Active Devices Vacuum tubes. Thomas Edison’s light bulb – Glass envelope – Vacuum – Filament.

Active Devices

Vacuum tubes

Vacuum tubes

• Thomas Edison’s light bulb– Glass envelope – Vacuum– Filament

FILAMENT

Vacuum tubes

• Thermionic emission– Some metals will emit electrons when heated• “Thermionic” = heat• “Emission” = emit or shed

Fleming Valve

• Ambrose Fleming– Added a cathode and plate to Edison’s bulb– Created a two element device– Diode = two elements– Called it the valve

CATHODE

PLATE

FILAMENT

Vacuum tube diode

• When the charge on the plate is positive – current will flow

• When the charge is negative – no current flows

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Vacuum tube triode

• Lee Deforrest added a third element to the diode

• Triode = three elements

CATHODE

PLATE

GRID

Vacuum tube triode

• Third element called a grid• Triode amplifies• Deforrest called his triode the Audion

Vacuum tube triode

• Small fluctuations in current on the grid are amplified or magnified on the plate

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10v

100vBias voltage

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10v

100vAudio in

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