INTERELECTRODE CAPACITANCE

As you know, capacitance exists when two pieces of metal are separated by a dielectric.

You should also remember from your studies that a vacuum has a dielectric constant of 1. As the elements of the triode are made of metal and are separated by a dielectric, capacitance exists between them. This capacitance is called interelectrode capacitance, and is schematically represented in figure 1-29.

Figure 1-29. - Schematic representation of interelectrode capacitance.

Notice that there are three interelectrode capacitances involved in a triode. The capacitance between the plate and grid, designated C _pg, is the largest, because of the relatively large area of the plate, and therefore has the greatest effect on triode operation. The grid-to-cathode capacitance is designated C_kg.

The total capacitance across the tube is designated C _pk.

As we said earlier, C_pg has the greatest effect on the tube operation. This is because this capacitance will couple part of the ac signal from the plate back to the grid of the tube. The process of coupling the output of a circuit back to the input is called FEEDBACK. This feedback affects the gain of the stage. It may be desirable in some applications. In others, the effects must be neutralized. The effects of C_pk are greater at higher frequencies where X_c is lower.

DEVELOPMENT OF THE TETRODE

Interelectrode capacitance cannot be eliminated from vacuum tubes, but it can be reduced. The easiest method found to reduce interelectrode capacitance is to split the capacitance between the grid and plate (C_pg) into two capacitors connected in series. This is done by placing an extra grid, called the SCREEN GRID, between the control grid and the plate. This is shown in figure 1-30.

Figure 1-30. - Effect of the screen-grid on Interelectrode capacitance.

Remember from your study of capacitance that connecting capacitors in series reduces the total capacitance to a value smaller than either of the capacitors. This is mathematically summed up as follows:

The addition of the screen grid has the effect of splitting C _pg into two capacitances (C1 and C2) connected in series. Therefore, the total interelectrode capacitance between the control grid and the plate is greatly reduced.

OPERATION OF THE BASIC TETRODE CIRCUIT

Figure 1-31 depicts a basic tetrode circuit. While the circuit may look complicated, it isn't. You are already familiar with most of the circuit. Only three components have been added: the screen grid, the screen grid dropping resistor, and the screen grid bypass capacitor (C_sg).

Figure 1-31. - Basic tetrode circuit.

The problem now is: at what voltage and polarity should the screen grid be operated? If the screen grid were operated at a potential that would make it negative in relation to the control grid, it would act as a negative screen between the plate and control grid. As a result, gain would be reduced. If the screen grid were operated at plate potential, it would draw current from the electron stream when the tube conducts. Because of this, the value of R_sg is normally selected to cause the screen grid to be positive in relation to the control grid, but not as positive as the plate.

Despite this precaution, the screen grid still draws some current from the electron stream. Any signal applied to the control grid will appear at the screen grid inverted by 180 from the input signal. This is undesirable, as it reduces the gain of the tube. Consider the effect if the control grid goes positive. Conduction through the tube increases, and since the screen grid is in the electron stream, it will draw some current. This causes the screen grid to go toward negative potential (less positive). The effect then, is to place a negative-going electrode between the plate and positive-going control grid. The plate becomes partially screened by the negative-going screen grid, and again, gain will decrease. Because the signal at the screen grid is always 180 out of phase with the control grid, its effect will always be to oppose the effect of the control grid.

To overcome this, a bypass capacitor (C_sg) is connected between the screen grid and ground. The addition of this capacitor will shunt, or pass, the ac variations on the screen grid to ground while maintaining a steady dc potential on the screen grid. In other words, C _sg moves all of the undesired effects mentioned in the previous paragraph.

One very useful characteristic of the tetrode tube is the relationship between the plate and screen grid. The screen grid will lessen the effect that a decreasing plate potential (negative-going signal) has on conduction through the tube. In a triode, when the grid goes positive, the plate goes negative. This decreases electrostatic attraction across the tube and tends to decrease the gain somewhat.

In a tetrode, the screen grid has the ability to isolate the effect that ac variations on the plate have on the electron stream.

The positively charged screen grid will accelerate electrons across the tube even though the plate is negative going. As long as the plate remains positive in relation to the cathode, it will draw off these accelerated electrons. As a result, conduction through the tube when the plate is going negative will not be decreased. This is another big advantage of screen-grid tubes.

TETRODE CHARACTERISTICS

Because the screen grid is in the electron stream, it will always draw some current. The current drawn by the screen grid will be lost to the plate. This means that the transconductance of a tetrode, which is based on the amount of plate current versus control-grid voltage, will be lower in tetrodes than in triodes. The formula for transconductance of a triode,

must be adjusted for screen-grid current, and becomes

As you can see, the transconductance for a tetrode can never be as high as that of a triode of similar construction.

While lowered transconductance in a tetrode is an undesirable characteristic, it is not the reason that tetrodes have found little acceptance in electronics. The factor that severely limits the operation of tetrodes is

SECONDARY EMISSION.

Because the screen grid is positively charged, electrons traveling from the cathode to the plate are accelerated. Electrons are accelerated to such an extent that they dislodge electrons from the plate when they strike it. This is similar to the manner in which a high-velocity rifle bullet fired into a pile of sawdust throws sawdust about. Some of these electrons are fired back into the tube, where they tend to accumulate between the screen grid and the plate. This effect is most pronounced when the signal at the control grid is going positive and conduction through the tube is increasing. The plate in this situation is going negative in answer to the control-grid signal. This causes the electrons accumulating between the plate and screen grid to be attracted to the screen grid. The current that is drawn by the screen grid is lost to the plate and gain is decreased. Gain is also decreased in another way. The negative charge accumulated by secondary emission causes some of the electrons (from the cathode) to be repelled from the plate, which further reduces gain.

Another undesirable characteristic of tetrodes associated with secondary emission is that the outputs are NOISY. What this means is that small sporadic signals appear on the main output signal, as shown in figure 1-32. When electrons are knocked from the plate, they represent losses of plate current and corresponding positive pulses on the output. Electrons falling back to the plate represent increases in plate current and cause negative-going pulses to appear in the output.

Figure 1-32. - Noise in a tetrode circuit.

For these reasons tetrodes are only used in very specialized applications of electronics.

Q.30 How does the addition of a screen grid in a tetrode reduce interelectrode capacitance?
Q.31 What undesirable effect does the screen grid in a tetrode create?