Video Amplifier

video amplifier [′vid·ē·ō ′am·plə‚fī·ər]

(electronics)

A low-pass amplifier having a band width on the order of 2-10 megahertz, used in television and radar transmission and reception; it is a modification of anRC-coupled amplifier, such that the high-frequency half-power limit is determined essentially by the load resistance, the internal transistor capacitances, and the shunt capacitance in the circuit.

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Video amplifier

A low-pass amplifier having a bandwidth in the range from 2 to 100 MHz. Typical applications are in television receivers, cathode-ray-tube computer terminals, and pulse amplifiers. The function of a video amplifier is to amplify a signal containing high-frequency components without introducing distortion.

Modern video amplifiers use specially designed integrated circuits. With one chip and an external resistor to control the voltage gain, it is possible to make a video amplifier with a bandwidth between 50 and 100 MHz having voltage gains ranging from 20 to 500. See Amplifier, Integrated circuits

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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.

REFERENCES

Kreitser, V. L. Videousiliteli. Moscow, 1952.
Lur’e, O. Usiliteli videochastoty, 2nd ed. Moscow, 1961.

A. IA. KLOPOV