UNIT IV ELECTRONIC CIRCUIT INTRODUCTION TO PN JUNCTION DIODE A PN junction is formed by suitably joining a P-type semiconductor and a Ntype semiconductor. A PN junction is shown in fig. Fig Formation of PN junction Diode The P- Type semiconductor has more holes and less electrons. The N-type semiconductor has more electrons and less holes. Therefore at the junction, the electrons in the N-side have tendency to move towards the P-side. Similarly the holes on the P-side have tendency to move towards the N-side. According to that, the electrons and holes recombine with each other, to form a region at the junction. It is called “Depletion Region”. These two charges, on either sides, make a potential across the depletion region called “barrier potential”. Working of a PN Junction Diode: The conduction of any diodes, depends on their biasing. There are two types of biasing, known as Forward biasing and Reverse biasing. (i) Forward biasing In forward biasing, the positive terminal of the battery is connected to the Ptype and the negative terminal of the battery is connected to the N-type materials of the diodes, shown in fig. Under the forward bias condition, the applied positive pote tial repels the holes in P-type region. The negative potential repels the electrons in N-type region. Now the electrons in N-type region and the holes in the P type region move towards the junction. This reduces the width of the depletion region and also the barrier potential. Fig PN junction under forward bias If the applied potential is greater than barrier potential, the majority carriers on both regions move towards the junction. It makes current flow through the junction. The VI characteristics of a PN junction diode under forward bias condition is shown in fig. Fig Forward characteristics (ii) Reverse biasing In reverse biasing, the positive terminal of the battery is connected to the N-type and the negative terminal of the battery is connected to the P-type materials of the diode, shown in fig. Under reverse bias condition, the majority carriers with P and N regions are moved towards the battery respectively. The holes in P-type and the electrons in N type regions move to the negative and positive terminals of the battery respectively. Hence the width of the depletion region is increased. Fig PN junction under reverse bias When the applied voltage is slowly increased, the minority carriers(electrons) in P-region and the minority carriers(holes) in N-region makes a small amount of current flow through the junction. This current is called reverse saturati n current shown in fig Fig Reverse characteristics 4.1 VI Characteristics of a Diode: The complete VI characteristic of a diode is the combination of its forward as well as reverse characteristics. Fig 4.1 VI characteristics of a Diode In forward characteristics, it is seen initially forward current is small as long as the bias voltage is less than the barrier potential. At a certain voltage close to barrier potential, current increases rapidly. The voltage at which diode current starts increasing rapidly is called as cut in voltage. It is denoted by Vγ. Below this voltage, current is less than 1% of maximum rated value of diode current. The cut in voltage for germanium is about 0.3V while for silicon it is 0.7V. It is important to note that the breakdown voltage is much higher and practically diodes are not operated in the breakdown condition. The voltage at which breakdown occurs is called reverse breakdown voltage denoted as VBR. 4.1.1 VI CHARACTERISTICS OF TYPICAL Ge AND Si DIODES The complete Vi characteristics of a typical Ge and Si diodes are shown in fig 4.1.1.,As mentioned earlier, the barrier potential for germanium (Ge) diode is about 0.3V while for silicon (Si) diode is about 0.7V. The reverse saturation current in a germanium diode is higher than reverse saturation current of a silicon diode. The reverse breakdown voltage for Si diode is higher than that of the Ge diode of a comparable rating. Fig 4.1.1 Characteristic of typical Ge and Si diode RECTIFIERS: Definition: The process of a.c to d.c conversion is known as rectification .A circuit which is used for rectification is called as rectifier. A rectifier is a circuit which is used to convert a.c voltage into pulsating d.c voltage .It can be classified as follows. Rectifier Half wave Rectifier Full wave Rectifier Full wave rectifier With centre taped transformer Bridge rectifier 4.2 Half –wave rectifier: 4.2.1 Definition: In half wave rectification, the rectifier conducts current only during positive half –cycles of input a.c supply. 4.2.2 Circuit diagram: The basic circuit diagram of half-wave rectifier is shown in fig.4.2.1 This circuit contains. (1) A step down transformer (2) One diode. (3) Load resistor RL A diode D is connected between secondary of transformer and resistor ,RL Fig 4.2.1 Halfwave Rectifier 4.2.3 Operation of half wave rectifier circuit: In a.c supply voltage there are two half cycles, one is positive half cycle the other is negative half cycle. 4.2.4 During positive half-cycle: Fig 4.2.2 During positive half cycle  During the positive half cycle of the input signal, terminal A is positive with respect to terminal B.  Diode D conducts in forward bias so the current flows from terminal A to B through diode D and load resistor RL.  It produces the output voltage. 4.2.5 During negative half –cycle:  During the negative half –cycle of the input signal, terminal B is positive with respect to terminal A.  Diode D conducts in reverse bias so no current flows through the diode and load resistor,  Now the output voltage is zero. Fig 4.2.3 During negative half cycle 4.2.6 Waveforms of the half-wave Rectifier: Fig 4.2.4 Input and Output Waveforms 4.2.7 Analysis of Halfwave Rectifier: D.C output Current: Input signal a.c voltage =V msin 8 Load resistance=RL Input signal a.c current=Imsin 8 Hence the output d.c. current Idc= 1 2n f 2n 0 id8 = n f id8 + 1 1 2n 0 f 2n id8 2n n In the above equation the current that flows through the load is zero during the period from n to 2n. 1 n Idc= f id8 2n 0 n = 1 f Im sin 8 d8 2n 0 = Im 2n [cos 8] =Im [−(cos n − cos 0)] 2n = Im = Im 2n [−(−1 − 1)] n Idc = Im n D.C Voltage: D.C Voltage Vdc = IdcRL = Vdc V = IR ImRL Vm= I m RL n Vm = n A.C INPUT CURRENT: 1 2n I2rms = f i 2 d8 2n 0 1 n 1 2n 2n 0 2n n = f i2d8+ f i2d8 No current flows during the period from n to 2n I2rms 1 2n 2 i d8 = f 2n 0 n = 1 f (Im sin 8) 2 d8 2n 0 = = = Im 2 n 1–cos 28 f [ 2n 0 Im 2 4n Im2 4n 2 ] d8 [ fd8 − f cos 28 d8] [8 − sin 28 n 2 ] 0 = Im2 4n [(n − sin 2n sin 2 X 0 2 2 ) − (0 − )] 2 =Im [n − 0] 4n I2rms = Irm = s Im2 4 Im 2 RMS Load Voltage: Vrms Vrms = IrmsRL = ImRL = Vm D.C output power = Since Vm= Im RL 2 2 I we know that P= I2R R dc L Im = ( ) 2R L n Pdc A.C input power Pac Im =( n )2R L = I2rms( rf + RL) = Im = Im 2 4 2 4 ( r f + RL) ( r f + RL) Rectifier Efficiency: %5 = Pdcx 100% Pac Im 2 ( = Im2 4 = ) RL G ( rF+ RL) x 100% IN 2 ×RL ×4 n2 ×I2m ×(rF +RL ) = 4 ( RL ) % n2 rF +RL = 40.6( RL ) % rF +RL 5 = 40.6% Ripple Factor: × 100% 2 Ripple Factor= J(Irms ) − 1 Idc Im/ = J(I m/ 2 2 )−1 n 2 =Jn − 1 4 = 1.21 Ripple Factor= 1.21 Ripple Frequency: The frequency of the pulsating load voltage is known as ripple frequency. It is equal to the frequency of supply voltage in the halfwave rectifier. f = fs 4.2.8 Advantages: 1.Simple Circuit 2.Low Cost 4.2.9 Disadvantage of Halfwave Rectifier: 1.Low efficiency(41%) 2.High ripple factor (121%) 3.Low dc output voltage and current 4.3 Half Wave Rectifier with Capacitor Filter Output of half wave rectifier is not a constant DC voltage. From the output diagram that its a pulsating dc voltage with ac ripples. In real life applications, we need a power supply with smooth wave forms. In other words, we desire a DC power supply with constant output voltage. A constant output voltage from the DC power supply is very important as it directly impacts the reliability of the electronic device we connect to the power supply. The output of half wave rectifier smooth by using a filter (a capacitor filter or an inductor filter) across the diode. In some cases an resistor-capacitor coupled filter (RC) is also used. The circuit diagram below shows a half wave rectifier with capacitor filter. Fig 4.3.1 Half Wave Rectifier with Capacitor Filter – Circuit Diagram & Output Waveform Full-Wave Rectifier (FWR) Definition: In full wave rectifier circuit current flows through the load in the same direction for both half-cycles(full wave) of input a.c voltage. Types of full wave rectifier: The following two circuits are commonly used for full-wave rectification. 1.Full wave rectifier with center tapped transformer. 2.Full wave bridge rectifier. 4.4 Full wave rectifier with center tapped transformer: 4.4.1 Definition:In full wave rectifier circuit current flows through the load in the same direction for both half-cycles(full wave) of input a.c voltage. 4.4.2 Circuit diagram: The circuit diagram of full-wave rectifier is shown in fig. This circuits contains 1. Centre tapped transformer 2. Two Diodes (D1 & D2) 3. Load Resistor Two diode D1 and D2 are used as the switching elements, one diode acts for half cycle and second diode for another half cycle. Fig 4.4.1 FWR with center tapped transformer 4.4.3 Operation of the circuit: In a.c supply voltage, there are two half cycle, one is positive half cycle the other is negative half cycle. 4.4.4 Duration of Positive half cycle: Fig 4.4.2 During positive half cycle  During the positive half cycles of the input voltage, terminal A is positive and B is negative with respect to terminal O.  The diode D1 conducts in forward bias and diode D2 conducts in reverse bias.  The current I1 flows from the terminal A to the load through diode D1.  No current flows through the diode D2. 4.4.5 Duration of Negative half cycle:     Fig 4.4.3 During Negative Half cycle During the negative half cycles of the input voltage, terminal B is positive and A is negative with respect to terminal O. The diode D2 conducts in forward bias and diode D1 conducts in reverse bias. The current I2 flows from the terminal B to the load through diode D2. No current flows through the diode D1. 4.4.6 Input and Output Waveforms: Fig 4.4.4 Input and Output Waveforms 4.4.7 Analysis of FWR with Center tapped Rectifier: D.C output Current: Input signal a.c voltage =Vmsin 8 Load resistance =RL Input signal a.c current=Imsin 8 Hence the output d.c. current 1 2n Idc= f id8 2n 0 = 1 n f id8 + 1 2n 0 f 2n id8 2n n The variation between 0 and n is equal to n and 2n. 1 n Idc 1 n = f id8 + f id8 2n 0 1 = 2n 0 × 2[ nfid8] 0 2n n = 1 f Im sin 8 d8 n0 = Im n [cos 8] =Im [−(cos n − cos 0)] n = Im = 2Im Idc = n [−(−1 − 1)] n 2Im n D.C Voltage: D.C Voltage Vdc = IdcRL = Vdc = using 2ImRL n 2Vm using V = IR Vm= I m RL n A.C INPUT CURRENT: 1 2n I2rms = f i2 d8 2n 0 1 n 1 2n 2n 0 2n n = f i2d8+ f i2d8 The variation between1 0nand n is1 equal to n and 2n. n I2rms = f i2d8+ f i 2d8 2n 0 2n 0 = 1 × 2 n if2d8 2n 0 1 n = f (Im sin 8) 2 d8 n0 = Im 2 n 1–cos 28 f [ 2 ] n 0 d8 = Im 2 [ fd8 − f cos 28 d8] 2n = = Im2 2n Im2 2n [8 − sin 28 [(n − 2 ] sin 2n sin 2 X 0 2 2 ) − (0 − )] 2 =Im [n − 0] 2n Im2 I2rms = Irms 2 Im = √2 RMS Load Voltage: Vrms Vrms = IrmsRL = ImRL = Vm Vm= I m RL 2 √2 D.C output power = I2dcRL =( Pdc A.C input power = 2Im 2 ) RL n 4Im2 RL n2 = I2rms( rf + RL) = Im 2 2 Pac P= I2R = Im 2 2 ( r f + RL) ( r f + RL) Rectifier Efficiency: % 5 = = = Pdc Pac ( x 100% 2Im 2 ) RL G Im2 (r 2 x 100% +F R ) L 4×I N 2 ×RL ×2 n 2 ×I 2m ×(rF +RL) = 8 ( RL ) % n2 rF +RL × 100% = 81.2( RL ) % rF +RL If rf<

Unit 4: Electronic Circuit

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