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Diodes

Structure. Symbol

circuit symbol

directions for current

and voltage

physical structure

Ideal diodeKeeps only one property:

the electrical current passes in only one direction (unidirectional behavior)

Automatic switch:

totally blocks the current flow if the voltage across its terminals

(vD) is negative (off);

allows any value of the current to flow if the voltage across its

terminals (vD) tends to a positive value (on)

VTh=0V

Model of the ideal diode

off on

open-circuit short-circuit

Example

0Ov

0Dv

OID vvv

0Iv

IO vv

0Di

R

vi ID

0Iv

What is the VTC vO (vI)?

VTC Voltage Transfer Characteristic

0Di 0Dv

Ov0Iv

Iv 0Iv

0

Voltage transfer characteristic

Application:

Voltage rectifier

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Waveforms:

output voltage

input voltage

currenta) vo(t) for an input sine

wave with 3V amplitude

and 2V offset?

b) Peak forward current

through diode if R=2k?

Half-wave rectifier

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Constant-voltage-drop model

For forward bias vD>0:

when vD0

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The effect of voltage drop

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Switching DR multi-port networks

Maximum multi-port networks

AoBA vvoffDonDvv );(),(;0 21

BoAB vvonDoffDvv );(),(;0 21

0);(),(;0,0 21 oBA voffDoffDvv

)0,,max(0 BA vvv

For the diodes assume the ideal model

13 / 9

vO= max(vA , vB , 0).

vO= max(vA 0.7V; vB 0.7V; 0).

D - idealD constant-voltage-drop

Minimum multi-port networks

vO(t) ?

vO= min(vA, vB, VPS)

vO= min(vA+ 0.7V, vB+ 0.7V, VPS)

DR logic circuits

analog signal digital signal

Logic 0 false low

Logic 1 true high

0V logic 010Vlogic 1 CMOS logic family

supplied at +5V

2-input OR circuit

0V logic 05V logic 1

operating table truth table

3-input AND circuit

-VZK

VTh

)1( TD

nV

v

SD eIiIS - saturation current

q

KTVT thermal voltage

25TV mV (aprox 20O C)

n =2 discret diodes n=1 integrated diodes

SD Ii

T

D

nV

v

SD eIi

Exponential model of the diode

D is a rectifier diode, 1N400x with IS=14nA and n=2

mA8.16)1(1014 252700

9 eiD

Illustration

Usually, one can assume a voltage drop across the diode

V7.0Dv

The resulting forward current through diode is:

Numerical example for a DR circuit in permanent

conduction.