Extra grids

Chapter Extra grids

Teach Yourself Electricity and Electronics Third Edition Book
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Summary of Contents

Teach Yourself Electricity and Electronics Third Edition Book

  • A tube with a control grid, in addition to the cathode and plate, is a triode. This isillustrated schematically in Fig. 29-3. In this case, the cathode is indirectly heated; thefilament is not shown. This omission is almost standard in schematics showing tubeswith indirectly heated cathodes. When the cathode is directly heated, the filament sym-bol serves as the cathode symbol.542 Electron tubes29-3Schematic symbol of triodetube. The filament is notshown.Now perhaps you see the resemblance between a triode tube and an N-channelFET. The cathode is analogous to the source; the grid is analogous to the gate; the platecorresponds to the drain. The only major differences, in fact, between the triode tubeand the N-channel FET are that the tube is constructed differently, and the tube workswith much higher voltages.In a tube amplifier, the control grid is where the signal input is normally applied. Atriode amplifier or oscillator circuit looks just like an FET circuit, except that the tubeplate and grid bias voltages are far greater than the FET drain and gate voltages.Extra gridsMany tubes have multiple grids. These extra elements allow for improved gain and sta-bility in tube type amplifiers.The tetrodeA second grid can be added between the control grid and the plate. This is a spiral ofwire or a coarse screen and is called the screen grid or screen. This grid normally car-ries positive dc voltage, roughly 1/4 to1/3 that of the plate voltage.The screen grid reduces the capacitance between the control grid and plate, mini-mizing the tendency of a tube amplifier to oscillate. The screen grid can also serve as asecond control grid, allowing two signals to be injected into a tube. This tube has fourelements and is known as a tetrode. Its schematic symbol is shown at A in Fig. 29-4.The pentodeThe electrons in a tetrode can strike the plate with extreme speed, especially if the platevoltage is high. The electrons might bombard the plate with such force that some of thembounce back or knock other electrons from the plate. Imagine hurling a baseball into a