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Pronunciation Drill 2. While reading these words pay attention to the stress markings:

While reading these words pay attention to the stress markings:

signal require convenience summarize

model apply dimension definite

axis across proportion voltmeter

TEXT 8

TRIODE CHARACTERISTIC CURVES

/ 1. The relationships between the plate voltage, grid vol­tage and plate current in a triode, which we have explored in the last two texts, can be as in the case of the diode conve­niently summarized in the triode's characteristic performance curves.¹ Actually, a three-dimensional surface model² is required to represent the relation between all three quanti­ties at the same time, but for convenience two-dimensional cuts through this surface³ will give the relation between any two quantities, while the third is held constant.

2. Thus, we can plot a curve that shows the values of the
plate current Ia as a function of varying plate voltages Ea,
when the grid voltage Eg is held at some fixed value. This
is known as the plate current—plate voltage (I _a — E_a) char­
acteristic.⁴

3. Or we can show graphically the effect on the plate cur­
rent caused by varying the grid voltage i.e. the bias, while
holding the plate voltage at a constant value. This is called
the plate current-grid voltage (l_a — E_g) characteristic of
the triode.⁵

4. We can, of course, obtain a whole set of either of these
characteristics by assuming different values for the constant
quantity either plate or grid voltage and plotting a curve
between the remaining quantities (I_a — E_a, or I_a — E_g)
for each of these conditions. Such a set of characteristic
curves is known to be a family of static triode characteristics.

The term static denotes that the characteristics are obtained

when various steady voltages are applied to the tube's elec­trodes.

5. A circuit for obtaining the static characteristics of a

triode is illustrated in Fig. 3. Variable voltage dividers or potentiometers are connected across the plate voltage and grid voltage supplies denoted by E_aa and Egg, respectively, ' to permit ascertaining the effect of varying either voltage on the tube's plate current, while the remaining voltage is held constant. The potentials at the electrodes and the plate and grid currents resulting are then measured by suitable voltme­ters and amperemeters, inserted into the grid and plate circuits.

6. A grid family of charac- _ teristic curves for a type 6J5 triode is shown in Fig. 4. In these curves the plate current I_a has been plotted as a func­tion of the grid voltage E_g for various constant values of the plate voltage E_a. Note that each curve intersects the grid-voltage axis at a specific point that indicates the value of the negative grid voltage required to stop the plate current at the fixed value of the applied plate potential. This is the cut­off bias. As the plate voltage is increased, it may be seen that the negative bias required to cut off the plate current also increases.

7. It is also evident that each of the graphs in Fig. 4 is quite curved in the lower portion, near cut-off, while it is almost a straight line in the central and upper portions. Triodes are almost always operated in the straight-line, li­near portions of their characteristics and rarely in the

Curved or non-linear portion.⁶ This is so, because increases in the grid or signal voltage do not result in proportional increas­es of the plate current in the non-linear portion of the charac­teristic. But in the linear portion of the characteristic, equal changes in grid voltage cause equal changes in the plate current.

8. We can use the circuit of Fig. 3 to determine the family of static plate current-plate voltage characteris­tics for the 6J5 triode. The results are shown in Fig. 5. Here again, the curves have a similar shape, being curved in the lower por­tion near plate-cur­rent cut-off and fairly linear in the upper portions. Each curve shows the effect upon the plate current when the plate voltage is varied over a certain range, while holding the grid voltage fixed at a definite value. A new curve results each time when the grid voltage is changed to another fixed value, and when this is done over a representative range of constant grid voltages, the entire family of curves is obtained.

9. The effect of positive grid voltages on the I_a — E_a curves may be seen clearly. Even for low positive values of the grid voltage the plate current is seen to increase very rapidly with small increases in plate voltage. In addition the grid circuit of the tube draws a grid current I_g for posi­tive grid voltages. This grid current evidently decreases as the plate voltage is made large relative to the grid voltage, and the electrons are rapidly attracted to the plate.

Commentary

1. in the triode's characteristic performance curves — в семействе тоиодных характеристик

2. a three-dimensional surface model — трехмерная мо­
дель

3. two-dimensional cuts through this surface — двух­
мерное сечение этой поверхности

4. the plate current-plate voltage (/a— E_a) character­
istic — вольтамперная характеристика

5. the plate current-grid voltage (I_a — E_g) character­
istic of the triode — анодно-сеточная характеристика три­
ода

6. in the straight-line, linear portions of their characte­
ristics and rarely in the curved or non-linear portion — на
прямолинейных или линейных участках своих характерис­
тик и редко на изогнутых или нелинейных участках

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