12th Class Physics CHAPTER 7 ELECTRONICS Short Question Answers
Height of potential barrier is between p and n side of silicon diode. When electron recombines with a hole, 0.7 eV energy is released in the form of a photon. No 0. 7 eV = 0.7 * 1.6 * 1019j
So, E = 0.7 * 1.6 * 1019j
But E = hf = 6. 63 * 10-34 * f
Or f = E/6.63 * 10-34 = 0.7 * 1.6 * 1019
6.63 * 10-34
or f = 1.6 * 1014 Hz
λ = c/f = 3*108 = 1.77 * 10-6 m
1.6 * 1014
Or λ = 1770 * 10-9 m
= 1770 nm
Hence, photon emitted from Si diodes have wavelength much greater than visible light (wave length of visible light, 750 _ 400 nm). It means that wave length of the photons emitted from Si lies in infrared region which is not visible.
In an inverting amplifier the input signal voltage Vl to be amplified is applied at inverting terminal (-) through a resistor Rl and Vo is the input voltage. The non- inverting terminal (+) is grounded and its potential is zero. As open loop gain AoL is very high ( 105) so by knowing the relation
AoL = V0/ V+ – V– (output/input)
For any value of Vo, V+ – V . 0
Or = V+ V
Since V+ is zero is ground potential so V- is virtually at ground potential i. e V o
This is called principle of virtual ground
(B). Calculation of voltage gain. Current through R1 = l1 = Vl – V/Rl
Or ll = Vl/Rl…………………………………..(1)
Current through R2 = l2 = V – V0/R2
l2 = -V0/R2 …………………………..(2)
Using Kirchhoff s current rule
l1 = l2
Putting the values of l1 and l2 in equation (3) we get
Vl/ Rl = -V0/ R2
or V0/Vi = -R2/R1
Gain of inverting amplifier = output/ Input
Or G= V0/ Vi
G = –R2/R1
Hence, this is voltage gain of inverting amplifier.
Input of the gate are given below
A = 1 B = 0
(a). If output of the gate = o, then gate is AND gate or NOR gate or XNOR gate.
(b). When output of the gate = 1 then the gate is OR gate or NAND gate or XOR gate.
The germanium or silicon crystals formed after adding the pentavalent impurity (donor impurity) is known as N – type substance.
When a germanium a silicon crystal is formed after adding an impurity of an element from fifth group of the periodic table (i.e antimony, arsenic or phosphorous) the impurity donates free electrons as current carriers to the crystal.
P- type substance
A crystal of germ anent or silicon formed after adding a trivalent impurity (accepter impurity ) is known as P – type substance.
When a trivalent impurity such as boron, Gallium or Indium is added to a germanium or silicon crystal, the three valence electrons of each trivalent atom from covalent bonds with three of its neighbors and there exists a vacancy for an electron in the fourth covalent bond. This vacancy is known as a hole and provides the crystal as a positive.
In case of conductors, the highest energy bond with some electrons is unfilled as shown if fif.(a) these electrons can be exited from lower to higher energy level within the bond. Such excitation is possible on applicable of even weak electric field.
In an insulator the highest occupied level is completely filled. Also the forbidden energy band above the occupied band is wide. The electron cannot jump over from lower to upper permissible energy level (fig b)
In semi- conductor the gap between the filled energy band and next higher permitted energy band is small. It means that the forbidden band is small. The thermal energy, even at room temperature is sufficient to let some electrons from lower permitted band to higher permitted band i.e. conduction band (fig c) the electrons is conduction band are free to transport (carry) charge.
If a signal germanium crystal contains donor impurities (n – type impurities) on one side and accepter impurities (p – type impurities) on the other side, a p – junction is formed. Such a junction between p and a materials forms a semi- conductor diode as shown in fig (a)
Initially there is an excess of holes to the left of the junction (p- region) and excess of electrons to the right (n – region) . A diffusion of electrons and holes across the junction in opposite directions takes place as shown in the fig(b). This change takes place in a narrow region called as the depletion region. As a result, a layer of net positive charge (holes) appears on the n- side of the junction and layer of net negative charge (electrons) appears on the p Side of the junction. Due to these two layers having opposite charges in the depletion region and electric field or potential barrier is set up at the junction. This potential barrier further steps the diffusion (movement) of electrons and holes across the junction.
If we want to send current across the p – n junction, we have to reduce the height of the potential barrier at the junction. This can be done by connecting the positive terminal of the battery to the p- side of the junction and negative terminal to the n – side of the junction as shown fig (a) the polarity of concentrations repels the reduces the height of potential barrier and it gives easy way to flow of electric current through the junction. Hence a p – n junction or a semi- conductor diode conducts when it is forward biased.
On the order hand, if negative terminal of the battery is connected to the p- side of the junction and positive terminal to away from the junction, both the holes and the electrons move barrier at the junction due to attraction . Thus the height of potential barrier at the junction increases and depletion region becomes more wide as shown in fig (b)
In this state, no current flows through the junction and circuit. Hence semi – conductor diode does not conduct when it is reverse biased. We note that semi- conductor diode allows the current to flow only in one direction when it is forward biased. The flow of current is practically zero in the opposite direction when it is reverse biased. It acts as a tube diode.
LED stands for Light Emitting Diodes.
Definition:- Light emitting diodes is basically a p – n junction diode made from the semi – conductor gallium arsenide phosphate. When electric current passes through such a circuit, energy ids released in form of light (photons). It is called a light emitting diode (LED).
(ii). LASER:- Laser stand for light amplification by stimulated Emission of Radiation
Definition:- A laser is device which produces very narrow intense beam of light having the following properties.
(i). It is monochromatic (of one frequency or one wavelength)
(ii). It is coherent (crest and troughs of beam are in phase)
(iii). It is unidirectional (radiations of beam travel in the same direction)
The light emitted by an ordinary light source is not only incoherent but also emitted in all directions. So laser light is different from the ordinary light.
The materials which are neither good conductors nor good incoherent but are partly (intermediately) conductor are called semi- conductors
(i). At ok, a semi conductor acts like an insulator
(ii). The resistivity of semi – conductor to the flow of electric current lies between those of conductors (10-6 ohm_cm).
(iii) In the case of conductors the electronic resistance increases linearly with increase in temperature.
But in semi – conductors the resistance decreases experimentally with increase in temperature as shown in the graph.
A photo diode is a semi – conductor device usually made from silicon. It consists of a normal p – n junction with temperature window through which light can enter as shown in fig.
A p – type material is diffused on the n – type substance. The diffuse takes place through a window formed on the top of n – type material in silicon diode. This window is in contact with p – type region and serves as an electrode. The base is made of certain conducting metal on n – type material and it acts as another electrode. The window controls the area receiving the light.
Use: – Photo diodes are used as ‘fast’ counters which produce a current pulse, every time a been of light is light is interrupted.
A photo voltaic cell (solar cell) is a device which converts light energy into electrical energy.
When light is made to fail on this cell, the voltage across its terminals increase. The value of voltage increases with the increase of intensity of incident light. The photo – voltaic cell is basically a junction device. The cells are made of semi – conductor materials. It is generally made from cither silicon or Selenium.
Photo cells are used in satellites are space vehicles to convert solar energy into electrical energy which can be used to operate the other electrons equipments.
Transistor is a semi – conductor device which which can amplify both threw current as well as voltage. There are two types of transistors:-
(i). P – n –p transistor
(ii). N –p – n transistor
(i). P – n- p Transistor
In p –n- p transistor type material is sandwiched between two p – type material as shown in the fig. (b)
(ii). n- p –n Transistor:-
In n –p –n transistor p – type material is sandwiched between two n –type materials as shown in the fig (b).
Symbols of p –n – p and n – p-n transistors:-
The two types of transistors along with their circuit symbols are shown in the following figures. A transistor consists of three portions or regions. The three portions of the transistor are known as emitter, base and collector respectively.
(i). The middle portion of the transistor is called base.
(ii). The region on left hand side of transistor is called emitter.
(iii). The region on right hand side of transistor is called Collector.
The charge less region around the p – n junction is called depletion region.
(Biasing). The application of some electric potential across the diode is known as biasing
The ratio between forward bias voltage ΔVf and current Δlf is known as forward resistance of the p – n junction i.e.
Rf = ΔVf/Δlf
(i). They are used for automatic switching i.e. ‘ON’ or ‘OFF’ of a current.
(ii). They can be used to detect visible and invisible light.
(iii). They are used in logic circuits.
(iV). They are used in optical fibre communication equipment.
B = Vout/Vin
B = lc/lB = Constant
It is resistance between output terminal and ground terminal of the operational amplifier. Its values are very low.
It is resistance between positive and negative inputs of the op- amplifier. Its values are very high – of the order of several mega ohms.
ALo = Vo/Vi
G = 1 + Rl/R2
Where Rl and R2 are the external resistances of an amplifier.
The power Gain ‘G’ is intendment of the internal circuit of an amplifier but depends upon externally connected resistance R1 and R2.
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