Ideal and Real Diodes in Electronics
Ideal
Ideal
and
and
Real Diode
Real Diode
Ideal Diode
Ideal Diode
A diode is said to be an
Ideal Diode
when
it is
forward biased and acts like a
perfect
conductor
, with zero voltage drop across it
.
Similarly,
when the diode is reversed biased, it
acts as a
perfect insulator
with zero current
through it.
N
N
o
o
t
t
e
e
:
:
v
v
o
o
l
l
t
t
a
a
g
g
e
e
d
d
r
r
o
o
p
p
=
=
I
I
*
*
R
R
An ideal diode may be defined as a device which:
a)
When
forward biased
forward biased
, conducts with
zero
resistant,
act
act
as
short-circuit
short-circuit
, and there is no voltage
drop (even though current is there) since a short has
zero resistant.
b)
When
reverse biased
reverse biased
, there is no reverse
current because reverse resistant is
infinite
infinite
, and acts as
open-circuits.
It is helpful to think of an
ideal
ideal
diode as a
bistable switch
bistable switch
which is
close in the
forward direction
and
open in the reverse direction
.
Hence
Hence
it has two stable states: ON and OFF.
I-V characteristics of the Ideal diode
I-V characteristics of the Ideal diode
An Ideal diode acts like a switch
An Ideal diode acts like a switch
.
.
When the diode is forward biased it acts like a
closed
closed
switch
switch
as shown in the figure below:
Whereas, if the diode is reversed biased, it acts like an
open
open
switch
switch
,
as shown in the figure below:
The I-V characteristics of the Ideal diode are shown in the
following figures:
Example:
Example:
Solution :
Solution :
N
N
o
o
t
t
e
e
:
:
p
p
o
o
w
w
e
e
r
r
d
d
i
i
s
s
s
s
p
p
a
a
t
t
e
e
d
d
=
=
I
I
2
2
*
*
R
R
H.W:
H.W:
Real Diode:
Real Diode:
A
Real diode
Real diode
contains barrier potential
V
o
(
0.7 V for silicon
and 0.3 V for Germanium
) and a forward resistance
R
F
.
When a diode is
forward biased
and conducts a forward current
I
F
flows through it which causes a voltage drop
I
F
R
F
in the forward
resistance.
Hence
Hence
, the forward voltage
V
F
applied across the real diode for
conduction has to overcome the following:
1- Potential barrier
1- Potential barrier
2 - Drop in forward resistance
2 - Drop in forward resistance
For all the practical purposes, a diode is considered to be an open switch
when
reverse biased
reverse biased
. It is because the value of reverse resistance is so high
(R
R
> 100 MΩ)
that is considered to be an infinite for all practical purposes.
The
equivalent circuit
equivalent circuit
of the real diode under
forward bias
forward bias
condition is
shown below:
This circuit shows that a real diode still acts as a switch when forward
This circuit shows that a real diode still acts as a switch when forward
biased, but the voltage required to operate this switch is V
biased, but the voltage required to operate this switch is V
F
F
.
.
The I-V characteristic of the Real diode
The I-V characteristic of the Real diode
a) Forward Direction:
In this case, we
have
to take
two factors
into account:
1-
Forward current dose not start
flowing until the voltage applied to
the diode exceeds its threshold or
knee voltage
V
K
(
0.3V for Ge and
0.7V for Si
). Hence, a real diode is
shown as equivalent to an ideal
diode in series with a small
oppositely-connected battery of
V
K
as shown in figure 4(a).
2-
The forward dynamic or ac resistant (
r
ac
) offered by the circuit. So far,
we considered this resistant to be
zero
implying that forward
characteristic is a straight vertical line as in figure 4(a). If we take (
r
ac
)
into account, the forward characteristic becomes as shown in figure 4(b).
Here the reciprocal of the slope of this characteristic represent
r
ac
.
NOTE THAT:
NOTE THAT:
r
ac
or
r
d
= r
B
+ r
j
r
j
= 25
(mV) /
I
F
(mA)
…
for
Ge
=
50
(mV) /
I
F
(mA)
…
for
Si
In this case, the signal voltage is small as compared with
V
K
. Since
I
F
would be small,
r
j
would be very large as compared with
r
B
r
ac
=
r
j
+
r
B
≈
r
j
Large signal source are those whose voltage is much greater than the
diode knee voltage
V
K
( nearly equal to barrier potential
V
B
). Under such
conditions, forward current would be large, so that
r
j
would be negligible.
r
ac
=
r
j
+
r
B
≈
r
B
i) Large Signal operation:
ii) Small Signal operation:
b) Reverse Direction:
An actual or real diode does not have infinite
resistance in the reverse direction
because it will always have some
reverse saturation current prior to breakdown.
For example
, if with a V
R
= 50 V, I
R
is 10 μA, then R
R
= 5*10
6
Ω = 5MΩ.
silicon diode have reverse resistant of many thousands of mega ohms.
Hence,
an actual diode in the reverse direction can be thought of as
equivalent to a high resistor.
Example:
Example:
Solution :
Solution :
In electronics, an ideal diode behaves like a perfect switch with no voltage drop when forward biased and acts as an insulator with zero current when reverse biased. On the other hand, a real diode has a barrier potential and forward resistance to overcome when conducting current. The I-V characteristics of both ideal and real diodes showcase their behavior as switches in different biasing conditions, providing valuable insights for circuit analysis.
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Presentation Transcript
Ideal and Real Diode
Ideal Diode A diode is said to be an Ideal Diode when it is forward biased and conductor, with zero voltage drop across it. acts like a perfect Note Note: : voltage drop = voltage drop = I * I * R R Similarly, when the diode is reversed biased, it acts as a perfect insulator with zero current through it.
An ideal diode may be defined as a device which: a) When forward biased, conducts with zero resistant, act as short-circuit, and there is no voltage drop (even though current is there) since a short has zero resistant. b) When reverse biased, there is no reverse current because reverse resistant is infinite, and acts as open-circuits. It is helpful to think of an ideal diode as a bistable switch which is close in the forward direction and open in the reverse direction. Hence it has two stable states: ON and OFF.
I-V characteristics of the Ideal diode The I-V characteristics of the Ideal diode are shown in the following figures: An Ideal diode acts like a switch. When the diode is forward biased it acts like a closed switch as shown in the figure below: Whereas, if the diode is reversed biased, it acts like an open switch, as shown in the figure below:
Solution : Note Note: : power disspated = power disspated = I I 2 2 * * R R
Real Diode: A Real diode contains barrier potential Vo(0.7 V for silicon and 0.3 V for Germanium) and a forward resistance RF. When a diode is forward biased and conducts a forward current IF flows through it which causes a voltage drop IFRFin the forward resistance. Hence, the forward voltage VFapplied across the real diode for conduction has to overcome the following: 1- Potential barrier 2 - Drop in forward resistance
The I-V characteristic of the Real diode For all the practical purposes, a diode is considered to be an open switch when reverse biased. It is because the value of reverse resistance is so high (RR> 100 M ) that is considered to be an infinite for all practical purposes. The equivalent circuit of the real diode under forward bias condition is shown below: This circuit shows that a real diode still acts as a switch when forward biased, but the voltage required to operate this switch is VF.
a) Forward Direction: In this case, we have to take two factors into account: 1- Forward current dose not start flowing until the voltage applied to the diode exceeds its threshold or knee voltage VK(0.3V for Ge and 0.7V for Si). Hence, a real diode is shown as equivalent to an ideal diode in series with a small oppositely-connected battery of VK as shown in figure 4(a). 2- The forward dynamic or ac resistant (rac) offered by the circuit. So far, we considered this resistant to be zero implying that forward characteristic is a straight vertical line as in figure 4(a). If we take (rac) into account, the forward characteristic becomes as shown in figure 4(b). Here the reciprocal of the slope of this characteristic represent rac.
i) Large Signal operation: Large signal source are those whose voltage is much greater than the diode knee voltage VK( nearly equal to barrier potential VB ). Under such conditions, forward current would be large, so that rjwould be negligible. rac= rj+ rB rB ii) Small Signal operation: In this case, the signal voltage is small as compared with VK. Since IF would be small, rjwould be very large as compared with rB rac= rj+ rB rj NOTE THAT: NOTE THAT: racor rd= rB+ rj rj = 25 (mV) / IF(mA) for Ge = 50 (mV) / IF(mA) for Si
b) Reverse Direction: An actual or real diode does not have infinite resistance in the reverse direction because it will always have some reverse saturation current prior to breakdown. For example, if with a VR= 50 V, IRis 10 A, then RR= 5*106 = 5M . silicon diode have reverse resistant of many thousands of mega ohms. Hence, an actual diode in the reverse direction can be thought of as equivalent to a high resistor.
End End
