1. TRIODE TUBES

2. THE TRIODE TUBE The development of the diode or two-element tube merely opened the door to the field of electronics. The development of the triode tube early in the start of the 20th century touched off the rapid development in electronics that has occurred since then. The triode tube has three elements: a cathode, a grid, and a plate. The introduction of the grid which is placed between the cathode and the plate of the tube made it possible for a vacuum tube to amplify weak signals.

3. HOW THE GRID WORKS As the name implies, the grid is of open construction. The schematic symbol used to represent a grid is shown in Fig. 7(A). On some old diagrams, you might find the symbol shown in Fig. 7(B), which is obsolete.

4. Figure 7. Different ways of representing the grid on schematic diagrams.

5. Several different types of grid construction are shown in Fig. 8. Notice that in (A) the grid is made in the form of a spiral mesh, like a screen, whereas the grid shown in (B) is made up of a spiral-wound coil with the turns placed relatively close together. In (C), the same type of construction is used as in (B), but the space between the turns is much greater. In (D), the grid is more or less rectangular in shape and is supported by the U-shaped elements at the end so that the grid is held in a very rigid position. HOW THE GRID WORKS

6. Figure 8. Different types of grid structures.

7. HOW THE GRID WORKS This grid is supported by a frame so this type of construction is referred to as a frame grid. It has the advantage over the other types in that the grid wires can be placed very close together and very close to the cathode, which, as you will see later, makes it possible to make a tube with a much higher gain. Before we see how the tube amplifies, let’s consider the effect of the grid on the flow of plate current when different voltages are applied to it.

8. HOW THE GRID WORKS The first case we will take up is where the grid is connected directly to the cathode so that the voltage applied to it is zero. Zero Grid Voltage. When the grid is connected directly to the cathode, we have the arrangement shown in Fig. 9. When the cathode is heated, it will emit electrons and they will fly off into the space surrounding the cathode.

9. Figure 9. When there is no grid bias, an average number of electrons flow to the plate, and the rest form a space charge between the cathode and the grid.

10. HOW THE GRID WORKS These electrons will form a cloud of electrons around the cathode. This cloud of electrons is called a space charge. Some of the electrons in the space charge will fall back to the cathode, and others will be attracted by the positive voltage applied to the plate of the tube so they will be drawn through the grid wires to the plate. A few electrons in traveling from the cathode to the plate may accidentally strike the grid wires.

11. HOW THE GRID WORKS These electrons will flow through the external circuit from the grid back to the cathode. As long as the grid is connected directly to the cathode, the tube acts very much like a diode and the amount of plate current flowing will depend primarily upon the voltage applied between the plate and cathode of the tube, and upon the spacing between the plate and cathode.

12. HOW THE GRID WORKS Notice that we’ve labeled the battery connected between the plate and cathode with the letter B. This comes from the early days of radio when all radios manufactured were battery operated. The battery used to heat the filament was called the A battery. The battery used between the plate and cathode was called the B battery, and a third battery connected in the grid circuit was called the C battery.

13. FIGURE 10 Making the grid positive greatly increases the number of electrons moving to the plate.

14. POSITIVE GRID VOLTAGE Now if we modify the Circuit shown in Fig. 9 by adding a small C battery in the grid circuit as shown in Fig. 10, we will have a positive voltage on the grid of the tube. This positive voltage will attract electrons from the space charge on the cathode and start them speeding toward the plate. By the time they reach the grid, most of them will be traveling at such a high speed that they will pass right through the grid and come under the influence of a high positive voltage on the plate of the tube. Most of the electrons will continue traveling toward the plate of the tube until they eventually strike the plate.

15. POSITIVE GRID VOLTAGE The number of electrons reaching the plate will be much higher,than in the preceding case where no voltage was applied to the grid. The positive voltage on the grid increases the number of electrons flowing to the plate because the grid is placed very close to the cathode, and even though there is only a very low positive voltage applied to the grid, it is sufficient to pull negative electrons from the electron cloud and start them on their way to the plate. If the positive voltage to the grid is made higher, the grid will start to attract more and more electrons.

16. POSITIVE GRID VOLTAGE Eventually a point will be reached where the grid will be taking many of the electrons that would normally flow over to the plate. When this happens, instead of causing the plate current to increase, the large number of electrons flowing to the grid will starve the plate “so that the plate current will be less than it would be if the grid were operated at zero potential.

17. NEGATIVE GRID VOLTAGE. If instead of applying a positive voltage on the grid, you apply a negative voltage on it, you will have the circuit shown in Fig 11. Now the negative potential on the grid of the tube repels the electrons coming from the space charge and drives them back to the space charge so that the number of electrons getting through the grid and reaching the plate is reduced.

18. FIGURE 11 Making the grid negative reduces the number of electrons moving to the plate.

19. NEGATIVE GRID VOLTAGE. As a matter of fact, if the negative voltage is made high enough, all electron movement between the cathode and the plate will be stopped. Then there will be no flow of electrons from the cathode to the plate of the tube.

20. AMPLIFICATION FACTOR. Current is actually movement of electrons. Since the grid controls the electrons flowing from a cathode to the plate of a tube, the grid can control the current flowing through the tube. Current flowing in the plate circuit is called a plate current. Changing the plate voltage of a triode will cause the plate current to change, but because the grid is closer to the cathode than the plate, the grid exerts a greater effect on plate current than the plate does.

21. AMPLIFICATION FACTOR. As a matter of fact, we may have to change the plate voltage on a tube as much as 100 volts to get the same change in plate current that will be obtained by changing the grid voltage only 1 volt. The exact ratio between the change in plate voltage to the change in grid voltage needed to get the same change in plate current is called the amplification factor. The amplification factor of a tube is a good indication of how much voltage gain you can expect to obtain from an amplifier stage using the tube.

22. AMPLIFICATION FACTOR. The total amplified voltage produced by the tube is equal to the amplification factor times the signal voltage applied between the grid and the cathode of the tube. It is not possible to get all this amplified voltage out of the tube because the tube has internal resistance, and part of the voltage will be dropped across this resistance. However, in general, the higher the amplification factor of a tube, the greater the gain we can expect to obtain from the tube.

23. AMPLIFICATION FACTOR. To provide a short form for expressing the amplification factor of a tube, the Greek letter mu, which is pronounced “mew,” and written μ, is used as a symbol to represent the amplification factor. The amplification factor is often referred to as the mu of the tube. Thus, a high-mu tube is a tube with a high amplification factor.

24. HOW A TRIODE AMPLIFIES The triode tube can be used to amplify a signal voltage by connecting it in a circuit such as shown in Fig. 12. Notice that we have the A battery to provide the power required to operate the heater and hence heat the cathode. The B battery is connected through R2 between the cathode and plate of the tube. This. will place a positive voltage on the plate. The C battery is connected through R1 between the cathode and the grid.

25. Figure 12 A triode amplifier circuit.

26. HOW A TRIODE AMPLIFIES It is connected to place a negative potential on the grid. If we measure the voltage between the plate of the tube and ground, we’ll find that we have a positive voltage on the plate of the tube. The voltage will be equal to the B battery voltage, minus the voltage drop across R2. This is shown in Fig. 13(A).

27. FIGURE 13 WAVEFORMS FOR CIRCUIT SHOWN IN FIGURE 12

28. HOW A TRIODE AMPLIFIES Now if we apply an ac input signal to the input, we’ll have an input signal similar to that shown in Fig. 13(B). The input signal will be in series with the C battery between the grid and cathode of the tube. Notice we’ve only shown one cycle. Up to point 1 on the curve the voltage is the C battery voltage.

29. HOW A TRIODE AMPLIFIES At point 1, the voltage begins increasing in a positive direction so that if will be subtracting from the C voltage until it reaches point 2 where the signal voltage is a maximum value. If the signal voltage is subtracting from the C battery voltage, we have the effect of reducing the negative voltage applied between the grid and cathode of the tube. When this happens, the plate current flowing through the tube will increase. When the plate current increases, the voltage drop across R2 will increase so that the plate voltage will swing in a negative direction as shown between points 1 and 2 on the curve in Fig. 13(C).

30. HOW A TRIODE AMPLIFIES From point 2 to point 3, the positive half-cycle of the input signal is decreasing until, at point 3, it reaches zero. At this point, the grid voltage will be equal to the voltage of the C battery. As the grid voltage swings from point 2 to point 3, the plate current will begin to decrease and when this happens, the voltage drop across R2 will begin decreasing. Therefore, the voltage on the plate of the tube will increase so that the plate voltage will follow the curve from point 2 to point 3, as shown in Fig. 13(C).

31. HOW A TRIODE AMPLIFIES Now the input signal reverses polarity and begins to swing in a negative direction from point 3 to point 4 where it reaches its maximum negative value. When this happens, it will add to the C battery voltage making the grid more negative. This will cause the current flowing from the cathode to the plate of the tube to decrease so that the voltage drop across R2 will decrease. When this happens, the plate voltage will swing in a positive direction as shown by the curve between points 3 and 4 in Fig. 13(C).

32. HOW A TRIODE AMPLIFIES At this point, the negative voltage on the grid begins dropping from point 4 to point 5 where once again it reaches zero. As the negative voltage begins to decrease, the negative grid voltage will begin decreasing so that the current flow through the tube will start increasing. When this happens, the voltage drop across R2 will begin to increase so that the plate voltage will follow the curve from point 4 to point 5 in Fig. l3(C).

33. HOW A TRIODE AMPLIFIES The current flowing through the tube can be considered a dc current with an ac current superimposed on it. It is the ac current that produced the varying voltage drop across R2 and hence the varying voltage between the plate of the tube and ground. If we block the dc voltage at the plate by means of a capacitor, and take the signal from the output terminal in Fig. 12, the output voltage will be the amplified signal shown in Fig. l3(D).

34. HOW A TRIODE AMPLIFIES The signal in the output will be an amplified version of the input because the small input signal controls the flow of plate current, causing the current through the tube to increase when the signal swings in a positive direction and to decrease when the signal swings in a negative direction. This ac signal current flowing through the large value resistor in the plate circuit causes an amplified signal voltage to appear between the plate of the tube and ground. The resistor in the plate circuit is often called the plate load resistor or simply the load resistor.

35. HOW A TRIODE AMPLIFIES Notice that when the input signal swings in a positive direction, the output signal swings in a negative direction. In other words, the output signal is 180° out-of-phase with the input signal. Remember that this is the same situation we found in a transistor amplifier using the common-emitter circuit. The circuit shown in Fig. 12 is called a common-cathode circuit and is the most widely used circuit in vacuum tube amplifiers.

36. QUESTION Why does the grid voltage have a greater effect on plate current than the plate voltage?

37. ANSWER The grid is much closer to the cathode than the plate and therefore the voltage applied to the grid has a much greater effect on plate current than the same voltage applied to the plate will have.

38. QUESTION If the voltage applied between the grid and cathode of a triode tube makes the grid positive with respect to the cathode, what effect will this have on the current flow through the tube?

39. ANSWER The positive voltage applied to the grid will cause the number of electrons flowing from the cathode to the plate to increase. Most of these electrons will flow through the grid structure to the plate because the plate will normally have a much higher positive voltage than the grid. However, some of the electrons will be attracted by the grid and cause some current to flow in the grid circuit.

40. QUESTION If the grid is made negative with respect to the cathode, and the voltage is slowly increased, what will happen to the plate current?

41. ANSWER As the negative grid voltage is increased, the plate current will decrease until eventually the grid voltage will become negative enough to prevent any electrons from reaching the plate.

42. QUESTION What do we mean by the amplification factor?

43. ANSWER The amplification factor of a tube is the ratio of the change in plate voltage to the change in grid voltage required to produce the same change in plate current.

44. QUESTION Between what values would you expect the amplification factor of a triode to fall?

45. ANSWER Between 5 and about 100.

46. QUESTION When the signal voltage is amplified by a tube in a grounded-cathode circuit, will the amplified signal be the same as the input signal?

47. ANSWER The amplified signal will normally be the same as the input signal except it will be 180 degrees out-of-phase.

48. QUESTION What do we mean by ac plate current?

49. ANSWER The ac plate current is the change in plate current produced by the input signal. It acts like an ac current superimposed on the dc plate current that flows through the tube when the signal is zero.