Electron Emission and Vacuum Tubes

Introduction to Electronic Devices and Circuits

• The module focuses on the fundamentals of vacuum tubes.
• The course is ECE 2119 at the University of Santo Tomas.

Electronic Emission

• Electronic emission is the liberation of electrons from an electrode into the surrounding space, usually under the influence of heat, light, or a high electric field.
• The potential barrier is the force that restrains electron emission.

Methods of Electronic Emission

1. Thermionic Emission

• Thermionic emission is the process of emitting electrons by supplying heat energy.
• Charge carriers like electrons move over a surface or a potential-energy barrier; applying thermal energy overcomes the binding potential of the material.
• In thermionic emission, electrons gain enough energy to escape the metal electrode and become free electrons.

2. Photoelectric Emission

• Photoelectric emission is the process of emitting electrons when light radiation falls on a metal surface.
• The energy of the light radiation is transferred to the free electrons within the metal.
• Sufficient energy transfer speeds up the electrons enabling them to leave the surface.
• Emission depends on the intensity of the light striking the metal surface.

3. Field Emission

• A strong electric field, created by a high positive voltage, pulls electrons out of the cathode surface.

4. Secondary Emission

• High-speed electrons striking a metallic surface transfer their kinetic energy to other electrons and atoms within the metal.
• This collision ejects these electrons from the surface.

Vacuum Tube

• Also known as electron tube.
• An electronic device comprising an electrode structure inside a glass or metal container (evacuated).

Edison Effect

• Named after Thomas Edison.
• Current flows through a vacuum between two metal wires, one heated.
• Current flows only when the heated wire is negative, as free electrons are released from the wire.
• It is the fundamental principle of vacuum tubes.
• The heated wire is more negative and the current flows in this case.

Vacuum Tube Components

• A cathode (emitter) emits electrons.

• An anode (collector) collects the emitted electrons.

• The applied anode voltage must be positive relative to the cathode to attract electrons.

• A cathode is heated in order to emit electrons.

Vacuum Tube - Additional Notes

• Air is removed from the tube (evacuated) after assembly to maintain a vacuum.

• The heated filament inside can easily oxidize and burn in air.

• Air molecules can be ionized under certain conditions; maintaining a vacuum ensures that only emitted electrons travel to the plate.

• Two ways to heat the cathode:

• Direct heating— the filament itself is heated.

• Indirect heating— a separate heater element inside the tube heats the cathode

Vacuum Tube Uses

• Vacuum tubes can amplify input signals.
• They can convert AC input to pulsating DC output.
• They can generate AC output from power supply without AC input.

Types of Vacuum Tubes- Diode

• Invented by J.A. Fleming in 1904.
• Also called a Fleming Valve or Thermionic Valve.
• It is the simplest vacuum tube, comprising two electrodes: the plate (anode) and the cathode.
• Electrons are emitted from the cathode either directly or indirectly by supplying heat.
• A cloud of free electrons, known as space charge, forms near the cathode during heating.
• With a positive voltage applied to the plate, free electrons are attracted to the plate; the vacuum tube diode behaves like a closed switch or short circuit in this case.
• If the plate voltage is negative with respect to the cathode, the electrons are repelled back, resulting in zero plate current, akin to an open circuit.

Types of Vacuum Tubes - Diode (cont.)

• Diodes conduct current in one direction.
• Diodes act as rectifiers, converting AC to pulsating DC.
• Diode operation involves a characteristic curve displaying plate current versus voltage, with cutoff and saturation points.

Types of Vacuum Tubes - Triode

• Invented by Lee De Forest in 1906, also known as the De Forest tube.
• A device capable of amplification.
• It shares a similar structure to a diode, but with an additional electrode—the control grid.
• The control grid is positioned between the cathode and the plate.
• Grid voltage controls the current flow.
• Smaller negative grid voltage increases plate current.
• Greater negative grid voltage decreases plate current, achieving amplification.
• The ability of the control grid (to vary the plate current) enables small AC signals to be amplified.

Types of Vacuum Tubes - Triode (cont.)

• Controlling grid voltage has a considerable effect on electron control, more notable than the plate.
• The amplification factor (mu)— ratio of the plate voltage change to the grid voltage change, represents the effectiveness of the grid voltage in altering the plate current.
• Plate resistance (rp) is measured by the ratio of voltage change to the current change when the grid voltage is held constant and reflects the tube's opposition to electron flow.
• Transconductance (gm)— measures the effectiveness of the grid voltage in altering the plate current, determined as the ratio of current change to the grid voltage change, when the plate voltage is constant.

Types of Vacuum Tubes - Tetrode

• Addition of a screen grid between the control grid and plate.
• The screen grid reduces the grid to plate capacitance and increases the electron flow to the plate.
• Secondary emission occurs when electrons strike the plate.

Types of Vacuum Tubes - Pentode

• Enhances a tetrode by adding a suppressor grid.
• The suppressor grid repels secondary electrons back to the plate, suppressing secondary emission currents.
• The suppressor grid blocks the secondary electrons.

Vacuum Tube Disadvantages

• Bulky
• High power consumption
• High operating voltages needed
• Less suitable for portable products

Solution to Vacuum Tube Problems

• Semiconductor Technology replaced vacuum tubes.

Additional notes

• ENIAC, the first electronic general-purpose computer, utilized vacuum tubes.
• Interelectrode capacitances are capacitances formed between electrodes in the vacuum tube.
• The addition of more grids (screen and suppressor grids) in vacuum tube designs address certain issues, reducing capacitances and optimizing operation.

Explore electron emission methods like thermionic, photoelectric, and field emission. Understand vacuum tube diodes, their components, and inventors. Learn about space charge effects and oscillator functions.