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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
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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.