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TOPIC 3: ELECTRONICS | PHYSICS FORM 6

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PHYSICS FORM SIX FULL NOTES ENVIRONMENTAL PHYSICS ATOMIC PHYSICS TOPIC 3: ELECTRONICS | PHYSICS FORM 6 CURRENT ELECTRICITY ELECTROMAGNETISM III

TOPIC 3: ELECTRONICS | PHYSICS FORM 6

1. Conductors

Posses free electrons

Metals are all good conductors due to having low resistance to the flow of current.

2. Insulators

They do not have free electrons for conduction. They have high resistance to the flow of current.

All non metals are bad conductors. Eg. dry wood, paper and air.

3. Semiconductors

These are class of materials whose conductivity is between that of good conductors and insulators

Silicon and Germanium are examples of semiconductors elements widely used in electronic industry.

INTRINSIC SEMICONDUCTORS

These are pure semiconductors.

EXTRINSIC SEMICONDUCTORS

These are impure semi conductors material.

DOPING

Is a process of introducing a tiny amount of impurity into a semiconductors material to form extrinsic semiconductors shells.

 N-SEMI CONDUCTORS
Silicon and germanium atoms are tetravalent

-They have four electrons in their outermost shell.
-When a doner atom with fine electrons in its outer most shell (ie Arsenic) is added to a silicon crystal, the fifth electrons becomes a free change carriers since there is production of large number of negative charge carriers(electrons) the impure semiconductors is called N-Semiconductors


E:\..\..\..\thlb\cr\tz\__I__Images__I__\nsemiconductor.png

P-SEMICONDUCTORS​​ 

A-P- Semiconductor is made by adding a trivalent atom (an acceptor) such as B or on to pure semi conductor such as germanium. Since there is a production of large number of holes (positive charges) the impure semiconductor is called P- Semiconductor.

E:\..\..\..\thlb\cr\tz\__I__Images__I__\psemiconductor.png

P-N=JUNCTION/DIODE:

This is formed when P and N semiconductors are melted to form a junction between them

E:\..\..\..\thlb\cr\tz\__I__Images__I__\p8.png

The marrow region at the P-n junction which contains the negative and positive charge is called depletion layer.

A barrier dip is a p.d which oppose more diffusion of charges across the junction.

This is produced when the flow of +ve and -ve Charges ceases

 

P – N JUNCTION AS A RECTIFIER:

FORWARD BIAS.
Is said to be forward biased when its P- semiconductor is connected to the +ve terminals of the battery and its N- Semi conductors is connected to the -ve terminal at the battery.

In this case electrons and holes flow across the P-n junction. This happen because the +ve pole of the battery repel the +ve charge and –Ve pole rel the –ve charges.

 E:\..\..\..\thlb\cr\tz\__I__Images__I__\p9.png

REVERSE BIAS

A-P-N junction is said to be reverse biased when its P. Semiconductor is connected to the negative pole junction of a battery and N. Semiconductor is connected to the +ve [p;e pf the battery in this case only a very small a current flows.

E:\..\..\..\thlb\cr\tz\__I__Images__I__\reverse_Bias.png

                                      P-N JUNCTION AS RECTIFIERS

 E:\..\..\..\thlb\cr\tz\__I__Images__I__\pnrectfier.jpg

The graph shows that P-N junction acts as a rectifier, it has low resistance in one direction of P.d (ie + v) and higher resistance in the opposite direction of P.d (-v)

 

RECTIFIER CIRCUITS:​​ 

HALF-WAVE RECTIFIER CIRCUIT​​ 

 

 E:\..\..\..\thlb\cr\tz\__I__Images__I__\moja.jpg

A rectifier is a circuit which allow the flow of current I P.d in one direction only:

FULL –WAVE RECTIFIERS CIRCUITS:​​ 

a)     Using centre –tapped transformer.

E:\..\..\..\thlb\cr\tz\__I__Images__I__\p10.png

b)    Using bridge circuits

 E:\..\..\..\thlb\cr\tz\__I__Images__I__\using_Bridge_Circuits1.png

On one half of a cycle when P is +v relative to Q only diode D1​​ conducts

On one other half the same cycle only the diode D2​​ conducts.

In both cases the current gees through resistor RL in the same direction.

The large capacitor C is used for stabilizing the marying d.c voltage.

B)    TRANSISTORS​​ 

Transistor is a component which amplifies current. It is made from three layers of P and n. Semiconductors. The layers are called the emitter (E) base (B) an collector (C)

There are two types of transistors.

       I.            n.p.n transistor

     II.            p.n.p transistor

E:\..\..\..\thlb\cr\tz\__I__Images__I__\transistors.png

E:\..\..\..\thlb\cr\tz\__I__Images__I__\transistor21.png

Formation of a transistor

A transistor is formed by putting the doped semiconductors together in such a way that two junction are formed.

 The pnp transistor (bipolar transistor).

E:\..\..\..\thlb\cr\tz\electronics_Files\image024.jpg

-Bipolar means n p n and p n p transistor as they have two opposite polarity of doped semiconductors and voltages across terminals

P n p
E:\..\..\..\thlb\cr\tz\__I__Images__I__\electro1.png

n p n
E:\..\..\..\thlb\cr\tz\__I__Images__I__\electro2.png

Transistor configuration

There are 3 basic configurations

1. Common emitter configuration

2. Common base configuration

3. Common collector configuration

1. Common emitter configuration (n p n)

Under thus configuration the transistor has both voltage gain and current gain.

E:\..\..\..\thlb\cr\tz\electronics_Files\image025.jpg

To get volt you need a resistance​​ RL

E:\..\..\..\thlb\cr\tz\electronics_Files\image029.gif= IE:\..\..\..\thlb\cr\tz\electronics_Files\image027.gif

Ring is used for injecting only a small current for great amplification on E by C

Current gain

    E:\..\..\..\thlb\cr\tz\electronics_Files\image030.gif = very large​​ E:\..\..\..\thlb\cr\tz\electronics_Files\image031.gif

 E:\..\..\..\thlb\cr\tz\__I__Images__I__\230.png

E:\..\..\..\thlb\cr\tz\__I__Images__I__\319.png

E:\..\..\..\thlb\cr\tz\__I__Images__I__\412.png

E:\..\..\..\thlb\cr\tz\electronics_Files\image039.gif

But

E:\..\..\..\thlb\cr\tz\__I__Images__I__\510.png

Also  ​​
E:\..\..\..\thlb\cr\tz\__I__Images__I__\69.png

E:\..\..\..\thlb\cr\tz\electronics_Files\image046.gifis the reflection of  ​​ E:\..\..\..\thlb\cr\tz\electronics_Files\image047.gif

Common base configuration (PnP)

E:\..\..\..\thlb\cr\tz\electronics_Files\image048.jpg

Under this configuration the transistor has voltage gain but no current gain

Earthing puts the common line at p.d=0

 E:\..\..\..\thlb\cr\tz\electronics_Files\image049.gif

E:\..\..\..\thlb\cr\tz\electronics_Files\image050.gif=E:\..\..\..\thlb\cr\tz\electronics_Files\image051.gif

E:\..\..\..\thlb\cr\tz\__I__Images__I__\710.png

Common collector configuration

Under this configuration the transistor has current gain but no voltage gain

E:\..\..\..\thlb\cr\tz\electronics_Files\image054.gif=Amplification factor

           ​​ E:\..\..\..\thlb\cr\tz\electronics_Files\image055.gif=E:\..\..\..\thlb\cr\tz\electronics_Files\image056.gif

            ​​ E:\..\..\..\thlb\cr\tz\electronics_Files\image017.gif=E:\..\..\..\thlb\cr\tz\electronics_Files\image057.gif

E:\..\..\..\thlb\cr\tz\__I__Images__I__\810.png

E:\..\..\..\thlb\cr\tz\electronics_Files\image062.jpg

E:\..\..\..\thlb\cr\tz\__I__Images__I__\96.png

For common emitter

                           Vo =​​ IC​​ R2

                             Vi =​​ IBRB

E:\..\..\..\thlb\cr\tz\__I__Images__I__\103.png

From, IE =​​ IB​​ + IC

E:\..\..\..\thlb\cr\tz\__I__Images__I__\1117.png

E:\..\..\..\thlb\cr\tz\electronics_Files\image077.gif

E:\..\..\..\thlb\cr\tz\electronics_Files\image078.gif

Common emitter characteristic curve

 

E:\..\..\..\thlb\cr\tz\electronics_Files\image079.jpg

 

The circuit above is for investigating the variation of current with voltage in the input and output circuits.

OUTPUT CHARACTERISTICS
IC-VCE​​ with IB​​ constant .​​

The results are plotted below.
The knee of the curves shown corresponds to a low P.d(0.2) the output for higher P.d the output IC​​ varies linearly with VCE​​ for a given value of base current IB.

The linear part of the characteristic is the one used in the audio frequency (a.f) amplifiercircuits so that the output is undistorted.

E:\..\..\..\thlb\cr\tz\__I__Images__I__\146.png

INPUT CHARACTERISTICS

IB-VBE​​ with VCE​​ constant​​

The results are as follows:-
The input characteristics is non-linear

E:\..\..\..\thlb\cr\tz\__I__Images__I__\1311.png

TRANSFER CHARACTERISTICS
IC-IB  with VCE​​ constant

The results are as shown below:-
The output current IC​​ varies linearly with the input current IB. The current transfer ratio or current gain is given by
E:\..\..\..\thlb\cr\tz\__I__Images__I__\153.png
​​ E:\..\..\..\thlb\cr\tz\__I__Images__I__\164.png

In the figure  below​​
E:\..\..\..\thlb\cr\tz\__I__Images__I__\182.png

 E:\..\..\..\thlb\cr\tz\__I__Images__I__\1216.png​​

E:\..\..\..\thlb\cr\tz\__I__Images__I__\18.1_.png

Questions

1.   An npn transistor has a current gain (Beta) value of 200. Calculate the base current​​ E:\..\..\..\thlb\cr\tz\electronics_Files\image018.gif required to switch a resister of 4µA.

E:\..\..\..\thlb\cr\tz\electronics_Files\image084.jpg

E:\..\..\..\thlb\cr\tz\__I__Images__I__\192.png

E:\..\..\..\thlb\cr\tz\electronics_Files\image088.gif

2. An npn transistor has a dc base bias voltage​​ E:\..\..\..\thlb\cr\tz\electronics_Files\image089.gif of and an input base resistor​​ E:\..\..\..\thlb\cr\tz\electronics_Files\image090.gif of 100kΩ.What will be the of base current into the transistorE:\..\..\..\thlb\cr\tz\electronics_Files\image091.gif

(The transistor is a silicon type)​​

E:\..\..\..\thlb\cr\tz\electronics_Files\image092.jpg

Data

From KirchhoffE:\..\..\..\thlb\cr\tz\electronics_Files\image093.gif law

For silicon

E:\..\..\..\thlb\cr\tz\electronics_Files\image089.gif =10v​​

E:\..\..\..\thlb\cr\tz\electronics_Files\image090.gif  = 100kΩ

E:\..\..\..\thlb\cr\tz\electronics_Files\image094.gif =0.6v (wasted voltage)

Solution

E:\..\..\..\thlb\cr\tz\electronics_Files\image089.gifE:\..\..\..\thlb\cr\tz\electronics_Files\image095.gifE:\..\..\..\thlb\cr\tz\electronics_Files\image094.gif=0

E:\..\..\..\thlb\cr\tz\electronics_Files\image089.gifE:\..\..\..\thlb\cr\tz\electronics_Files\image094.gif=E:\..\..\..\thlb\cr\tz\electronics_Files\image095.gif

E:\..\..\..\thlb\cr\tz\__I__Images__I__\202.png

Example

Given the circuit below, determine

E:\..\..\..\thlb\cr\tz\electronics_Files\image099.jpg

E:\..\..\..\thlb\cr\tz\electronics_Files\image100.gif

E:\..\..\..\thlb\cr\tz\electronics_Files\image015.gif

E:\..\..\..\thlb\cr\tz\electronics_Files\image101.gif

E:\..\..\..\thlb\cr\tz\electronics_Files\image102.gif

E:\..\..\..\thlb\cr\tz\electronics_Files\image103.gif

The transistor has​​ E:\..\..\..\thlb\cr\tz\electronics_Files\image104.gif=150

The transistor of silicon type

Solution

Second Kirchhoff’s law in the input circuit

E:\..\..\..\thlb\cr\tz\electronics_Files\image105.gif×E:\..\..\..\thlb\cr\tz\electronics_Files\image106.gif = 0

5VE:\..\..\..\thlb\cr\tz\electronics_Files\image107.gif

E:\..\..\..\thlb\cr\tz\electronics_Files\image108.gif

E:\..\..\..\thlb\cr\tz\electronics_Files\image109.gifA

E:\..\..\..\thlb\cr\tz\electronics_Files\image110.gifA

E:\..\..\..\thlb\cr\tz\electronics_Files\image111.gif

E:\..\..\..\thlb\cr\tz\electronics_Files\image112.gif

E:\..\..\..\thlb\cr\tz\electronics_Files\image113.gif

E:\..\..\..\thlb\cr\tz\electronics_Files\image114.gif

E:\..\..\..\thlb\cr\tz\electronics_Files\image115.gif           

E:\..\..\..\thlb\cr\tz\electronics_Files\image116.gif=0.6v

E:\..\..\..\thlb\cr\tz\electronics_Files\image117.gif

10E:\..\..\..\thlb\cr\tz\electronics_Files\image118.gif

E:\..\..\..\thlb\cr\tz\electronics_Files\image119.gif=​​ 3.4V

E:\..\..\..\thlb\cr\tz\electronics_Files\image120.gif

E:\..\..\..\thlb\cr\tz\electronics_Files\image121.gif=5+ (​​ E:\..\..\..\thlb\cr\tz\electronics_Files\image122.gif)

E:\..\..\..\thlb\cr\tz\electronics_Files\image123.gif=5+ (10000×4.4×10-4)​​ E:\..\..\..\thlb\cr\tz\electronics_Files\image124.gif (6.6)​​

E:\..\..\..\thlb\cr\tz\electronics_Files\image089.gif=2.8v

Example

1. A common emitter amplifier has​​ E:\..\..\..\thlb\cr\tz\electronics_Files\image127.gif= 1.2kΏ​​ and supply Voltage of V=12v. Calculate the maximum collector current​​ E:\..\..\..\thlb\cr\tz\electronics_Files\image015.gif following throughout resistor when switched fully on (saturation assumeE:\..\..\..\thlb\cr\tz\electronics_Files\image128.gif.Also find​​ E:\..\..\..\thlb\cr\tz\electronics_Files\image129.gifwith a voltage drop of 1v across it, the transistor silicon.

            Solution   ​​ 

E:\..\..\..\thlb\cr\tz\electronics_Files\image130.gif

E:\..\..\..\thlb\cr\tz\__I__Images__I__\2112.png

E:\..\..\..\thlb\cr\tz\electronics_Files\image132.gif

   ​​ E:\..\..\..\thlb\cr\tz\electronics_Files\image133.gifA                      ​​

Quiescent point:

It’s a point when the current flow is smooth i.e. not being clicked (excess) and transistor functions.

Saturation point:

 If​​ E:\..\..\..\thlb\cr\tz\electronics_Files\image018.gif=0 transistor is in the cutoff region, there is a small current collector leakage, CEO

E:\..\..\..\thlb\cr\tz\electronics_Files\image134.jpg

Normally​​ E:\..\..\..\thlb\cr\tz\electronics_Files\image135.gifis neglected so thatE:\..\..\..\thlb\cr\tz\electronics_Files\image136.gif  =E:\..\..\..\thlb\cr\tz\electronics_Files\image137.gif

In cutoff both the base emitter and base collector junction are reverse based.

When base emitter becomes forward based.​​ E:\..\..\..\thlb\cr\tz\electronics_Files\image138.gifis increase, then IC also increases when​​ E:\..\..\..\thlb\cr\tz\electronics_Files\image139.gif decreases as a result.

When​​ E:\..\..\..\thlb\cr\tz\electronics_Files\image139.gif reaches its saturation value BC junction becomes forward based and​​ E:\..\..\..\thlb\cr\tz\electronics_Files\image140.gif can increase no further even with continued increase in​​ E:\..\..\..\thlb\cr\tz\electronics_Files\image018.gif

At the point of saturation (E:\..\..\..\thlb\cr\tz\electronics_Files\image141.gif) not longer valid)

     ​​ VCE(Sat)​​ for a transistor occurs somewhere below the knees of the collector curve.

  The saturation value for​​ E:\..\..\..\thlb\cr\tz\electronics_Files\image139.gif (Sat) is usually a few tenth of volt for silicon transistors.

  The DC load line, the cutoff and saturation can be illustrated by the load line.​​

  Between the cutoff point and the saturation point is where the transistor is active and as most active at the quiescent point.

Self biasing /fixed bias

E:\..\..\..\thlb\cr\tz\electronics_Files\image143.jpg

E:\..\..\..\thlb\cr\tz\electronics_Files\image144.gif

E:\..\..\..\thlb\cr\tz\__I__Images__I__\2210.png

E:\..\..\..\thlb\cr\tz\electronics_Files\image146.gif

Outer loop

E:\..\..\..\thlb\cr\tz\electronics_Files\image147.gif

E:\..\..\..\thlb\cr\tz\electronics_Files\image148.gif

E:\..\..\..\thlb\cr\tz\electronics_Files\image149.gifA  ​​

 E:\..\..\..\thlb\cr\tz\electronics_Files\image022.gif

 E:\..\..\..\thlb\cr\tz\electronics_Files\image150.gif

E:\..\..\..\thlb\cr\tz\electronics_Files\image151.gifA    ​​

E:\..\..\..\thlb\cr\tz\electronics_Files\image152.gif

10 = 9.4×10-3×100 +​​