Basic Electronic Devices and Circuits EE 111 Electrical Engineering Majmaah University 2 nd Semester 1432/1433 H. Chapter 2. Diodes and Applications

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1 Basic Electronic Devices and Circuits EE 111 Electrical Engineering Majmaah University 2 nd Semester 1432/1433 H Chapter 2 Diodes and Applications 1

2 Diodes A diode is a semiconductor device with a single pn junction and metal connections to leads. It has the ability to pass current in only one direction. p n Depletion region 2

3 Forward bias Forward bias is the condition which allows current in the diode. The bias voltage must be greater than the barrier potential. I F (ma) V F R I F VBIAS 0 0 A B C 0.7 V Knee V F 3

4 Quiz Q. The forward biased knee voltage in a semiconductor diode is approximately equal to the a. bias supply voltage b. breakdown voltage c. output voltage d. barrier potential 4

5 Reverse bias Reverse bias is the condition in which current is blocked. V BIAS V R V BR Knee 0 0 I = 0 A R V BIAS I R 5

6 Approximations Three diode approximations are: I F I F I F V R V F V R 0.7 V V F V R 0.7 V V F I R I R slope > 0 Ideal Practical Complete I R In addition, the complete model includes the effect of a large reverse resistance that accounts for a tiny current when reverse-biased. 6

7 1. Ideal Model 7

8 2. Practical Model 8

9 3. Complete Model 9

10 Example Use the practical model to determine the current in the circuit: R V BIAS 12 V 3.3 kω I VR = VBIAS 0.7 V= 12 V 0.7 V= 11.3 V I V R 11.3 V 3.3 kω R = = = 3.4 ma 10

11 Quiz Q. Using the ideal diode model, the current in the circuit shown is a ma R b ma c ma d. 1.2 ma V BIAS 8.0 V 10 kω 11

12 Quiz Q. Using the practical diode model, the current in the circuit shown is a ma R b ma c ma d. 1.2 ma V BIAS 8.0 V 10 kω 12

13 Quiz Q. The diode model which includes the large reverse resistance is the a. ideal model b. practical model c. complete model d. all of the above 13

14 Typical diode packages Some common configurations are K K A K A A A A K K K K A K A K K A K A K A 14

15 The Basic DC Power Supply 15

16 Half-wave Rectifier The diode conducts during the positive half cycle. It does not conduct during the negative half cycle. V in 0 t I R L 0 0 t1 2 t 0 t1 V out t t 2 I = 0 A V V out in 0 R t L 0 0 t1 t 2 t0 t1 t 2 What is the output if the diode is reversed? See next slide 16

17 Half-wave Rectifier V in 0 t I V out R L 0 0 t1 2 t 0 t1 t t 2 What is the output if the diode is reversed? 17

18 Average Value of the Half-Wave Output Voltage 18

19 Example 19

20 Effect of the Barrier Potential on the Half-Wave Rectifier Output 20

21 Half-wave Rectifier The peak inverse voltage (PIV) is equal to the peak input voltage and is the maximum voltage across the diode when it is not conducting. V 0 V p(in) t p PIV at t p I = 0 Notice that the PIV can be found by applying Kirchhoff s Voltage Law. The load voltage is 0 V, so the input voltage is across the diode at t p. R L 21

22 Full-Wave Rectification 22

23 Average Value of the Full-Wave Output Voltage 23

24 Full-wave Rectifier F D 1 A center-tapped transformer is used with two diodes that conduct on alternating halfcycles. V in 0 During the positive half-cycle, the upper diode is forward-biased and the lower diode is reverse-biased. V in 0 F I D 2 D 1 During the negative half-cycle, the lower diode is forward-biased and the upper diode is reverse-biased. D 2 I R L V out 0 R L V out 0 24

25 Full-Wave Rectification 23

26 Average Value of the Full-Wave Output Voltage 24

27 Full-wave Rectifier F D 1 A center-tapped transformer is used with two diodes that conduct on alternating halfcycles. V in 0 During the positive half-cycle, the upper diode is forward-biased and the lower diode is reverse-biased. V in 0 F I D 2 D 1 During the negative half-cycle, the lower diode is forward-biased and the upper diode is reverse-biased. D 2 I R L V out 0 R L V out 0 25

28 Effect of the Turns Ratio Transformer Turns Ratio = 1 V p(pri) : peak value of the primary voltage. 26

29 Effect of the Turns Ratio Transformer Turns Ratio = 2 27

30 Remember! Reverse bias Reverse bias is the condition in which current is blocked. V BIAS V R V BR Knee 0 0 I = 0 A R V BIAS I R 28

31 Peak Inverse Voltage (PIV) The PIV can be shown by applying KVL around the green loop shown for the reversebiased diode. V in 0 F V p(sec) 2 D 1 Apply KVL Notice that the peak secondary voltage will be across the reversebiased diode. D 2 R L 29

32 Peak Inverse Voltage (PIV) D 1 forward-biased D 2 reverse-biased 30

33 The Bridge Full-Wave Rectifier The Bridge Full- Wave rectifier uses four diodes connected across the entire secondary as shown. V in F I D 3 D 1 V out D R L out 2 D 4 Conduction path for the positive half-cycle. F 0 I D 3 D 1 V in D R L V out 2 D 4 0 Conduction path for the negative half-cycle. 31

34 Bridge Output Voltage 32

35 The Bridge Full-Wave Rectifier Determine the peak output voltage and current in the 3.3 kω load resistor if V sec = 24 V rms. Use the practical diode model The peak output voltage is: V V ( ) = 1.41Vrms = 33.9 V p sec F 120 V D 3 D 1 ( ) = Vp ( sec ) 1.4 V V (sec) = 24 Vrms R = 32.5 V L V p(out ) p out Applying Ohm s law, I p(out) = 32.5 / 3.3 = 9.8 ma D 2 D kω 33

36 Remember! Reverse bias Reverse bias is the condition in which current is blocked. V BIAS V R V BR Knee 0 0 I = 0 A R V BIAS I R 34

37 Peak Inverse Voltage (PIV): Ideal 35

38 Peak Inverse Voltage (PIV): Practical 36

39 Remember: Peak of a Sine Wave There are several ways to specify the voltage of a sinusoidal voltage waveform. The amplitude of a sine wave is also called the peak value, abbreviated as V P for a voltage waveform. 20 V 15 V The peak voltage of this waveform is 20 V. 10 V 0 V V P t ( µ s) -10 V -15 V -20 V 37

40 Remember: Peak-to-Peak & RMS The voltage of a sine wave can also be specified as either the peak-to-peak or the rms value. The peak-topeak is twice the peak value. The rms value is 1/ times the peak value. 20 V The peak-to-peak voltage is 40 V. The rms voltage is 20/ V. 15 V 10 V 0 V -10 V -15 V V PP V rms t ( µ s) -20 V 38

41 The Bridge Full-Wave Rectifier Determine the peak output voltage and current in the 3.3 kω load resistor if V sec = 24 V rms. Use the practical diode model The peak output voltage is: V V ( ) = 1.41V rms = 33.9 V p sec F 120 V D 3 D 1 ( ) = V p ( sec ) 1.4 V V (sec) = 24 Vrms R = 32.5 V L V p(out ) p out Applying Ohm s law, I p(out) = 32.5 / 3.3 = 9.8 ma D 2 D kω 39

42 Remember! Reverse bias Reverse bias is the condition in which current is blocked. V BIAS V R V BR Knee 0 0 I = 0 A R V BIAS I R 40

43 Peak Inverse Voltage (PIV): Ideal 41

44 Peak Inverse Voltage (PIV): Practical 42

45 Remember! The Basic DC Power Supply 43

46 Power Supply Filters Filtering is the process of smoothing the ripple from the rectifier. V in V OUT Full-wave 0 V Filter 0 rectif ier (Ripple is exaggerated.) The capacitor input filter is widely used. A half-wave rectifier and capacitor-input filter are shown: V in V C R L 44

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49 Ripple Voltage 47

50 Period and Frequency 48

51 Half-Wave vs. Full Wave 49

52 Ripple Factor where V r(pp) is the peak-to-peak ripple voltage, and V DC is the dc (average) value of the filter s output voltage. The lower the ripple factor, the better the filter. The ripple factor can be lowered by: increasing the value of the filter capacitor or increasing the load resistance. 50

53 Power Supply Filters R = V / I C = Q / V R C = Q / I = Q / (Q/t) = t Ω F = s How is the ripple affected by the RC time constant? V in V C R L A longer time constant will have less ripple for the same input voltage and frequency. 51

54 Remember! The Basic DC Power Supply 52

55 Power Supply Regulators A voltage regulator can furnish nearly constant output with excellent ripple rejection. Three-terminal regulators require only external capacitors to complete the regulation portion of the circuit. F 1 T 1 SW1 D 3 D 1 Voltage D 2 D 4 regulator C 1 C 2 53

56 Power Supply Regulators Regulation performance is specified in two ways. Line regulation specifies how much the dc output changes for a given change in regulator s input voltage. The next formula is based on a dc input voltage change to the regulator due to a change in the ac line voltage. V V OUT Line regulation = 100% Assume the dc input to a regulator changes by 1.0 V due to a change in the ac line voltage. If the output changes by 1.5 mv due to the change, what is the line regulation? IN V 1.5 mv = = VIN 1.0 V OUT Line regulation = 100% 100% 0.15% 54

57 Power Supply Regulators Load regulation specifies how much change occurs in the output voltage for a given range of load current values, usually from no load (NL) to full load (FL). V V V NL FL Load regulation = 100% FL Assume the dc output of a regulator changes from 5.00 V to 4.96 V when the output is varies from no load to full load. What is the load regulation? V V 5.00 V 4.96 V = = VFL 4.96 V NL FL Load regulation = 100% 100% 0.8 % 55

58 Quiz Q. The formula to calculate the load regulation is, a. b. c. d. V NL Load regulation = 100% VFL V OUT Load regulation = 100% VIN V OUT Load regulation = 100% VOUT VIN V V V NL FL Load regulation = 100% FL 56

59 Diode Limiting (Clipping) Circuits A diode limiter (clipper) is a circuit that limits (or clips) either the positive or negative part of the input voltage. A biased limiter is one that has a bias voltage in series with the diode, so that a specific voltage level can be selected for limiting. A positive limiter is shown. R L is normally >> R 1 to avoid loading effects. The output will be clipped when the input voltage overcomes the bias voltage and the forward voltage of the diode. R 1 V in V BIAS 0.7 V 0 R L 0 V BIAS 61

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62 Diode Limiting (Clipping) Circuits What is the output of positive limiter shown? V in 10 V 1.0 kω 0 R 1 R L 3.0 V V BIAS = kω 2.3 V The diode is forward-biased when the output tries to go above =3.0 V. This causes the output to be limited to voltages less than 3.0 V. 64

63 Quiz Q. The bias voltage is set to 4.3 V. The output of the biased limiter shown will be clipped a. above 3.6 V R 1 b. below 3.6 V c. above 5.0 V d. below 5.0 V 10 V 1.0 kω V in 0 V BIAS = 4.3 V R L 100 kω 65

64 Diode Limiting (Clipping) Circuits What happens in the previous circuit if the diode is reversed? 10 V 1.0 kω R 1 V in 0 R L V BIAS = kω 1.6 V 2.3 V The diode is forward-biased when the output tries to go below =1.6 V. This causes the output to be limited to voltages greater than 1.6 V. 66

65 Quiz Q. The bias voltage is set to 4.3 V. The output of the biased limiter shown will be clipped a. above 3.6 V R 1 b. below 3.6 V c. above 5.0 V 10 V 1.0 kω V in 0 V BIAS = 4.3 V R L 100 kω d. below 5.0 V 67

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67 Diode Clamping Circuits A clamper (dc restorer) is a circuit that adds a dc level to an ac signal. A capacitor is in series with the load. A positive clamper is shown. The capacitor is charged to a voltage that is one diode drop less than the peak voltage of the signal. V p(in) 0.7 V V p(in) 0.7 V V p(in) V out 0 R L V 69

68 Positive Clamper Operation Reverse -biased 70

69 Positive Clamper Operation When the input voltage initially goes negative, the diode is forward-biased, allowing the capacitor to charge to near the peak of the input. Just after the negative peak, the diode is reverse-biased. The capacitor can only discharge through the high resistance of R L. So, from the peak of one negative half-cycle to the next, the capacitor discharges very little. The amount that is discharged depends on the value of R L. If the RC time constant is 100 times the period, the clamping action is excellent. An RC time constant of 10 times the period will have a small amount of distortion. The net effect of the clamping action is that the capacitor retains a charge approximately equal to the peak value of the input less the diode drop (0.7 V). The capacitor voltage acts essentially as a battery in series with the input voltage. The dc voltage of the capacitor adds to the input voltage by superposition. 71

70 Diode Clamping Circuits Reversing the diode forms a negative clamper. V p (in) -0.7 V p(in) V 0 R L V out V p (in) 0.7 V 72

71 Quiz Q. The circuit shown is a a. negative clipping circuit b. positive clipping circuit c. negative clamping circuit R L d. positive clamping circuit 73

72 Voltage Multipliers Voltage multipliers use clamping action to increase peak rectified voltages. Half-Wave Voltage Doubler (neglecting the diode drop) 74

Full-Wave Voltage Doubler The full-wave voltage doubler works by charging a capacitor to the positive peak voltage on one cycle of the sine wave and a second capacitor on the negative peak voltage.

73 Full-Wave Voltage Doubler The full-wave voltage doubler works by charging a capacitor to the positive peak voltage on one cycle of the sine wave and a second capacitor on the negative peak voltage. The output is (ideally) doubled by taking it across both capacitors in series. (neglecting the diode drop) 0 V p D 1 I D 2 Reverse-biased C 1 C 2 V p 0 Reverse-biased V p I D 1 C 1 C 2 D 2 V p V p 2V p 75

74 Quiz Q. The circuit shown is a a. full-wave rectifier D 1 b. full-wave voltage doubler C 1 c. positive clamping circuit d. negative clamping circuit D 2 C 2 76

75 Selected Key Terms Rectifier Filter Regulator Ripple Voltage An electronic circuit that converts ac into pulsating dc; one part of a power supply. In a power supply, the capacitor used to reduce the variation of the output voltage from a rectifier. An electronic device or circuit that maintains an essentially constant output voltage for a range of input voltage or load values; one part of a power supply. The small variation in dc output voltage of a filtered rectifier caused by charging and discharging of the filter capacitor. 77

76 Selected Key Terms Line Regulation Load Regulation Limiter Clamper The change in output voltage of a regulator for a given change in input voltage, normally expressed as a percentage. The change in output voltage of a regulator for a given range of load currents, normally expressed as a percentage. A diode circuit that clips off or removes part of a waveform above and/or below a specified level. A circuit that adds a dc level to an ac voltage using a diode and a capacitor. 78

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Electronic Devices. Floyd. Chapter 2. Ninth Edition. Electronic Devices, 9th edition Thomas L. Floyd

Electronic Devices. Floyd. Chapter 2. Ninth Edition. Electronic Devices, 9th edition Thomas L. Floyd Electronic Devices Ninth Edition Floyd Chapter 2 Agenda Diode Circuits and Applications Half-wave Rectifier Full-wave Rectifier Power Supply Filter Power Supply Regulator Diode Limiting Circuits Diode