Transistors

TRANSISTORS

A transistor is a semiconductors device commonly used to amplify or switch electronic signals.

• A transistor made from three layers of p – type and n-type semiconductors 

TYPES OF TRANSISTORS

There are two (2) types of transistors

          i.            Bipolar transistors (BJT – transistors)

          ii.            Field –Effect Transistors(FETs)

BIPOLAR TRANSISTORS (BJT)

Is the transistor consists of a pair of p-n junction diodes that are joined back- to- back.

LAYERS OF TRANSISTORS

There are three layers of transistors.

             i.            Base

             ii.            Emitter

             iii.            Collector

BASE

Controls the flow of charge carriers from the emitter to the collector

COLLECTOR:

Receives charge carriers from the emitter.

EMITTER

Supply charge carriers to the collector.

The arrow on the emitter shows the direction of the convection current

TYPES OF BIPOLAR TRANSISTORS

There two types of bipolar transistors.

             i.            NPN transistor

             ii.           PNP transistors

NPN TRANSISTOR

Is the transistor where p-type semi-conductor piece is sandwiched between two pieces of n – type semiconductor layers

The charge carriers in NPN transistor are electrons.

MODE ACTION (OPERATION) OF NPN TRANSISTORS

For the npn transistor to operate, the base emitter junction is forward biased.

The forward bias causes the electrons in n-type emitter to flow towards the base which constitute emitter current (Ie). These electrons cross into the p-type base, they try to combine with holes but the base is lightly doped and very thin. Therefore only few electrons combine with holes and remaining electrons cross into the collector and generates collector current (I_c)

I_e = I_b + I_c

PNP TRANSISTOR

Is the transistor when n-type semiconductor piece is sandwiched between two pieces of p-type semiconductors.

The majority charge carriers are holes.

MODE OF ACTION (OPERATION) OF PNP TRANSISTORS

For the pnp transistor to operate, the base-emitter junction is forward biased.

     

The forward bias causes the holes in p- type emitter to flow towards the base which constitute emitter current (Ie). These holes cross into the n-type base, they try to combine with electrons but the base is lightly doped and very thin. Therefore, only few holes combine with electrons and remaining holes cross into the collector and generates collector current (Ic)

I_e = I_b + I_c

Note: -

Most transistors in used today are n-p-n transistors because the majority carriers are electrons which move faster than holes in p-n-p transistors.

USES OF TRANSISTORS

Transistors are usually used in many electronic devices such as calculators, televisions, radios, computers, biomedical instruments etc.

Used in: -

                                             i.            Switching circuits.

                                           ii.            Amplifier circuits.

                                          iii.            Oscillator circuits.

                                          iv.            Voltage regulator circuits.

                                           v.            Logic gates/digital logic. Integrated circuits.

TRANSISTOR AS AMPLIFIER

Amplifier is a circuit that increases the amplitude of a given input signets.

SINGLE – STAGE AMPLIFIERS

These are amplifiers with only one amplifying device.

CONFIGURATIONS OF SINGLE STAGE AMPLIFIERS

There are three configurations:-

                                          i.            The common – Base amplifier.

                                        ii.            The common – Emitter amplifier.

                                      iii.            The common – collector amplifier.

THE COMMON – BASE AMPLIFIER (CB)

In the common Base configuration, the Base in common to both input signal and output signal. The input signals being applied between the base and the emitter terminals. The output signal being taken between the base and the collector terminals.

The common – Base configuration has voltage gain but no current gain.

This configuration is not very common due to its unusually high voltage gain characteristics.

THE COMMON – COLLECTOR (CC) AMPLIFIER

In the common – collector configuration, the collector is common to both input signals and output signals.

This configuration (CC) is also called voltage follower or Emitter follower.

The emitter – base is forward biased and collector base is reverse – biased.

Note: -

The common – collector has current gain but no voltage gain.

COMMON – EMITTER (CE) AMPLIFIER

In the common emitter amplifier, the input signal is applied between the base and emitter while the output signals is taken between the collector and emitter.

Note: -

The common – emitter configuration is commonly used because.
    i.    Has both current gain and voltage gain.
   ii.   The current gain and voltage gain is very high.
   iii.  Produce low input impedance as it connected to forward biased PN – Junction, while the output impedance is HIGH when it is taken from a reverse – biased PN – Junction.

FULL STABILIZED COMMON EMITTER AMPLIFIER

Is the common – emitter arrangement for the need to stabilize them for excessive temperature rise. This helps to prevent transistor from damage due to temperature rise.

COMPONENTS

i.    Potential divider (R₁ and R₂ )

• Provides the base bias voltage as per requirement
•  Controls base current.

ii.   Coupling capacitors (C₁ and C₂)

• Separates A.C signals from D.C biasing voltage.
• They are also called “STAGE”

iii.   Load resistor (collector resistor) – R₂

• Produce/control output voltage

iv.  Emitter resistor(Gain resistor)- R₃

• Controls “thermal runaway” which can burn up a transistor.
• Controls voltage gain or amplification.

v.   By pass capacitor (C₃)

•  Prevents undesirable feedback of the amplified signal to the base – emitter circuit.
• Prevent or decrease negative feedback of the amplified signals.