TOPIC 3: ELECTRONICS | PHYSICS FORM 6
Posses free electrons
Metals are all good conductors due to having low resistance to the flow of current.
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.
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.
These are pure semiconductors.
These are impure semi conductors material.
Is a process of introducing a tiny amount of impurity into a semiconductors material to form extrinsic semiconductors shells.
–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
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.
This is formed when P and N semiconductors are melted to form a junction between them
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:
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.
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.
P-N JUNCTION AS RECTIFIERS
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)
HALF-WAVE RECTIFIER CIRCUIT
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.
b) Using bridge circuits
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.
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
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).
-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
n p n
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.
To get volt you need a resistance RL
Ring is used for injecting only a small current for great amplification on E by C
= very large
is the reflection of
Common base configuration (PnP)
Under this configuration the transistor has voltage gain but no current gain
Earthing puts the common line at p.d=0
Common collector configuration
Under this configuration the transistor has current gain but no voltage gain
For common emitter
Vo = IC R2
Vi = IBRB
From, IE = IB + IC
Common emitter characteristic curve
The circuit above is for investigating the variation of current with voltage in the input and output circuits.
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.
IB-VBE with VCE constant
The results are as follows:-
The input characteristics is non-linear
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
In the figure below
1. An npn transistor has a current gain (Beta) value of 200. Calculate the base current required to switch a resister of 4µA.
2. An npn transistor has a dc base bias voltage of and an input base resistor of 100kΩ.What will be the of base current into the transistor
(The transistor is a silicon type)
From Kirchhoff law
=0.6v (wasted voltage)
Given the circuit below, determine
The transistor has =150
The transistor of silicon type
Second Kirchhoff’s law in the input circuit
× = 0
=5+ ( )
=5+ (10000×4.4×10-4) (6.6)
1. A common emitter amplifier has = 1.2kÎ and supply Voltage of V=12v. Calculate the maximum collector current following throughout resistor when switched fully on (saturation assume.Also find with a voltage drop of 1v across it, the transistor silicon.
It’s a point when the current flow is smooth i.e. not being clicked (excess) and transistor functions.
If =0 transistor is in the cutoff region, there is a small current collector leakage, CEO
Normally is neglected so that =
In cutoff both the base emitter and base collector junction are reverse based.
When base emitter becomes forward based. is increase, then IC also increases when decreases as a result.
When reaches its saturation value BC junction becomes forward based and can increase no further even with continued increase in
At the point of saturation () not longer valid)
VCE(Sat) for a transistor occurs somewhere below the knees of the collector curve.
The saturation value for (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
10 = 9.4×10-3×100 +