Power amplifier

Voltage amplifier with small signal inputs
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SPECIAL UHF TUBES

In the earlier discussion of conventional-electron tubes, you learned some of the limitations of tubes. One of these limitations was that the conventional tube was not able to operate (amplify) at extremely high frequencies such as those used in radar equipment. Even at frequencies lower than those used in radar equipment, problems occur. For example, at ultrahigh frequencies (300 MHz to 3000 MHz), transit time effects make the operation of a conventional-electron tube impossible. For this reason, the special ultrahigh frequency tubes were developed to operate within this frequency range.

Before we discuss the way in which special uhf tubes counter the effects of transit time, you should understand the manner in which transit time affects conventional tubes.

LIMITATION OF TRANSIT TIME

We will explain the limitation of transit time by using figure 2-9. In view A, the positive-going alternation of a uhf ac signal is applied to the grid of a conventional-triode tube. The first positive-going alternation reduces the negative bias on the grid, and electrons start to move toward the grid. Since the input is an ultrahigh frequency signal, the majority of the electrons cannot pass the grid before the input signal progresses to the negative alternation. The electrons that have not yet passed the grid are either stopped or repelled back toward the cathode. This is shown in view B. Before these electrons can move very far, the second positive alteration reaches the grid, and causes even more electrons to move from the cathode (view C). At the same time, the electrons that were repelled from the grid toward the cathode by the first negative alternation feel the effect of the positive-going grid. These electrons reverse direction and again move toward the grid. Because these electrons had to first reverse direction, they are now moving slower than the electrons that are attracted from the cathode by the second positive alteration. The result is that the electrons from the cathode catch up to the slower moving electrons and the two groups combine (view C). This action is called BUNCHING.

Figure 2-9. - Effect of transit time at ultrahigh frequencies.

In effect, the area between the grid and cathode becomes highly negatively charged, as shown in view D. This negative charge is surrounded by an electrostatic field. The electrostatic field cuts the grid and repels electrons that are present in the grid. As electrons are forced from the grid, the grid tries to go positive. Unfortunately, this tendency toward a positive charge attracts electrons from the mass or bunched charge. Thus, as an electron is forced from the grid; it is replaced by another from the massed charge. Electrons forced from the grid represent grid current (I _g), as shown in view E. The grid current flows from the grid through R_g, to the cathode, from the cathode, to the massed charged, and back again to the grid, The movement of current in this manner is, in effect, a path for current flow from the cathode to the grid. Because current flows between the cathode and grid, the resistance (rgk) between these elements is lowered to the point of a short circuit. The grid, in effect, is short circuited to the cathode and ceases to function; and this, in turn, lowers tube efficiency dramatically. This is shown in view F of figure 2-9.

Transit time may be decreased by reducing the spacing between electrodes or by increasing the electrode voltages, which in turn increases electron velocity through the tube.

The problem with the last solution is that the tube does not present an infinite resistance to current flow. If the operating voltage is raised to an operating potential that is too high, arcing (arcover) occurs between the cathode and the plate and, most likely, will destroy the tube. For this reason, the effects of transit time are reduced in uhf tubes by placing the tube elements very close together.

UHF TUBE TYPES

Uhf tubes have very small electrodes placed close together and often are manufactured without socket bases. By reducing all the physical dimensions of the tube by the same scale, the interelectrode-capacitance and transit time effects are reduced, without reducing the amplification capability of the tube. A disadvantage to this type of tube construction is that the power-handling capability of these tubes is also reduced due to the close placement of the tube elements.

Uhf tubes are placed in three broad categories based on their shape and/or construction; ACORN, DOORKNOB, and PLANAR tubes.

Acorn and Doorknob Tubes

ACORN TUBES, as shown in figure 2-10, are available for use as diodes, triodes, or pentodes. Acorns are very small tubes that have closely spaced electrodes and no bases. The tubes are connected to their circuits by short wire pins that are sealed in the glass or ceramic envelope. Because of their small size, acorn tubes are usually used in low-power uhf circuits.

Figure 2-10. - Acorn tubes.

The DOORKNOB TUBE is an enlarged version of the acorn tube. Because of its larger physical size, it can be operated at higher power than the acorn tube.