INTRODUCTION TO ELECTRON TUBES

Figure 1-15.—DeForest's experiment.In view (B), you see De Forest's tube with +3 volts applied to the control grid. When De Forestapplied this voltage, he found that plate current, I p , increased by a large amount. (We'll say it doubled tosimplify the explanation.) You already know that the only way to double the plate current in a diode is toincrease the plate voltage by a large amount. Yet, De Forest had doubled plate current by applying only 3volts positive to the control grid!The reason for this is fairly easy to understand. It's the old principle of "opposites attract." When thecontrol grid was made positive, electrons surrounding the cathode (negative charges) were attracted to thegrid. But remember, the grid is a metal mesh. Most of the electrons, instead of striking the grid wires,were propelled through the holes in the mesh. Once they had passed the grid, they were attracted to thepositive charge in the plate.You might wonder why the grid would make that much difference. After all, the plate has 300 voltson it, while the grid only has 3 volts on it. Surely the plate would have a greater effect on current flowthan a grid with only one one-hundredth the attractive potential of the plate. But remember, in your studyof capacitors you discovered that opposites attract because of electrostatic lines of force, and that thestrength of electrostatic lines of force decreased with distance. In his tube, DeForest had placed the gridvery close to the cathode. Therefore, it had a greater effect on current flow from the cathode than did theplate, which was placed at a much greater distance from the cathode. For this reason, De Forest was ableto double the current flow through the tube with only +3 volts applied to the grid.DeForest had certainly hit on something. Now the problem was to find out what would happen whena negative potential was applied to the grid. This is shown in view (C) of figure 1-15. When De Forestapplied -3 volts to the grid, he found that plate current decreased to half of what it was when the grid hadno voltage applied. The reason for this is found in the principle of "likes repel." The negatively chargedgrid simply repelled some of the electrons back toward the cathode. In this manner, the attractive effect ofthe plate was decreased, and less current flowed to the plate.Now De Forest was getting somewhere. Using his new tube (which he called a triode because it had3 elements in it), he was able to control relatively large changes of current with very small voltages. But!was it amplification? Remember, amplification is the process of taking a small signal and increasing itsamplitude. In De Forest's circuit, the small input signal was 3 volts dc. What De Forest got for an output1-20