Chapter 2: Diode Applications

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Load-Line Analysis 2 The load line plots all possible combinations of diode current (I D ) and voltage (V D ) for a given circuit. The maximum I D equals E/R, and the maximum V D equals E. The point where the load line and the characteristic curve intersect is the Q-point, which identifies I D and V D for a particular diode in a given circuit.

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Series Diode Configurations Constants Silicon Diode: V D = 0.7 V Germanium Diode: V D = 0.3 V Analysis (for silicon) V D = 0.7 V (or V D = E if E < 0.7 V) V R = E – V D I D = I R = I T = V R / R 3 Forward Bias

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Series Diode Configurations Diodes ideally behave as open circuits Analysis V D = E V R = 0 V I D = 0 A 4 Reverse Bias

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Parallel Configurations 5

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Half-Wave Rectification 6 The diode only conducts when it is forward biased, therefore only half of the AC cycle passes through the diode to the output. The DC output voltage is 0.318V m, where V m = the peak AC voltage.

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky PIV (PRV) 7 Because the diode is only forward biased for one-half of the AC cycle, it is also reverse biased for one-half cycle. It is important that the reverse breakdown voltage rating of the diode be high enough to withstand the peak, reverse-biasing AC voltage. PIV (or PRV) > V m PIV = Peak inverse voltage PRV = Peak reverse voltage V m = Peak AC voltage

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Full-Wave Rectification 8 V dc = 0.318V m Half-wave: V dc = 0.318V m V dc = 0.636V m Full-wave: V dc = 0.636V m The rectification process can be improved by using a full-wave rectifier circuit. Full-wave rectification produces a greater DC output:

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Full-Wave Rectification 9 Bridge Rectifier Four diodes are connected in a bridge configuration V DC = 0.636V m

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Full-Wave Rectification 10 Center-Tapped Transformer Rectifier Requires Two diodes Center-tapped transformer V DC = 0.636V m

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Summary of Rectifier Circuits 11 V m = peak of the AC voltage. I In the center tapped transformer rectifier circuit, the peak AC voltage is the transformer secondary voltage to the tap. Rectifier Ideal V DC Realistic V DC Half Wave Rectifier DC V DC = 0.318V m DCm V DC = 0.318V m – 0.7 Bridge Rectifier DC V DC = 0.636V m DC V DC = 0.636V m – 2(0.7 V) Center-Tapped Transformer Rectifier DC V DC = 0.636V m DC V DC = 0.636V m – 0.7 V

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Diode Clippers 12 series clipper The diode in a series clipper “clips” any voltage that does not forward bias it: A reverse-biasing polarity A forward-biasing polarity less than 0.7 V (for a silicon diode)

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Biased Clippers 13 Adding a DC source in series with the clipping diode changes the effective forward bias of the diode.

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Parallel Clippers 14 parallel clipper The diode in a parallel clipper circuit “clips” any voltage that forward bias it. DC biasing can be added in series with the diode to change the clipping level.

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Summary of Clipper Circuits 15 more…

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Summary of Clipper Circuits 16

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Clampers 17 A diode and capacitor can be combined to “clamp” an AC signal to a specific DC level.

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Biased Clamper Circuits 18 The input signal can be any type of waveform such as sine, square, and triangle waves. The DC source lets you adjust the DC camping level.

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Summary of Clamper Circuits 19

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Zener Diodes 20 The Zener is a diode operated in reverse bias at the Zener Voltage (V z ). When V i  V Z –The Zener is on –Voltage across the Zener is V Z –Zener current: I Z = I R – I RL –The Zener Power: P Z = V Z I Z When V i < V Z –The Zener is off –The Zener acts as an open circuit

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Zener Resistor Values 21 If R is too large, the Zener diode cannot conduct because the available amount of current is less than the minimum current rating, I ZK. The minimum current is given by: The maximum value of resistance is: If R is too small, the Zener current exceeds the maximum current rating, I ZM. The maximum current for the circuit is given by: The minimum value of resistance is:

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Voltage-Multiplier Circuits Voltage Doubler Voltage Tripler Voltage Quadrupler 22 Voltage multiplier circuits use a combination of diodes and capacitors to step up the output voltage of rectifier circuits.

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Voltage Doubler 23 This half-wave voltage doubler’s output can be calculated by: V out = V C2 = 2V m where V m = peak secondary voltage of the transformer

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Voltage Doubler 24 Positive Half-Cycle oD 1 conducts oD 2 is switched off oCapacitor C 1 charges to V m Negative Half-Cycle oD 1 is switched off oD 2 conducts oCapacitor C 2 charges to V m V out = V C2 = 2V m

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Voltage Tripler and Quadrupler 25

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Practical Applications Rectifier Circuits –Conversions of AC to DC for DC operated circuits –Battery Charging Circuits Simple Diode Circuits –Protective Circuits against –Overcurrent –Polarity Reversal –Currents caused by an inductive kick in a relay circuit Zener Circuits –Overvoltage Protection –Setting Reference Voltages 26