Thermal emission The diode The triode How do triodes - - PowerPoint PPT Presentation

Agenda

• A brief history
• Thermal emission
• The diode
• The triode
• How do triodes amplify?

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In 1802, Humphrey Davy invents the electric lamp - 1

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In 1875, American, G.R. Carey invents the phototube.

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In 1878, Englishman Sir William Crookes invents the Crookes tube - 2…

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In 1895, German, Wilhelm Roengten invents an X-ray tube.

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In 1897, German, Karl Ferdinand Braun invents the cathode ray tube. - 5

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In 1904, John Ambrose Fleming invents the first practical valve called the 'Fleming Valve‘ / ‘Kenotron’. - 3

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In 1906, Lee de Forest invents the Audion later called the

• triode. - 4

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 One of the most famous Crookes tubes.  Demonstrates that electrons go in a straight line

and are stopped by metal, hence you see the shadow of the cross on the end glass.

 After a while due to fatigue of the glass the glow is

weaker

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Important discoveries stemming from the Crookes tube.

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Wilhelm Röntgen discovered X-rays using the Crookes tube in 1895.

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In 1897 J. J. Thomson identified “cathode rays” as negatively charged particles, later renamed “electrons.”

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Now Edison …

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 Edison was facing a problem with his light bulbs due

to their short life.

 Although the filament life was a problem, the major

limiting factor was that the bulbs quickly became blackened.

 Initially this was attributed to atoms of carbon from the

filament hitting the glass.

 Despite this …

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1880 Edison patent

U.S. Patent #223898 issued to Thomas Edison on January 27, 1880 for an incandescent lamp Twenty-two other people also devised versions of the light bulb, Edison's outstrip the others because

• Effective incandescent material,
• Higher vacuum
• High resistance made power

distribution from a centralized source economically viable.

• Photos ….

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 Very early Edison lamp  Not very practical  But some were attractive..

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The early lamps were attractive but very inefficient Edison effect…

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Particles leaving the filament were known to be negatively charged, so experiments were carried out to prevent them from hitting the glass.

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One method that Edison tried involved placing a second electrode in the envelope.

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He reasoned that if he placed a positive charge on the second electrode, particles would be attracted away from hitting the glass

• f the bulb.



Edison experimented with the polarity of the charge on the second electrode and he noticed that when the second element was made positive with respect to the filament a current flowed in the

• circuit. When the potentials were reversed this did not happen.



This became known as the Edison effect. Used by Flemming…

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 J.A. Fleming was a consultant to the Marconi  Coherers and Magnetic Detectors  In November 1904 whilst he was walking along Gower

Street in London’s West End he had what he called “a very happy thought".

 He wondered if the Edison Effect could be used to

rectify the "feeble to and fro motions of electricity from an aerial wire".

 Fleming set up an experiment and was able to prove

that the idea worked. Patented in 1905…

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Flemmings Patent 1905 The Kenotron…

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 Patent original  Others not so

attractive….

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Actual hand blown Valves used in Flemming’s 1904 Experiments Then the first practical diode…

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Actual diode valve Used by Marconi in 1904

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There are 3 types of electron Emission

 Secondary – High velocity particles – X rays  Photo – Photons of light – photo cells  Thermionic / Thermal Emission…

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 At ambient temperature outer shell electrons

have insufficient velocity to escape

 As temperature increases the molecular

vibration increases

 Rotational velocity increases  Electrons “Boil” off into surrounding space…

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Material Normal Operating Efficiency in Temperature mA of emission °K per watt heater power Tungsten 2450 to 2600 3 to 15 Thoriated Tungsten 1900 62.5 Oxide coated 1100 to 1170 50 to 125 All materials developed to produce maximum electron flow…

Two requirements for electron flow

 Filament heated to

correct temperature to achieve thermal emission

 Positive anode potential  Require to plot curves…

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

A dynamic curve must be plotted to see how the diode

• works. This requires :-

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A Fixed heater voltage

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A Variable Anode voltage

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We get an Ia /Va curve…

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From this curve we calculate the Ra….

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 The workshop of Alfred Charles Cossor in

Clerkenwell London

 Latter to become a leading British valve

manufacturer.

 Photograph taken in 1896  Some of his workers (children!) preparing

glass tubes.

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 Lee DE Forest  1906 works to improve diode  1908 patents at three electrode valve the

Audion…

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 Early Audion

1906

 Latter versions..

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 The Audion valve

1908

 The Grid…

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 Fitted between heater and anode…

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Electrically it looks like…

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Like the diode we need to plot some curves….

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With this circuit we can plot the Ia/Va and Ia/Vg curves…

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 Input voltage  Output current

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 Input voltage  Output current

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 Input voltage  Output current  To output voltage

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 Directly heated – Battery powered sets  Indirectly heated – Mains powered sets  Directly heated specials – large rectifiers and

transmitting valves

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 The R' valve (1922)  The first widely used triode  Note the thin wire filament

suspended inside helical wire grid, inside tubular anode.

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Ediswan AR (1922)

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Cossor P2 (1922)

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Cosmos SP18 (1925)

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The Cossor device used an unusual 'hat' construction to avoid infringing patents which specified tubular electrodes.

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