Physics 101 Vacuum Tubes Notes PDF

Physics 101, Handout Number 3: Notes on VACUUM TUBES 1. Vacuum tubes are the first devices to attempt some form of control of currents, and to apply this control to various devices. Electron Emission 2. For a current in a traditional circuit, electrons are set in motion within the circuit; elec...

Physics 101, Handout Number 3: Notes on VACUUM TUBES 1. Vacuum tubes are the first devices to attempt some form of control of currents, and to apply this control to various devices. Electron Emission 2. For a current in a traditional circuit, electrons are set in motion within the circuit; electrons do not leave the wire. 3. The motion of the electrons (and atoms) is affected by temperature. If enough heat is applied, electrons can acquire enough kinetic energy to be emitted into the surrounding space. The electron emission due to heat is called thermionic emission. 4. There are four possible ways by which electrons can gain energy to escape: a. By evaporation or application of heat b. By bombardment of small, high speed particles like other electrons c. By photoelectric effect d. By placing it near an object of high potential 5. In a tube (normally in vacuum), the electron “current” is forced to fly off in space, emitted by the cathode (negative) and captured by the anode (positive). Note: Recall difference between conventional current and electron flow. The Diode Tube 6. Diodes have two basic parts: (a) an emitter of electrons (cathode) and (b) a receiver of electrons (anode). There are also two types of diode tube cathodes: (a) directly heated, referred to as filament cathode and (b) indirectly heated, referred to as cathode. 7. The plate consists of the anode that receives the emitted electrons. When the plate receives the electrons, it can heat up (and “glow”) because of: a. The heat from the filament or cathode b. The electrons striking the plate 8. Too much heating may cause the plate to itself release electrons. If electrons travel both ways, then the diode is now acting like a resistor. Heat of the plate is controlled by: (a) dull, black finish (blackbody radiation); (b) fitted radiator fins; or (c) water cooling. 9. To achieve reliable cooling, diode tubes must be “evacuated” (air is removed, thus a vacuum tube) to prevent the creation of ionized gas. 10. The characteristic diode curve of current versus voltage is shown below. Note that at 0V there is still a current because of “stray” electrons and that the current reaches a maximum value called the plate current saturation. In this case, there is no electron cloud around the plate; the electrons are immediately absorbed by the plate. Increasing the voltage once plate saturation point is achieved results in the plate itself releasing electrons. The Triode Tube 11. The triode tube consists of three elements: cathode, plate, and grid. While diodes were used to control currents only, triodes could be used as amplifiers because of the grid. The drawings below represent the schematics of a triode (top is anode, middle is grid, bottom is cathode). 12. A grid functions by “screening” the electrons hitting the plate. A grid attached to the positive terminal of a voltage source will increase the flow of electrons. A grid attached to the negative terminal of a voltage source will reduce the flow of electrons. 13. The grid controls the flow of electrons from cathode to plate. It means that it also controls the plate current and voltage. Changing the voltage of the source attached to the grid changes the plate voltage. For example, by changing the grid voltage by 1 volt, the plate voltage can be changed by 50 volts; this is an amplification of 50:1. We can measure the amplified voltage by: 𝑉𝑎𝑚𝑝𝑙𝑖𝑓𝑖𝑒𝑑 = 𝜇 𝑥 𝑉𝑠𝑖𝑔𝑛𝑎𝑙 , 𝑤ℎ𝑒𝑟𝑒 𝜇 𝑖𝑠 𝑡ℎ𝑒 𝑎𝑚𝑝𝑙𝑖𝑓𝑖𝑐𝑎𝑡𝑖𝑜𝑛 𝑓𝑎𝑐𝑡𝑜𝑟