Fig. 4-27 Pole positions for a staggered video amplifier. Each amplifier gives a 1-sec rise time and 1 per cent overshoot. (All coordinates in radians per second.)

the grid. Thus it is impractical to use the advantage of the large load resistance to the fullest extent.

There is some advantage in using a stage with high r; for the input stage, too, since this gives large stage gain to help overcome noise in subsequent stages. As a consequence of these two considerations, the stages with large amounts of peaking (the stages with the largest m and lowest rt) are best placed in the middle of the amplifier.

The pole-zero diagrams for these stagger-peaked amplifiers show that the poles lie very nearly on a line of constant a. Three examples of the pole-zero diagrams are given in Fig. 4-27. Each of these sets of pole locations yields an amplifier with 1 sec rise time and 1 per cent overshoot. The amplitude response for the two-stage (each stage peaked) example is shown in Fig. 4-28.

Presumably the same sort of improvement in multistage amplifiers could be achieved in a transistor amplifier. However, the situation with transistors is much more complicated because the position of the zero in a shunt-peaked stage does not bear a constant relation to the real part of a complex pole as it does in the vacuum-tube case. In a shunt-peaked vacuum-tube stage Z\ = 2 Re pi, where and pi are the zero and complex pole positions. Hence the Muller data cannot, in general, be directly applied to the transistor case.

4-11 Output Stages. Most amplifiers have, in addition to the requirement for a certain amount of gain between input and output terminals, a requirement for the amount of voltage (or power) that may be needed at the output. The maximum output voltage is limited by the amount of plate current that can flow through the load resistor of the last stage. The highest value that can be achieved—without regard to the requirements of linearity—is that of a step function which carries the plate current from zero to the maximum rated value for the tube. This rated value differs from tube to tube, and one would tend to choose a tube with a high current rating. But since the rise time of the output stage enters into the total rise time of the amplifier, one must consider this factor as well. Several tubes can therefore be compared on the basis of a new figure of merit suitable for output stages as proposed by Wallman;1 this is the ratio of maximum voltage output to rise time and depends upon the output capacitance C0 of the tube and the capacitance of the load Cl■ As an example, several

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