Transistors

Objectives

  • Define the basic function of a transistor
  • Examine the types of transistors
  • Discuss techniques for troubleshooting transistors

What is a Transistor?

At its most basic, a transistor is a semiconductor device that is used to control or amplify an electrical current.

Transistors are divided into groups:

Bipolar Junction Transistors (BJT)
Field Effect Transistors (FET)

The difference between the two groups is in the way that they control electrical current.

Bipolar Junction Transistors

A bipolar junction transistor (BJT) is essentially two diodes combined together.

This is done by sandwiching a P-type semiconductor material between two N-type semiconductor materials to create an NPN transistor.
(The NPN type is by far the most common type of bipolar junction transistor.)

Alternately by sandwiching an N-type semiconductor material between two P-type semiconductor materials creates a PNP transistor.

Collector
Base
Emitter

A bipolar junction transistor has three leads: the collector, the emitter, and the base.

In NPN BJT's the current flows from collector to emitter.
Controlled by base-emitter current.

In PNP BJT's, the current flows from emitter to collector.
Controlled by emitter-base current.

BJT Schematic Symbol

The schematic symbol for a NPN transistor.

Collector
Base
Emitter

The schematic symbol for a PNP transistor.

Collector
Base
Emitter
The schematic symbol for a BJT transistor is designed to remind the reader that the base is what is controlling the current flow across the device.

The line with the arrow on it is always the emitter.
The direction of the arrow lets you know whether you are dealing with NPN transistor or a PNP transistor.

A little trick to help you remember:

NPN means Not Pointing iN.

BJT Operation

The transister functions like a faucet for an electric current.

Base
Collector
Emitter

The base acts as the control valve, limiting how much flow is allowed to pass.

Let's look at how this works in a simple circuit.

VCE
RC
RB
Collector
Base
Emitter
Switch
VBE
Ground

When the switch is closed a path is created for current to flow from the signal source (VBE), across the base-emitter junction, to the ground.

Collector
Base
Emitter
P-N Junction

Once a sufficent amount of current begins to flow through a P-N junction, (in this case the base-emitter junction), that junction resistance becomes effectively 0Ω. This is the same way that a diode works.

With the resistance of the base-emitter junction now reduced, current can now flow directly from the collector across to the emitter.

The current across the collector to the emitter is proportionally greater than the current that is flowing across the base to to the emitter.

The ratio of the two current flows is dependant on the specific material that the transister is made from.

This is how a small controlling current is able to control a much greater working current.

When the switch is opened again the current flow through the base-emitter junction stops and that junction reasserts its resistance. Current flow from the collector to the emitter is no longer possible.

Try It Out!

Use the Switch button to activate the circuit.

Then use the Signal slider to change the value of the resistor.

This will change the amount of current flowing through the lamp.

Use the Switch button to activate the circuit.
Then use the Signal slider to change the value of the resistor.
This will change the amount of current flowing through the lamp.
Notice that as the resistance goes down the current increases and the light gets brighter.

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