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Transcript 
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
PLATE
CATHODE
FILAMENT
Vacuum tube diode
• When the charge on the plate is positive –
current will flow
• When the charge is negative – no current
flows
_
- - -- - - - --
+
- - -- - - - --
Vacuum tube triode
• Lee Deforrest added a third element to the
diode
• Triode = three elements
PLATE
GRID
CATHODE
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
+
Bias voltage
100v
10v
+
+
+
+
- - - - - - --
+
+
+ + + +
- - - - - - --
Audio out
Audio in
100v
10v
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