Transistors

1. Introduction

The bipolar transistor is a semiconductor, it is an electronic component which has 3 legs:

  • E: Transmitter
  • C: Collector
  • B: Basic

There are two types of transistor: PNP and NPN.

The main characteristic of the transistor is its amplification capacity.

 

2. Shapes

Transistors de faible puissance
 

Transistors can be found in different forms.

 
 

3. Analogy with hydraulics

Analogie avec l hydraulique

In the figure above, we can make the analogy of the operation of a transistor with that of a valve.

NPN: When a positive force (up and down) is applied to the valve lever, flow occurs from the manifold which is up (+) to the emitter which is down (-). The greater the force on the valve, the more the flow between the manifold and the emitter will be amplified.

PNP: When a negative force (from bottom to top) is applied to the valve lever, a flow appears from the emitter which is in height (+) towards the collector which is in bottom (-). The greater the force on the valve, the more the flow between the emitter and the collector will be amplified.

 

4. PNP transistor operation

Transistor pnp 1
Test transistor pnp
 
  1. Fig. A

PNP: The base is driven by a negative (-).

When the emitter is powered by a (+), the switching between the emitter and the collector takes place.

  1. Fig. B

To test a PNP transistor, you must use the " diode tester  " function of the multimeter.

For a PNP transistor in good working order:

  • The test is passing from the transmitter to the base.
  • The test is non-passing from the base to the transmitter.
  • The test is passing from the collector to the base.
  • The test is non-passing from the base to the collector.
 

5. NPN transistor operation

Transistor npn 1
Test transistor npn
 
  1. Fig. C

NPN: The base is driven by a positive (+).

When the base is powered by a (+), the switching between the collector and the emitter takes place.

.

  1. Fig. B

To test an NPN transistor, use the "diode tester" function of the multimeter .

For an NPN transistor in good working order:

  • The test is passing from the base to the transmitter.
  • The test is non-conducting from the transmitter to the base.
  • The test is passing from the base to the collector.
  • The test is not passing from the collector to the base.
 

6. PNP transistor connection

Connexion transistor pnp
 

PNP: The base is driven by a negative (-), the "collector" signal output is positive (+).

By closing the switch (NO), we see that the bulb (L1) lights up. The light intensity changes according to the position of the potentiometer (Pot). The resistor (R) is here to protect the transistor from overcurrent.

The emitter/base pass current creates a current multiplied by the gain of the transistor between the emitter and the collector.

Gain: Ability of a device to increase the strength of a signal.

 

6. NPN transistor connection

Connexion transistor npn

NPN: The base is driven by a positive (+), the "transmitter" signal output is negative (-).

By closing the switch (NO), we see that the bulb (L1) lights up. The light intensity changes according to the position of the potentiometer (Pot). The resistor (R) is here to protect the transistor from overcurrent.

The base/emitter pass current creates a current multiplied by the gain of the transistor between the collector and the emitter.

Gain: Ability of a device to increase the strength of a signal.

 
 

 
 

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