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 Junction

anode

cathode

Forward bias

Reversed bias

- -+ +

Polarization of the pn Junction

Forward biasexamples

(1) (2)

(3) (4)

Polarization of the pn Junction

Reversed biasexamples

(1) (2)

(3) (4)

PPNN

PP NN

1. DMM = 0

2. DMM =

Diode Ohms CheckDiode Ohms Check:

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

DMM

+-

Diode Voltages

A 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 Puzzle

Which lamps are alight? Some may not be full brightness.

+-

+-

A Diode Puzzle

Which 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 Characteristic

A diode is a nonlinear device and typical linear circuit analysis methods do not apply!

circuit

DRRp

Ev V reading

A reading

Diode Characteristic for Small-Signal Diodes

less than 1mA at 300K

When the temperature is increasing the knee voltage Vknee decreases by about 2mV/K

q

kTV

nV

vIi

T

T

dsD

1exp

Vknee

n ~ 1-2VT ~ 26 mV

Analysis of Diode Circuits

Nodal analysisMesh analysisKirchhoff’s voltage lawThevenin & Norton theorems

DDss vRiV

Vth/RTh

Vth

Slope=-1/RTh

Example 10.1

Analysis of Diode Circuits

Do vV

Theveninequivalent

+

-

+

-

io

Vo vD

iD

KVL

KCL Do ii Their characteristics intersect

Analysis of Diode Circuits

Nodal analysisMesh analysisKirchhoff’s voltage lawThevenin & Norton theorems

DDss vRiV

Vth/RTh

Vth

Slope=-1/RTh

Example 10.1

Load-Line AnalysisProblemIf the circuit shown below has Vss=2V and R=1kand a diode with ch-tic shown, find the diode voltage and current at the operating point

DDss vRiV

Repeat for:Vss=10V and R=10k

VDQ=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)

Kirchhoff’s voltage law

Vss+ RiD+vD=0

ProblemFind the output voltage for Vss=15V and Vss=20V if R=1k and a Zener diode has the ch-tic shown below.

Load Line analysis

Reverse bias region

Slope 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=1.2k and RL=6k

Problem

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

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=1.2k and RL=6k

VT=Vss*(RL/(R+RL))=20VRT=(RRL)/(R+RL)=1k

Thevenin equivalent

VT + RTiD + VD = 0

Load line equation

Finally iS=(VSS-VL)/R=11.67 mA (from circuit “a”)

Exercise 10.4 & 10.5

iD=-10mA

VL=-VD=10V

Ideal diode ModelUseful for circuits with more than one diode

(1)Assume 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.

ProblemAnalyze the circuit shown below using the ideal diode model. Start by assuming the D1 is off and D2 is on.

7V

Not consistent with the assumption that D2 if off

-3V

Exercise 10.6 & 10.7 & 10.8

ProblemAnalyze the circuit shown below using the ideal diode model. Start by assuming the D1 is off and D2 is on.

7V

-3V

Not consistent with the assumption that D1 is off

ProblemAnalyze the circuit shown below using the ideal diode model. Start by assuming the D1 is off and D2 is on.

7V

-3V

This 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 source

v = Rai + Va

ProblemFind circuit models for the Zener-diode volt-ampere ch-tic shown in figure below using the piecewise-linear diode model.

Draw a line

Look for intercept (0.6V) & the reciprocal of the slope (1/R)(1.6V-0.6V)/100mA=10

Repeat for the reverse bias

Open circuit approximation

Exercise 10.7