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Ch02

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Ch02

2. 2. Slide 1 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Load Line The Load Line plots all possible current (I D ) conditions for all voltages applied to the diode (V D ) in a given circuit. E/R is the maximum I D and E is the maximum V D . Where the Load Line and the Characteristic Curve intersect is the Q point , which specifies a particular I D and V D for a given circuit.
3. 3. Slide 2 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Diode Approximations In Forward Bias: Silicon Diode: V D = .7V Germanium Diode: V D = .3V In Reverse Bias: Both diodes act like opens V D = source voltage and I D =0A
4. 4. Slide 3 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Diode in DC Series Circuit: Forward Bias <ul><li>The diode is forward biased. </li></ul><ul><ul><ul><li>• V D = .7V (or V D = E if E < .7V) [Formula 2.4] </li></ul></ul></ul><ul><li> • V R = E – V D [Formula 2.5] </li></ul><ul><li>• I D = I R = IT = V R /R [Formula 2.6] </li></ul>
5. 5. Slide 4 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. <ul><ul><ul><li>Diode in DC Series Circuit: Reverse Bias </li></ul></ul></ul><ul><li>The diode is reverse biased. </li></ul><ul><ul><ul><li>• V D = E </li></ul></ul></ul><ul><li>• V R = 0V </li></ul><ul><li>• I D = I R = I T = 0A </li></ul>
6. 6. Slide 5 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. <ul><ul><ul><li>Diode in any DC Circuit </li></ul></ul></ul><ul><li>Solve this circuit like any Series/Parallel circuit , </li></ul><ul><li>knowing V D = .7V (or up to .7V) in forward bias </li></ul><ul><ul><ul><li>and as an open in reverse bias. </li></ul></ul></ul><ul><ul><ul><li>V D1 = V D2 = Vo = .7V </li></ul></ul></ul><ul><ul><ul><li>V R = 9.3V </li></ul></ul></ul><ul><li>Diodes in parallel are used to limit current: </li></ul><ul><li>I R = E – V D = 10V - .7V = 28mA </li></ul><ul><li>R .33k  </li></ul><ul><li>I D1 = I D2 = 28mA/2 = 14mA </li></ul>
7. 7. Slide 6 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. <ul><ul><ul><li>Diodes in AC Circuits </li></ul></ul></ul>The diode only conducts when it is in forward bias, therefore only half of the AC cycle passes through the diode.
8. 8. Slide 7 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. <ul><li>Diodes convert AC to DC in a process called rectification . </li></ul><ul><li>The diode only conducts for one-half of the AC cycle. The remaining half is either all positive or all negative. This is a crude AC to DC conversion. </li></ul><ul><li>The DC Voltage out of the diode : </li></ul><ul><ul><ul><li> V DC = 0.318Vm [Formula 2.7] </li></ul></ul></ul><ul><li>where V m = the peak voltage </li></ul>Half Wave Rectifier
9. 9. Slide 8 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. PIV (PRV) <ul><li>Because the diode is only forward biased for one-half of the AC cycle, it is then also off for one-half of the AC cycle . It is important that the reverse breakdown voltage rating of the diode be high enough to withstand the peak AC voltage. </li></ul><ul><ul><ul><ul><li>PIV (PRV) > V m [Formula 2.9] </li></ul></ul></ul></ul><ul><li>PIV = Peak Inverse Voltage; PRV = Peak Reverse Voltage </li></ul><ul><li>V m = Peak AC Voltage </li></ul>
10. 10. Slide 9 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Full Wave Rectification The rectification process can be improved by using more diodes in a Full Wave Rectifier circuit. Full Wave rectification produces a greater DC output.
11. 11. Slide 10 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. <ul><li>There are two Full Wave Rectifier circuits: </li></ul><ul><ul><ul><li>• Bridge Rectifier </li></ul></ul></ul><ul><li>• Center –Taped Transformer Rectifier </li></ul>Full Wave Rectifier Circuits
13. 13. Slide 12 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Operation of the Bridge Rectifier Circuit For the positive half of the AC cycle: For the negative half of the AC cycle:
14. 14. Slide 13 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Center–Tapped Transformer Rectifier Circuit Two diodes and a center-tapped transformer are required. V DC = 0.636(V m ) Note that V m here is the transformer secondary voltage to the tap.
15. 15. Slide 14 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Operation of the Center–Tapped Transformer Rectifier Circuit For the positive half of the AC cycle: For the negative half of the AC cycle:
16. 16. Slide 15 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Note: V m = peak of the AC voltage. Be careful, in the center tapped transformer rectifier circuit the peak AC voltage is the transformer secondary voltage to the tap. Rectifier Circuit Summary
17. 17. Slide 16 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Diodes “clip” a portion of the AC wave. The diode “clips” any voltage that does not put it in forward bias. That would be a reverse biasing polarity and a voltage less than .7V for a silicon diode. Clipper Diode Circuit
18. 18. Slide 17 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. By adding a DC source to the circuit, the voltage required to forward bias the diode can be changed. Variations of the Clipper Circuit
19. 19. Slide 18 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. By taking the output across the diode, the output is now the voltage when the diode is not conducting. A DC source can also be added to change the diode’s required forward bias voltage. Changing Output Perspective