University of Santo Tomas Module 1.2 - Vacuum Tubes PDF

UNIVERSITY OF SANTO TOMAS Electronic Devices and Circuits MODULE 1: Fundamentals of Vacuum Tubes Electronics Engineering Department | ECE 2119  UNIVERSITY OF SANTO TOMAS Introduction Electronic Emission The liberation of electrons from an electrode into the...

UNIVERSITY OF SANTO TOMAS Electronic Devices and Circuits MODULE 1: Fundamentals of Vacuum Tubes Electronics Engineering Department | ECE 2119  UNIVERSITY OF SANTO TOMAS Introduction Electronic Emission The liberation of electrons from an electrode into the surrounding space, usually under the influence of heat, light, or a high electric field. Potential Barrier It is the force that restrains the emission of electrons. Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Electronic Emission Methods of Electronic Emission 1. Thermionic Emission It is the process of emitting electrons by supplying heat energy. It is the process by which charge carriers, such as electrons, move over a surface or over a potential- energy barrier by applying thermal energy in order to overcome the binding potential of the material. In thermionic emission, electrons gain sufficient energy to escape from the metal electrode and become free electrons. Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Electronic Emission Methods of Electronic Emission 2. Photoelectric Emission It is the process of emitting electrons wherein the energy of light radiation falling upon the metal surface is transferred to the free electrons within the metal and speeds up sufficiently to enable them to leave the surface. The emission of electrons is dependent on the intensity of light striking the metal surface. Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Electronic Emission Methods of Electronic Emission 3. Field Emission A strong electric field set up by a high positive voltage pulls the electrons out of the cathode surface. Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Electronic Emission Methods of Electronic Emission 4. Secondary Emission When a high-speed electron strikes a metallic surface they give up their kinetic energy to the electrons and atoms which they strike. Bombarding electrons collide with the electrons in the metal consequently ejecting them out of the surface. Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube It is also known as “electron tube”. It is an electronic device which consists of an electrode structure enclosed in a glass or metal container that is evacuated. Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Edison Effect - It was named after Thomas Edison. - It is the flow of current through a laboratory vacuum, between two metal wires, one of which is heated. - The current flows only when the heated wire is more negative, because it is carried by free electrons released from the wire by its heat. - It is the principle behind vacuum tubes. Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube - It consists mainly of two parts: 1. Cathode (Emitter) 2. Plate/Anode (Collector) Cathode - the electrode that emits electrons Anode - the electrode that collects the emitted electrons - has a positive potential applied with respect to the cathode, so that the emitted electrons are attracted to provide plate current Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube - After the tube is assembled, air is exhausted from the envelope (vacuum). - Reasons why vacuum is necessary: - The heated filament would oxidize in air and burn. - The cathode emits more electrons in a vacuum. - When it is desired that only the electrons emitted from the cathode travel to the plate, without ionizing air molecules, the electrodes must be in a vacuum. Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Among the methods of electron emission, thermionic emission is most commonly used in vacuum tubes. - Two ways of applying heat to the cathode: 1. Direct Heating – electric current is applied to a filament wire, which also serves as the cathode; the emitter is usually made of either tungsten or oxide- coated material 2. Indirect Heating - the cathode is electrically heated by a separate heater element located inside it Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Direct Heating Indirect Heating Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Uses of Vacuum Tubes: 1. Amplifier - amplifies its input signal 2. Rectifier - changes its ac input to pulsating dc output 3. Oscillator - generates ac output from its power supply, without any ac input signal Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Types of Vacuum Tubes 1. DIODE - invented by J.A. Fleming in 1904 - also called as Fleming Valve or Thermionic Valve - simplest type of vacuum tube that consists of two metal electrodes: the plate/anode and the cathode Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Types of Vacuum Tubes 1. DIODE Operation: Electrons are emitted from the cathode by supplying heat either directly or indirectly. During the heating process, a cloud of free electrons called space charge is formed near the cathode. Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Types of Vacuum Tubes 1. DIODE Operation: By applying a positive voltage to the plate with respect to the cathode, the free electrons repelled by the cathode are attracted towards the plate. In this state, the vacuum tube diode acts like a closed switch or a short circuit. Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Types of Vacuum Tubes 1. DIODE Forward-biased Condition + _ Thermionic Emission Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Types of Vacuum Tubes 1. DIODE Operation: However, if the voltage at the plate is made negative with respect to the cathode, free electrons are pushed back to the cathode and no current flows, like an open-circuit. Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Types of Vacuum Tubes 1. DIODE Reverse-biased Condition Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Types of Vacuum Tubes 1. DIODE Due to the nature of the vacuum tube diode to conduct current in a particular direction and act as an open circuit when impressed with a voltage of the other direction, it is used as a rectifier. Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Types of Vacuum Tubes 1. DIODE Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Types of Vacuum Tubes 1. DIODE Cut-off Point - It is the minimum point at which Characteristic Curve the tube no longer operates. Below, this value, no plate current flows. Saturation Point - it is the maximum point wherein raising the plate voltage no longer increases the plate current Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Types of Vacuum Tubes 2. TRIODE Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Types of Vacuum Tubes 2. TRIODE - It was invented by Dr. Lee De Forest in 1906. - It is also known as De Forest Tube. - It is a device that provides amplification and it is considered as the first amplifier. - It is similar in construction to the vacuum tube diode except that a third electrode, called control grid, is placed between the cathode and the plate. Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Types of Vacuum Tubes 2. TRIODE - De Forest discovered that current in the triode could be controlled by adjusting the grid potential with respect to the cathode. - This device was found to be capable of amplifying small signal voltages, a discovery of such great practical importance in the electronics industry. Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Types of Vacuum Tubes 2. TRIODE Operation: The potential applied to the control grid is usually a small negative voltage with respect to the cathode. The grid voltage is usually made negative so that it cannot attract electrons. Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Types of Vacuum Tubes 2. TRIODE Operation: The function of the control grid is not to provide current but to have its voltage control the plate current. Since the control grid is placed nearer the cathode, it has a much larger effect on the control of electrons than the plate when a potential is placed in it. Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Types of Vacuum Tubes 2. TRIODE If VAK = VGG , the number of electrons at the grid is equal to the number of positive charges at the plate, hence no current will flow. Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Types of Vacuum Tubes 2. TRIODE The effects of the grid voltage on plate current can be summarized as follows: 1. Less negative grid voltage increases the plate current. 2. More negative grid voltage decreases the plate current. 3. If the grid voltage is made negative enough, there will be no plate current even with the plate positive. Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Types of Vacuum Tubes 2. TRIODE The ability of the control grid to vary the plate current makes the triode amplify small AC signal. Amplification takes place since a small change in the input signal applied at the grid results in a large variation on the plate current and on the resulting voltage across the plate and cathode. Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Dynamic Parameters of the Vacuum Tube Triode - For amplifier tubes, the main characteristics are the amplification factor, the internal plate resistance and the grid-plate transconductance. - These parameters are for the tube itself, not the amplifier circuits. Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Amplification factor, 𝜇 - The ratio of the change in plate voltage to the change in grid voltage for a constant plate current ∆ 𝐸𝑝 𝜇= (Ip constant) ∆ 𝐸𝑐 - This ratio compares the effectiveness of the control grid voltage to the plate voltage in changing the plate current Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Plate resistance, rp - The ratio of the change in plate voltage to the change in plate current when the grid potential is kept constant ∆ 𝐸𝑝 rp = (Ec constant) ∆ 𝐼𝑝 - It is the opposition of the tube to the flow of electrons from cathode to plate Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Transconductance, ɡm - This factor states how effective the grid voltage is in changing the plate current. Defined also as the ratio of a small change in plate current to the small change in control grid voltage producing it when the plate voltage is held constant. ∆ 𝐼𝑝 ɡm = (Ep constant) ∆ 𝐸𝑐 Relationship of 𝜇 , rp and ɡm 𝜇 = ɡm x rp Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Example The ff. data was observed while the tube was undertest. Grid Voltage, Ec Plate Voltage, Ep Plate current, Ip mA -5 V 100 V 15 mA -3 V 150 V 20 mA Solve for the tube parameters. Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Interelectrode Capacitances - Any two conductors in space have a capacitance between them. Thus, a capacitance exists between each pair of electrodes in a vacuum tube. - They are also called stray or parasitic capacitances. Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Types of Vacuum Tubes 3. TETRODE - The addition of another grid, called screen grid, reduces the grid-to-plate stray capacitance. Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Types of Vacuum Tubes 3. TETRODE - The screen grid is placed between the control grid and the plate and is close to the plate. - It is not used to control the plate current but has a steady positive dc voltage to help accelerate electrons to be collected by the plate. - Secondary emission occurs in tetrode. Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Types of Vacuum Tubes 3. TETRODE Operation: - A positive voltage, with a lower value compared to the plate voltage, is applied to the screen grid relative to the cathode to help accelerate the flow of electron from cathode to plate. Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Types of Vacuum Tubes 3. TETRODE Operation: - When the high speed electrons strike the metal plate, some of the loosely attached electrons within the plate are ejected out of the surface towards the screen grid as secondary electrons. - These secondary electrons are collected by positive screen grid, thus, increasing the screen grid current. Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Types of Vacuum Tubes 3. TETRODE Operation: Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Types of Vacuum Tubes 4. PENTODE - It is simply a vacuum tube tetrode with an additional electrode called the suppressor grid inserted between the screen grid and the plate to overcome the undesirable effects of secondary emission. Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Types of Vacuum Tubes 4. PENTODE - Usually, the suppressor grid is connected internally to the cathode. - Any secondary electrons emitted by bombardment of the plate are repelled back, before they can be attracted to the positive screen grid. Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Did you know? ENIAC (Electronic Numerical Integrator and Computer) - first electronic general- purpose computer Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Vacuum Tube Disadvantages of Vacuum Tubes - Bulky - High power consumption - High operating voltages required - Less suitable for portable products Solution: SEMICONDUCTOR TECHNOLOGY! Electronics Engineering Department | ECE 2119 |  UNIVERSITY OF SANTO TOMAS Semiconductor Technology Electronics Engineering Department | ECE 2119 |