Friday, 8 June 2018
Wednesday, 6 June 2018
Unijunction Transistor (UJT)
UNIJUNCTION TRANSISTOR (UJT)
A UJT is a three terminal semiconductor device used for number of applications such as switching pulse generator,saw-tooth generator etc.
CONSTRUCTION
UJT consists of an n-type silicon bar with an electrical connection on each end.The leads of these connections are called bases B1 and B2.
- The device has one junction and three leads ,it is therefore called as unijunction transistor.
- With only one p-n junction,the device is really a form of diode.Because the two base terminals are taken from one section of the diode,this device is also called double-based diode.
OPERATIONS OF UJT
Generally UJT has B2 positive w.r.t B1.Let us consider following cases:
Generally UJT has B2 positive w.r.t B1.Let us consider following cases:
- When emitter is open :If the voltage VBB is applied between B2 and B1 with emitter open,a voltage gradient is established along the n-type bar.Since the emitter is located near to B2,more than half of VBB appears between the emitter and base B1.Now the voltage V1 between emitter and B1 reverse biases the p-n junction and hence the emitter current is cut off.However, a small current flows from B2 to emitter due to minority carriers.
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- When a positive voltage is applied to the emitter :In this case ,the p-n junction will remain reverse biased so long the input voltage is less than V1.Now if the positive voltage applied to the emitter is increased,the p-n junction becomes forward biased when it exceeds V1.Under these conditions,holes are injected from p-type material into n-type bar.These holes are repelled by positve B2 terminal and attracted by negative B1 terminal.This accumalation of holes in the emitter to B1 region results in the decrease of the resistance of this section of the bar.Hence the emitter current IE increases.As more holes are injected,the emitter current further increases untill it is limited by emitter power supply.The UJT is now said to be in ON state.
- When a negative voltage is applied to the emitter :In this case ,the p-n junction is reverse biased and hence the emitter current is cut off.The device is then said to be in OFF state.
CHARACTERISTICS OF UJT
The curve plotted between voltage VE and emitter current IE at a given voltage VBB between the bases is known as the emitter characteristic of UJT.From this characteristics,following points are noted:
- initially,in the cut-off region, as VE increases,a small leakage current flows from B2 to emitter.This current is due to minority carriers.
- Above a certain value of VE,forward current IE begins to flow.This current increases until the peak voltage Vp and current Ip are reached at point P.
- After point P, emitter voltage VE decreases with sudden increase in emmiter current IE.This is a negative resistance portion of the curve and is of particular importance for the use of UJT in trigger circuits, saw tooth generators,timing circuits etc.
- The negative resistance portion of the curve lasts until the valley point V is reached.At valley point, the current is increased to such an extent that further increase in conductivity can takes place.Beyond valley point,the device behaves as a conventional p-n junction diode.
APPLICATIONS OF UJT
UJT is used for variety of applications.Some of these are:
1.Switching device.
2.Saw tooth generator.
3.Timing and trigger circuits.
4.Phase control.
5.Over voltage detector.
6.Voltage and current regulated supplies.
FOURIER SERIES
FOURIER SERIES
J.B.Fourier in 1822 published a very useful throrem by which any complex periodic function can be analysed.
The theorem states that any complex periodic,continous or piecewise continous functiom f(x) may be expressed as a series and cosine in the form :
This trignometric series is known as Fourier series.Here a0,an and bn are constants called coffiecients of Fourier series.
DIRICHLET'S CONDITIONS
The conditions imposed on f(x) to make above equation valid are Dirichlet's conditions.These are :
These conditions are sufficient but not necessary.In a large number of physical problems where Fourier series representation of a function is required,these conditions are generally satisfied.
EVALUATION OF COFFICIENTS OF FOURIER SERIES
Let a periodic function f(x) be expressed by Fourier series as
EVALUATION OF COFFIECIENT an
EVALUATION OF COFFICIENT bn
ADVANTAGES OF FOURIER SERIES
J.B.Fourier in 1822 published a very useful throrem by which any complex periodic function can be analysed.
The theorem states that any complex periodic,continous or piecewise continous functiom f(x) may be expressed as a series and cosine in the form :
This trignometric series is known as Fourier series.Here a0,an and bn are constants called coffiecients of Fourier series.
DIRICHLET'S CONDITIONS
The conditions imposed on f(x) to make above equation valid are Dirichlet's conditions.These are :
- The function must be well defined and bounded.
- The function must be continous.
- The function must be single valued.
- The function must have a finite number of maxima and minima.
- The function must satisfy the periodicity condition.
These conditions are sufficient but not necessary.In a large number of physical problems where Fourier series representation of a function is required,these conditions are generally satisfied.
EVALUATION OF COFFICIENTS OF FOURIER SERIES
Let a periodic function f(x) be expressed by Fourier series as
EVALUATION OF COFFIECENT a0
EVALUATION OF COFFIECIENT an
EVALUATION OF COFFICIENT bn
ADVANTAGES OF FOURIER SERIES
- The Fourier series may represents a discontinous function.
- The Fourier series may represent a periodic function.
- In Fourier series representation the even and odd functions are conveniently expressed as cosine and sine series respectively.
- The Fourier series is helpful in the solution of equation of motion of an oscillating particle subject to a periodic driving force.The Fourier series expansion of the driving force then gives us the fundamental term and series of harmonics.
RECTIFIERS
RECTIFIERS
A rectifier is an electronic device which converts A.C voltage or current into a unidirectional voltage or current.
TYPES OF RECTIFIERS
There are two types of rectifiers :
1.HALF-WAVE RECTIFIER.
2.FULL-WAVE RECTIFIER.
HALF-WAVE RECTIFIER
A half wave rectifier consists of a transformer,a diode and a load resistor.The primary coil of transformer is connected to the A.C mains and the secondary coil to a load resistor R1 through the diode D.
During the positive half cycle of input A.C voltage,the end A of the secondary coil becomes positive and B is negative.This makes the diode forward biased and hence current flows through R1.The current produces a voltage across RL which has the same shape as the positive half cycle of the input voltage.During the negative half cycle of the input A.C voltage,the end A of the secondary coil becomes negative w.r.t end B.Under this condition,the diode is reverse biased.Hence no current flows through the circuit and no voltage develops across RL.Thus D.C output of half wave rectifier is unidirectional and pulsating.Since only half cycle of the input wave is used,so it is called a half wave rectifier.
FULL-WAVE RECTIFIER
A full wave rectifier consists of two diodes D1 and D2 connected to the centre tapped secondary coil of a transformer and a load resistor RL.The centre -tap is usually taken as the ground.
During the positive half cycle of the input A.C voltage,the end A of the secondary coil becomes positive and end B becomes negative.The diode D1 is forward biased and conducts.The diode D2 is reverse biased and does not conduct.The current flows through RL in the direction as shwon in figure.During the negative half cycle of the A.C input voltage,the end A of the secondary coil becomes negative and end B is positive.Therefore diode D2 being forward biased conduts while diode D1 being reversed biased and does not condut.The current flows through RL in the same direction as in the previous half cycle.Thus current flows during both half cycles of the input A.C voltage.The D.C output of full wave rectifier is unidirectional,continous and pulsating.However,it can be made smooth by using a filter circuit.
A rectifier is an electronic device which converts A.C voltage or current into a unidirectional voltage or current.
TYPES OF RECTIFIERS
There are two types of rectifiers :
1.HALF-WAVE RECTIFIER.
2.FULL-WAVE RECTIFIER.
HALF-WAVE RECTIFIER
A half wave rectifier consists of a transformer,a diode and a load resistor.The primary coil of transformer is connected to the A.C mains and the secondary coil to a load resistor R1 through the diode D.
During the positive half cycle of input A.C voltage,the end A of the secondary coil becomes positive and B is negative.This makes the diode forward biased and hence current flows through R1.The current produces a voltage across RL which has the same shape as the positive half cycle of the input voltage.During the negative half cycle of the input A.C voltage,the end A of the secondary coil becomes negative w.r.t end B.Under this condition,the diode is reverse biased.Hence no current flows through the circuit and no voltage develops across RL.Thus D.C output of half wave rectifier is unidirectional and pulsating.Since only half cycle of the input wave is used,so it is called a half wave rectifier.
FULL-WAVE RECTIFIER
A full wave rectifier consists of two diodes D1 and D2 connected to the centre tapped secondary coil of a transformer and a load resistor RL.The centre -tap is usually taken as the ground.
During the positive half cycle of the input A.C voltage,the end A of the secondary coil becomes positive and end B becomes negative.The diode D1 is forward biased and conducts.The diode D2 is reverse biased and does not conduct.The current flows through RL in the direction as shwon in figure.During the negative half cycle of the A.C input voltage,the end A of the secondary coil becomes negative and end B is positive.Therefore diode D2 being forward biased conduts while diode D1 being reversed biased and does not condut.The current flows through RL in the same direction as in the previous half cycle.Thus current flows during both half cycles of the input A.C voltage.The D.C output of full wave rectifier is unidirectional,continous and pulsating.However,it can be made smooth by using a filter circuit.
BRIDGE RECTIFIER
FULL WAVE BRIDGE RECTIFIER
It consists of four diodes D1,D2,D3 and D4 connected to form a bridge type arrangement.The A.C supply to be rectified is applied to the diagonally opposite ends of the bridge through the transformer.A load resistace RL is connected between other two ends of the bridge.
During the positive half cycle of input voltage,the end S1 of the secondary coil of the transformer is positive and the end S2 is negative.This makes diode D1 and D2 forward biased while diodes D2 and D4 are reverse biased.Therefore,only diodes D1 and D3 conduct.So a current flows along the direction ABEDCS2S1.
During the negative half cycle of input voltage,the end S1 becomes negative and S2 is positive.This makes the diodes D2 and D4 forward biased while diodes D1 and D3 are reverse biased.Therefore,only diodes D2 and D4 conduct and a current flows in a direction CBEDAS1S2.Thus in both half cycles,current flows through RL in the same direction.The wave form of input and output voltages are shown in figure given below.The wave form of the output is somilar to that of centre-tap full wave rectifier.
ADVANTAGES OF BRIDGE RECTIFIER
1.The transformer with centre-tapping is not needed.
2.The output is twice that of the centre-tap circuit for the same secondary voltage.
3.The peak inverse voltage is one half of that of the centre-tap full wave rectifier.
4.It is suitable for high voltage applications.
DISADVANTAGES OF BRIDGE RECTIFIER
1.The extra diodes are required.
2.As the diiodes are used in series.there is a large drop across the diodes.It reselts in poor rectification efficiency and poor voltage regulation.
It consists of four diodes D1,D2,D3 and D4 connected to form a bridge type arrangement.The A.C supply to be rectified is applied to the diagonally opposite ends of the bridge through the transformer.A load resistace RL is connected between other two ends of the bridge.
During the positive half cycle of input voltage,the end S1 of the secondary coil of the transformer is positive and the end S2 is negative.This makes diode D1 and D2 forward biased while diodes D2 and D4 are reverse biased.Therefore,only diodes D1 and D3 conduct.So a current flows along the direction ABEDCS2S1.
During the negative half cycle of input voltage,the end S1 becomes negative and S2 is positive.This makes the diodes D2 and D4 forward biased while diodes D1 and D3 are reverse biased.Therefore,only diodes D2 and D4 conduct and a current flows in a direction CBEDAS1S2.Thus in both half cycles,current flows through RL in the same direction.The wave form of input and output voltages are shown in figure given below.The wave form of the output is somilar to that of centre-tap full wave rectifier.
ADVANTAGES OF BRIDGE RECTIFIER
1.The transformer with centre-tapping is not needed.
2.The output is twice that of the centre-tap circuit for the same secondary voltage.
3.The peak inverse voltage is one half of that of the centre-tap full wave rectifier.
4.It is suitable for high voltage applications.
DISADVANTAGES OF BRIDGE RECTIFIER
1.The extra diodes are required.
2.As the diiodes are used in series.there is a large drop across the diodes.It reselts in poor rectification efficiency and poor voltage regulation.
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FOURIER SERIES FOURIER SERIES J.B.Fourier in 1822 published a very useful throrem by which any complex periodic funct...
