Devices comparision and LDR notes

 *** Write a short note on Applications of JFET

·        FET is used as a buffer in measuring instruments , receivers. Since it has high input impedance and low out put impedance.

·        FETs are used in RF amplifiers in FM tuners and communication equipment because of the low noise level.

·        FETs are used in cascade amplifiers in measuring and test equipment because the input capacitance of FET is low,

·        FETs are used as a voltage variable resistor in operational amplifiers and tone controls ,because FET  is voltage controlled device.

·        FETs are used in mixer circuits in FM ,TV receivers and communication equipment because inter modulation distortion is low.

·        FET s are used in oscillator circuits because frequency drift  is low.

·        FETs are used in low frequency amplifiers in hearing aids and inductive transducers because its coupling capacitor is small.

·        FETs are used in digital circuits in computers, LSD and memory circuits because of its small size.

Difference between FET and MOSFET

FET	MOSFET
FET is also called as JFET	MOSFET or IGFET is also called as Metal Oxide Semiconductor Field Effect  Transistor (or) Insulated Gate Field effect Transistor.
It operates depletion mode only	It operates both depletion mode and enhance mode.
Gate is not insulated from the channel.	Gate is insulated with SiO2 from the channel.
Input is reverse bias i.e the input resistance is high.	Input is insulated then the input resistance is very high.
These are used in Analog circuits.	These are used in Digital Circuits.
There are two types of FETs 1.N-channel FET  2.P-Channel FET	There are two types of MOSFET 1.N-channel MOS FET       2.P-Channel MOSFET
It is a three terminal device. The terminals in JFET is Source, Drain and Gate.	It is a four terminal device. The terminals in MOSFET is Source, Drain, Gate and Substrate.
The gate leakage current of JFET is in Nano amp.	The gate leakage current of MOSFET is in pico amp.
The input resistance of JFET is near to the order of 108 Ω.	The input resistance of JFET is near to the order of 1010 or 1015 Ω.
Normally it is a ON device.	Normally it is a OFF device.
More flat characteristic curve is observed in output.	Less flat characteristic curve is observed in output.
It has High input impedance.	It has Very high input impedance.
It has high Gate current.	It has low Gate current.
High Drain resistance.	Low  Drain resistance.
Conductivity is controlled by reverse biasing of Gate.	Conductivity is controlled by carriers induced in the channel.
Its Manufacturing process is simple.	Its Manufacturing process is difficult.
All FETs are not MOSFETs.	All MOSFETs are FETs
FETs work either positive input (P-Channel) or negative input (N-Channel).	MOSFETs work either positive and Negative voltage.
Signal handling capacity is less.	Signal handling capacity is more.
It is mainly used in low noise applications.	It is mainly used in high noise applications.
Cost is less comparative to MOSFET	Cost is high comparative to FET

*** Explain difference between BJT and UJT

BJT means Bi polar junction Transistor.	UJT means Uni junction Transistor.
The polarization is due to both majority and minority carriers .	The polarization is due to only Majority carriers.
It contains two PN junctions.	It contains single PN  junction.
In it the current is due to both majority and minority carriers.	In it the current  is due to majority carriers only.
It is a three terminal device.	It is a three terminal device.
Emitter, Base and Collector are the terminals of BJT.	Base-1,Base-2 and Emitter are the terminals of UJT.
The main application of BJT is Amplification.	The main application of UJT is  switching device.
It needs biasing	It need not any biasing.
BJT is current controlled device.	UJT is voltage control device.
It does not contain any negative resistance region.	It contains negative resistance region.
In BJT the current flows from Emitter to collecter teminals.	In UJT the current flows in between two base terminals.
The cost of BJT is low by comparing to UJT.	The cost of UJT is high by comparing to BJT.
Directly we are not connecting any battery to both Emitter and Collector in all cases.	Directly we connect battery to both the B1 & B2  terminals in all cases.
We are considering both the input and out put characteristics in BJT	We are considering only single characteristics in UJT.
Its manufacturing process is some more difficult  than UJT .	Its manufacturing process is simple .

*** Explain the differences between Half wave rectifier and Full wave rectifier

Half wave Rectifier	Full wave Rectifier
It conducts only in positive half cycle.	It conducts on both positive and negative half cycles.
Out put frequency is equal to frequency of input	Out put frequency is equal to twice of input.
Only one diode is required in it.	Two diodes are required in it.
Center tap transformer is not needed in it.	Center tap transformer is needed in it.
Its R.M.S value of output current is I0/2.	Its R.M.S value of output current is I0/root 2.
Its Efficiency  is 40.6%	Its Efficiency  is 81.2%
Its Ripple factor is  1.21	Its Ripple factor is  0.482
Its peak Inverse voltage is VMAX	Its peak Inverse voltage is 2VMAX
Its average value of current is  I0/π.	Its average value of current is  2I0/π.
Peak load current         IMAX= VMAX/(Rload+Rf )	Peak load current         IMAX= VMAX/(Rload+Rf )
DC output voltage VDC = IDC /Rload.	DC output voltage VDC = IDC /Rload.
Voltage regulation is good.	Voltage regulation is better.
Its form factor 1.57	Its form factor 1.11
Peak factor is 2	Peak factor is sq root 2
It gives discontinuous and pulsating dc output.	It gives continuous and pulsating dc output
It involves lot of wastage of power.	The wastage of power is not considering in this case.
Transformer utilization factor is0.286	Transformer utilization factor is0.692

***Explain LDR (Light  Dependent  Resistor)

                   The device which is used to convert light energy into electrical energy are called Photo Electric cells (or) Photo Conductive cells.

Principle :

                    The electrical resistance of a semi conductor materials decreases. When they are exposed to radiations the decrease in resistance depends up on the intensity and colour of the light.

Construction:  

                     The materials generally used for the construction of photo conductive cells are cadmium sulphide (cds), cadmium Solenoid (cdse)  

(or) lead sulphide (pbs) .

                     The  construction of cds cell as show in the above fig.

             In this cell, a thin film of cds is deposited on one side of an iron plate and placed and placed below a transparent metal film . A battery & resistance are induced to provide ( a direction to ) the flow of current.

Operation:

            When the light is fall on a transparent metal film. The electrical resistance of cds layer is decreases. Hence, its electrical conductance is increases.

 Therefore Current starts flowing in the circuit.

Characteristics :

            The Characteristics of a cds cell is drawn between resistance and intensity.

When not illuminated, the cell has resistance in the range of 100 KΩ. Which is known as dark resistance.

When illuminated with strong light, the cell resistance falls to only a few hundred ohms.

 Thus the ratio of “dark” to “light” resistance of the cell is about 1000:1

Symbol:

Application:

1.   To measure the intensity of illumination

2.   .To control street lighting.

3.    To work as ON/OFF switch.

4.   In counting application.

5.   In Burglar Alarm.