Diode Circuits

The left hand diagram shows reverse bias, with positive on the cathode and negative on the anode (via the lamp). No current flows.

The other diagram shows forward bias, with positive on the anode and negative on the cathode. A current flows.Practical Aspects of pn JunctionanodecathodeForward biasReversed bias--++

Polarization of the pn JunctionForward biasexamples(1)(2)(3)(4)

Polarization of the pn JunctionReversed biasexamples(1)(2)(3)(4)

Diode Ohms Check:

Checks preformed on Si diode, by reversing the leads on the Digital Voltage Mutimeter (DMM). DMM+-

Diode VoltagesA conducting diode has about 0.6 volts across if silicon, 0.3 volts if germanium.To forward bias a diode, the anode must be more positive than the cathode or LESS NEGATIVE. To reverse bias a diode, the anode must be less positive than the cathode or MORE NEGATIVE.

A Diode PuzzleWhich lamps are alight? Some may not be full brightness.+-+-

A Diode PuzzleWhich lamps are alight? Some may not be full brightness.+-+-

Which lamps are alight? Some may not be full brightness.+-+-Exercise - a Diode Puzzle

Which lamps are alight? Some may not be full brightness.+-+-Exercise - a Diode Puzzle

Diode CharacteristicA diode is a nonlinear device and typical linear circuit analysis methods do not apply! circuitDRRpEvV readingA reading

Diode Characteristic for Small-Signal Diodesless than 1mA at 300KWhen the temperature is increasing the knee voltage Vknee decreases by about 2mV/KVkneen ~ 1-2VT ~ 26 mV

Analysis of Diode Circuits Vth/RTh Vth Slope=-1/RThExample 10.1

Analysis of Diode CircuitsKVL

KCLTheir characteristics intersect

Analysis of Diode Circuits Vth/RTh Vth Slope=-1/RThExample 10.1

Load-Line AnalysisProblem If the circuit shown below has Vss=2V and R=1kW and a diode with ch-tic shown, find the diode voltage and current at the operating pointRepeat for:Vss=10V and R=10kWVDQ=0.68V and iDQ=0.93mA

Zener Diode - Voltage Regulator (reverse biased)A Zener diode is a type of diode that permits current not only in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the breakdown voltage known as "Zener knee voltage" or "Zener voltage".

Zener Diode - Voltage Regulator (reverse biased)Kirchhoffs voltage lawProblem Find the output voltage for Vss=15V and Vss=20V if R=1kW and a Zener diode has the ch-tic shown below. Load Line analysisReverse bias regionSlope of the load is -1/R

Load Line Analysis of Complex Circuits Thevenin Equivalent

Problem Consider the Zener diode regulator shown in figure (a). Find the load voltage vL and the source current iS if Vss=24V, R=1.2kW and RL=6kW.

Problem Consider the Zener diode regulator shown in figure (a). Find the load voltage vL and the source current iS if Vss=24V, R=1.2kW and RL=6kW.Exercise find Thevenin equivalent

Problem Consider the Zener diode regulator shown in figure (a). Find the load voltage vL and the source current iS if Vss=24V, R=1.2kW and RL=6kW.

VT + RTiD + VD = 0Load line equationFinally iS=(VSS-VL)/R=11.67 mA (from circuit a)

Exercise 10.4 & 10.5iD=-10mAVL=-VD=10V

Ideal diode ModelUseful for circuits with more than one diodeAssume a state for each diode, either on or off -2n combinations

(2) Assume a short circuit for diode on and an open circuit for diode off

(3) Check to see if the result is consistent with the assumed state for each diode (current must flow in the forward direction for diode on and the voltage across the diodes assumed to be off must be positive at the cathode reverse bias)

(4) If the results are consistent with the assumed states, the analysis is finished. Otherwise return to step (1) and choose a different combination of diode states.

Problem Analyze the circuit shown below using the ideal diode model. Start by assuming the D1 is off and D2 is on.7VNot consistent with the assumption that D2 if off-3VExercise 10.6 & 10.7 & 10.8

Problem Analyze the circuit shown below using the ideal diode model. Start by assuming the D1 is off and D2 is on.7V-3VNot consistent with the assumption that D1 is off

Problem Analyze the circuit shown below using the ideal diode model. Start by assuming the D1 is off and D2 is on.7V-3VThis is OK

Piecewise Linear Diode ModelsMore accurate that the ideal diode model and do not relies on nonlinear equation or graphical techniques.(1) Diode V-I ch-tic approximated by straight line segments(2) We model each section of the diode I-V ch-tic with R in series with a fixed voltage sourcev = Rai + Va

Problem Find circuit models for the Zener-diode volt-ampere ch-tic shown in figure below using the piecewise-linear diode model.Draw a lineLook for intercept (0.6V) & the reciprocal of the slope (1/R)(1.6V-0.6V)/100mA=10W

Repeat for the reverse bias Open circuit approximationExercise 10.7

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