Electron EmissionIntroduction The reader is familiar with current conduction (i.e.

flow of electrons)through a conductor. Examples are: current through power lines, windings of transformers and motors etc. Many electronic devices depend for what their operation on the movement of electrons in an evacuated space. For this purpose, the free electrons must be ejected from the surface of metallic conductor by supplying sufficient energy from some external source. This is known as electron emission. The emitted electrons can be made to move in vacuum under the influence of an electric field, thus constituting electric current in vacuum.

Metals are most suitable substances for electron emission because they contain a large number of free electrons. However, metal can not emit electrons from their surface under ordinary conditions. This is due to the fact that the electrons in the metal are free only to the extend that they may be transferred from one atom to another within the metal but they cannot leave metal surface to provide electron emission. However, if sufficient external energy is given to free electrons, their kinetic energy is increased and thus electrons will leave the metal surface. The additional energy required to emit electrons from a metallic surface is known as work function of the metal. The work function of pure metals varies roughly from 2eV to 6eV. It may be noted that it is desirable that metal used for electron emission should have low work function so that a small amount of energy is required to cause emission of electrons.

Types of Electron Emission. In order to liberate electrons from a metallic surface, external energy equal to work function of the metal must be supplied. The external energy supplied to the metal may be in several forms such as heat energy, energy stored in electric field, light energy or kinetic energy of the electric charges bombarding the metal surface. Accordingly, the following are the four principal methods of obtaining electron emission from the surface of a metal:

(1) Thermionic emission (2) Field emission

(3) Photo-electric emission (4) Secondary emission

The most commonly used type of emission is the thermionic emission. In this case, the emission from the metallic surface is caused by supplying thermal energy. The commonly used substances for thermionic emission are tungsten, thoriated tungsten, oxide-coated cathode ect. Although thermionic emitters should have the desirable characteristic of low work function, high melting point and high mechanical strength, yet their choice for a particular situation would depend upon the service requirements.

For instance, tungsten has high melting point (3560°C), high mechanical strength, high operating temperature (2500°C) and comparatively high work function (4.52 eV) and would, therefore, be suitable for applications involving high voltages (> 10 kV) e.g. in X-ray tubes. On the other hand, oxide coated cathodes have low work function (1.1 eV), low operating temperature (750°C) and comparatively low melting point and would, therefore, be suitable for applications involving small voltages (<1000V) e.g. in the tubes of radio receivers.

Important points about Electron Emission

Electron emission from a metallic surface will take place if external energy equal to or greater than the work function of the metal is supplied.

Generally, high temperature are required to cause thermionic emission.

Thermionic emitters are heated in vacuum by electrical methods because of convenience.

The thermionic emitter is also called cathode. The cathode can be of directly heated type or indirectly heated type. Indirectly heated cathodes are generally used in low voltage applications e.g. in the tubes of a radio receiver. However, directly heated cathodes are used in high voltage applications.

In field emission, the emission of electrons takes place by the application of strong electric field at the surface of a metal, usually at the room temperature. Generally, a voltage of the order of a million volts per centimeter distance between the metal and charged conductor is necessary to cause field emission.

In secondary emission, the emission of electrons from a metallic surface is obtained by the bombardment of high speed electrons or other particles. Secondary emission effects are undesirable in vacuum tubes because they lead to unfaithful amplification. However, secondary emission effects are utilized in some electronic devices e.g. electron multipliers etc.

 

Note : Thermionic emission is the most widely used type of emission because it is very convenient method of obtaining electron emission.

CHAPTER 2: VACUUM

TUBESBASIC ELECTRONICS SEPT. 11.2013

JOHNPAOLO R. NAPIZAJEWEL R. MARCELLABSE TLE III A.Y. 2013 - 2014

Prepared by:

ENGR. AQUILINO NOCEDA

Prepared to:

Introduction

CATHODE-

ANODE-

A vacuum tube is an electron device in which the flow of electrons is through vacuum. It usually contains cathode which is the electron emitter; an anode (anode called plate) which is the electron collector and one or more electrodes (called grids) for controlling the flow of electrons between cathode and plate.

These electrodes are housed in highly evacuated glass envelope. The plate is held at positive potential w.r.t. cathode so that the emitted electrons are attracted to plate to provide current in vacuum. The ability of vacuum tubes to conduct current in vacuum enables them to perform such functions as rectification, amplification etc.

Types of Vacuum Tubes

GRIDS The vacuum tubes are generally classified according to the number of electrodes present. There are two principal electrodes, namely cathode and plate present in every tube. The other electrodes, if any, called grids. It may be noted counted as electrodes because it is merely incandescent filament to heat the cathode electrically.

(i) DIODE

A vacuum diode is the simplest tube and contains cathode and anode(or plate) enclosed in a highly evacuated glass envelope. The plate is held at some high positive potential w.r.t. cathode. When the cathode is heated by passing electric current through the heater, it emits a large number current “Ib” . The plate current in a diode depends upon plate voltage and cathode temperature. Is used as rectifier to changed the alternating current to direct current. The major limitation of a vacuum diode is that plate current cannot be changed easily.

(ii) TRIODE A triode contains three electrode

cathodes, plate and control grid. The cathode and plate have similar construction as for a diode. The control grid consist of a fine mesh placed very close to the cathode. The placed is held at some positive potential w.r.t. cathode. The electrons emitted by the cathode go through the openings of the grid before they are captured by the plate.

Since the control grid is much closer to the cathode than the plate, a small voltage on the control grid has much more control on the electron flow than a comparatively high voltage on the plate. This places the control grid in a commanding position to control plate current in a triode.

The current controlling property of control grid permits a triode to act as amplifier. The weak signal is introduced in the grid circuit and amplified output is obtained in the plate circuit. The major limitations of triode are low amplification and feedback from plate to grid at high frequencies. For this reason, the use of triodes is only limited for low frequency applications.

(iii) TETRODE Is a four-electrode valve. It contains a

plate, cathode, control grid and screen grid. The potentials on cathode, plate and control grid are somewhat similar as for a triode. However, screen grid is held at some positive potentials (less than plate potentials) w.r.t. cathode. A tetrode also acts as an amplifier with the improved action of screen grid.

The presence of this grid increase the amplification and reduces the feedback from plate to grid. The limitations of tetrode is that during some positive part of the signal, plate current is decreased due to secondary emission effects.

(iv) PENTODE Is a five-electrode valve. It contains a

plate, cathode, control grid, screen grid and suppressor grid. The suppressor grid is generally held at cathode potential. The suppressor grid suppresses the harmful effects of secondary emission. Futher, it increases amplification and reduces the feedback from the plate grid. Therefore, pentode is an ideal tube to act as an amplifier.

Tube Co- efficients The most important tube cnstants are

amplification factor a.c. plate resistance and tranconductance.

Amplification factor, µ is at constant Ib a.c. plate resistance,, rp= at constant Ec

Transconductance, gm=at constant Eb The relations among these three constant is

given by; µ = rp x gm (it may be noted that rp is gerally measured in Ohms while gm is measured in mA / V [micromhos]).

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