Pentode

Plot of plate current lp versus plate voltage Ep for a typical pentode at various set control-grid potentials Ec. Adapted from Terman [5]. 

Figure 9.21 shows that the total emitter current Ip depends exponentially on the emitter voltage VE and is displaced to the left, as the collector voltage Vc increases. Figure 9.22 shows that when the npn transistor is designed properly, the collector current Ic is almost independent of the collector bias Vc and increases linearly as the emitter current IE is increased. The collector current Ic is also very close to the emitter current IE that is, the dimensions and conductivities of the three regions are so adapted that the base current IB is kept small. The curves in Fig. 9.22 for the npn transistor resemble the curves in Fig. 9.11 for the vacuum-tube pentode. 

In tha pentode pko phate pathway for degrading sugara, ribulose 5- phos.p(iate is converted to ribose Q-phosphate. Propo e a mechanism for the it merization. 

WILLIAMS Pathogenesisof Neoplasia aiid Influences of Pentodes... 

FIGURE 5.6 Four standard grid tube representations (a) triode, (b) tetrode, (c) pentode, (d) beam power, (e) cathode types. 

The dynamic operation of a pentode is described approximately by the relation... 

The value of /r is not normally provided on the specifications sheets for pentodes, and only the values for g ndrp are listed. Transconductance values are typically on the order of 5-6 mS, and plate resistance... 

Pentodes are sometimes operated in a triode connection, in which the screen grid is connected to the plate. In the 1950s, this mode of operation was popular in high-fidehty vacuum-tube audio power... 

TABLE 5.1 Operating Specifications for Hypothetical Beam Power Pentode Single-Tube Operation ... 

As is illustrated in Fig. 5.15, a DC load line is applied to the pentode plate characteristics in the same manner as for the triode. In the example shown, Rl = 1 and the Q-point values are Vp = 300 V, Ip = 200 mA, Vg —9 V. Note for this device that positive-grid operation is possible and that nonhnearities occur for large negative grid voltage because the curves are not evenly spaced. An AC load is added to the pentode plate characteristics in the same manner as for the triode case, and the graphical analysis proceeds in the same manner as for the triode. 

At low frequencies, the voltage gain of a pentode amplifier, such as the circuits shown in Fig. 5.16 is given by... 

FIGURE 5.15 Pentode plate characteristics with superimposed DC load line for Q-point calculations. 

FIGURE 5.16 Pentode audio amplifier circuits (a) audio amplifier with fixed bias, (b) audio amplifier cathode bias (self bias). 

Pentode Vacuum tube with five active electrodes cathode, control grid, screen grid, suppressor grid, plate. 

The suppressor grid of a pentode reduces the effects of secondary emission in the device. This attribute, thus, reduces the requirement to provide a reverse electron flow path for the screen grid power supply. The screen current requirement for a pentode may, however, be somewhat higher than for a tetrode of the same general characteristics. 

Neutralization of low-power push-pull tetrode or pentode tubes can be accomphshed with cross neutralization of the devices, as shown in Fig. 5.42. Small-value neutralization capacitors are used. In some cases, neutralization can be accomplished with a simple wire connected to each side of the grid circuit and brought through the chassis deck. Each wire is positioned to look at the plate of the tube on the opposite-half of the circuit. Typically, the wire (or a short rod) is spaced a short distance from the plate of each tube. Fine adjustment is accomplished by moving the conductor in or out from its respective tube. 

Single-ended tetrode and pentode stages can be neutralized using the method shown in Fig. 5.45. The input resonant circuit is placed above ground by a smaU amount because of the addition of capacitor Qn. 

In usual tetrode and pentode structures, the capacitance from screen-to-grid is approximately half the published tube input capacitance. The tube input capacitance is primarily the sum of the grid-to-screen capacitance and the grid-to-cathode capacitance. 

It should be noted that in the examples given, it is assumed that the frequency of operation is low enough so that inductances in the socket and connecting leads can be ignored. This is basically true in MF applications and below. At higher bands, however, the effects of stray inductances must be considered, especially in single-ended tetrode and pentode stages. 

At the self-neutralizing frequency, the tetrode or pentode is inherently neutralized by the circuit elements within the device itself, and external screen inductance to ground. When a device is operated... 

Optimum power output at these frequencies is obtained when the loading is greater than would be used at lower frequencies. Fortunately, the same condition reduces driving power and screen current (for the tetrode and pentode cases), and improves tube life expectancy in the process. 

The power output from a tetrode or pentode is sensitive to screen voltage. For this reason, any application requiring a high degree of linearity through the amplifier requires a well-regulated screen power supply. A screen dropping resistor from the plate supply is not recommended in such applications. 

The type and degree of protection required in an RF amplifier against circuit failure will vary with the type of screen and grid voltage supply. Table 5.3 lists protection criteria for tetrode and pentode devices. The table provides guidelines on the location of a suitable relay that should act to remove the principal... 

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