit and amplitude-frequency response of a one-stage tube video amplifier are shown in Figure 1.

Figure 1. Typical circuit of a one-stage tube video amplifier (the dotted line indicates the C_str connections) and its gain characteristic when various loads are applied to the tube’s plate circuit: (a) the resistor R_p, (b) the resistor R_p and the decoupling network R_fC_f, (c) the resistor R_p and the correcting inductances L_p and L_c; A is the relative amplification, which is equal to the ratio of the amplification at any frequency to the maximum gain; f is the frequency.

The gain increase (correction) at high frequencies is attained as a result of resonance phenomena in the oscillatory circuits formed by the inductances L_p and L_c with the stray (spurious) circuit capacitances C_str, and at low frequencies by the choice of the parameters for the plate circuit decoupling network R_fC_f.