Transistor Switches

Transistor Switch Operation
NPN Common Emitter
NPN Emitter Follower
Index

Electronics concepts

Digital Electronics

Reference
Horowitz & Hill
 
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Transistor Switch Example

The switch is open.

Close the switch

There is no current to the base, so the transistor is in the cut off condition with no collector current. All the voltage drop is accoss the transistor.

Transistor operation for switch conditions
Transistor Switches
Index

Electronics concepts

Digital Electronics

Reference
Horowitz & Hill
p52
 
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Transistor Switch Example

The switch is closed.

Open the switch

The base resistor is chosen small enough so that the base current drives the transistor into saturation.

In this example the mechanical switch is used to produce the base current to close the transistor switch to show the principles. In practice, any voltage on the base sufficient to drive the transistor to saturation will close the switch and light the bulb.

Transistor operation for switch conditions
Transistor Switches
Index

Electronics concepts

Digital Electronics

Reference
Horowitz & Hill
p52
 
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NPN Common Emitter Switch

For switching currents less than an ampere, the transistor switch can be used. Instead of a mechanical switch in the base circuit, an op-amp could be used. When the switch is open, no current flows in the base so the collector current is cut off. The resistor RB must be small enough to drive the transistor to saturation so that most of the voltage Vcc appears across the load.

In a configuration where the output is taken below the load resistor, this kind of switch can function as an inverting buffer in digital circuits.

Transistor Switches
Index

Electronics concepts

Digital Electronics

Reference
Diefenderfer/ Holton
p165
 
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NPN Emitter Follower Switch

Though more commonly in the common emitter configuration, the emitter follower can also be used for switching. The resistor RB must be small enough to drive the transistor to saturation so that most of the voltage Vcc appears across the load.

Transistor Switches
Index

Electronics concepts

Digital Electronics

Reference
Diefenderfer/ Holton
p166
 
HyperPhysics*****Electricity and magnetismR Nave
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