EE2301:
Basic Electronic Circuit
Let’s start with diode
EE2301: Block C Unit 1
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EE2301: Basic Electronic Circuit
Examples of Diode
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EE2301: Basic Electronic Circuit
The Basic Property of a Diode
Let’s have a demo
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EE2301: Basic Electronic Circuit
How does it work?
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EE2301: Basic Electronic Circuit
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Block C Unit 1 Outline
Semiconductor materials (eg. silicon)
>
Intrinsic and extrinsic semiconductors
How a p
-
n junction works (basis of diodes)
Large signal models
>
Ideal diode model
>
Offset diode model
Finding the operating point
Application of diodes in rectification
EE2301: Basic Electronic Circuit
Semiconductor Electronics
-
Unit 1: Diodes
6
Electrical Materials
Insulators
Conductors
Semi
-
Conductors
EE2301: Basic Electronic Circuit
Semiconductor Electronics
-
Unit 1: Diodes
7
Semiconductor Applications
Integrated Circuit
EE2301: Basic Electronic Circuit
Semiconductor Electronics
-
Unit 1: Diodes
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Semiconductor Applications
TFT (Thin Film Transistor)
EE2301: Basic Electronic Circuit
Semiconductor Electronics
-
Unit 1: Diodes
9
Intrinsic Semiconductor
Si
Si
Si
Si
Covalent Bonds
EE2301: Basic Electronic Circuit
Semiconductor Electronics
-
Unit 1: Diodes
10
Silicon Crystal Lattice
In 3
-
D, this looks like:
Number atoms per m
3
: ~ 10
28
EE2301: Basic Electronic Circuit
Semiconductor Electronics
-
Unit 1: Diodes
11
Growing Silicon
We can grow very pure silicon
EE2301: Basic Electronic Circuit
Semiconductor Electronics
-
Unit 1: Diodes
12
Conduction
EE2301: Basic Electronic Circuit
Semiconductor Electronics
-
Unit 1: Diodes
13
Currents in Semiconductor
Source: http://hyperphysics.phy
-
astr.gsu.edu/HBASE/solids/intrin.html
EE2301: Basic Electronic Circuit
Semiconductor Electronics
-
Unit 1: Diodes
14
Carrier Concentration
The number of free electrons available for a
given material is called the
intrinsic
concentration
n
i
.
For example, at room
temperature, silicon has:
n
i
= 1.5 x 10
16
electrons/m
3
1 free electron in about every 10
12
atoms
EE2301: Basic Electronic Circuit
Semiconductor Electronics
-
Unit 1: Diodes
15
Doping: n
-
type
1 Si atom substituted
by 1 P atom
P has 5 valence electrons
(1 electron more)
1 free electron
created
Si
Si
Si
P
Si
Si
-
Electrically neutral
EE2301: Basic Electronic Circuit
Semiconductor Electronics
-
Unit 1: Diodes
16
Doping: p
-
type
Si
Si
Si
B
Si
Si
1 Si atom substituted
by 1 B atom
+
B has 3 valence electrons
(1 electron short)
1 hole created
Electrically neutral
EE2301: Basic Electronic Circuit
Semiconductor Electronics
-
Unit 1: Diodes
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p
-
n Junction
EE2301: Basic Electronic Circuit
Semiconductor Electronics
-
Unit 1: Diodes
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Diode Physics
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EE2301: Basic Electronic Circuit
Semiconductor Electronics
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Unit 1: Diodes
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Diode Physics
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Website: http://www
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g.eng.cam.ac.uk/mmg/teaching/linearcircuits/diode.html
EE2301: Basic Electronic Circuit
EE2301: Block C Unit 1
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Biasing and Conventions
v
D
: Voltage of P (anode) relative to N (cathode)
i
D
: Current flowing from anode to cathode
EE2301: Basic Electronic Circuit
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Diode
Diode begins to conduct a significant amount
of current: Voltage
V
γ
is typically around 0.7V
Diode equation: I
D
= I
0
[exp(eV
D
/kT)
-
1]
EE2301: Basic Electronic Circuit
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Diode Symbol and Operation
Forward
-
biased
Current (Large)
Reverse
-
biased
Current (~Zero)
+
-
Forward Biased:
Diode conducts
-
+
Reverse Biased:
Little or no current
i
D
EE2301: Basic Electronic Circuit
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Real diode circuits
+
-
+
V
L
-
+ V
D
-
I
D
V
T
R
T
To find V
L
where V
T
and R
T
are known,
First apply KVL around the loop:
V
T
= V
D
+ R
T
I
D
Then use the diode equation:
I
D
= I
0
[
exp
(
eV
D
/
kT
)
-
1]
At T = 300K,
kT
/e = 25mV
We then need to solve these two simultaneous equations, which is not trivial.
One alternative is to use the graphical method to find the value of I
D
and V
D
.
EE2301: Basic Electronic Circuit
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Graphical method
Operating point is where
the load line & I
-
V curve
of the diode
intersect
Equation from KVL
EE2301: Basic Electronic Circuit
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Diode circuit models
Simplify analysis of diode circuits which
can be otherwise difficult
Large
-
signal models: describe device
behavior in the presence of relatively large
voltages & currents
>
Ideal diode model
>
Off
-
set diode mode
EE2301: Basic Electronic Circuit
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Ideal diode model
In other words, diode is
treated like a switch here
v
D
> 0: Short circuit
v
D
< 0: Open circuit
EE2301: Basic Electronic Circuit
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Ideal diode model
Circuit containing
ideal diode
Circuit assuming that the
ideal diode conducts
Circuit assuming that
the ideal diode does
not conduct
EE2301: Basic Electronic Circuit
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Ideal diode example 1
Problems 9.7 and 9.8
Determine whether the diode is conducting or not. Assume diode is ideal
Repeat for V
i
= 12V and V
B
= 15V
EE2301: Basic Electronic Circuit
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Ideal diode example 1 solution
This slide is meant to be blank
Option 1:
Assume diode is conducting and find the diode current direction
Outcome 1:
If diode current flows from anode to cathode, the assumption is true
Diode is forward biased
Outcome 2:
If diode current flows from cathode to anode, the assumption is false
Diode is reverse biased
Option 2:
Assume diode is not conducting and find the voltage drop across it
Outcome 1:
If voltage drops from cathode to anode, then the assumption is true
Diode
is reverse biased
Outcome 2:
If voltage drops from anode to cathode, then the assumption is false
Diode
is forward biased
EE2301: Basic Electronic Circuit
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Ideal diode example 1 solution
This slide is meant to be blank
Assume diode is conducting
Forward
-
bias diode current (ie anode to cathode)
= (10
-
12) / (5 + 10) =
-
2/15 A
Assumption was wrong
Diode is in reverse bias
Assume diode is not
-
conducting
Reverse
-
bias voltage (ie cathode referenced to anode)
= 12
-
10 = 2V
Assumption was correct
Diode is in reverse bias
EE2301: Basic Electronic Circuit
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Ideal diode example 1 solution
This slide is meant to be blank
Assume diode is conducting
Forward
-
bias diode current (ie anode to cathode)
= (15
-
12) / (5 + 10) = 1/5 A
Assumption was correct
Diode is in forward bias
Assume diode is not
-
conducting
Reverse
-
bias voltage (ie cathode referenced to anode)
= 12
-
15 =
-
3V
Assumption was incorrect
Diode is in forward bias
EE2301: Basic Electronic Circuit
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Ideal diode example 2
Problem 9.14
Find the range of V
in
for which D
1
is forward
-
biased. Assume diode is ideal
The diode is ON as long as forward bias voltage
is positive
Now, minimum v
in
for
v
D
to be positive = 2V
EE2301: Basic Electronic Circuit
EE2301: Block C Unit 1
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Offset diode model
EE2301: Basic Electronic Circuit
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Offset model example
Problem 9.19
The diode in this circuit requires a minimum current of 1 mA to be above the
knee of its characteristic. Use V
γ
= 0.7V
What should be the value of R to establish 5 mA in the circuit?
With the above value of R, what is the minimum value of E required to maintain
a current above the knee
EE2301: Basic Electronic Circuit
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Offset model example solution
This slide is meant to be blank
I
D
= (E
-
V
D
)/R
When the diode is conducting, V
D
= V
γ
I
D
= (5
-
0.7)/R
We can observe that as R increases, I
D
will decrease
To maintain a minimum current of 5mA,
R
max
= 4.3/5 = 860
Ω
Minimum E required to keep current above the knee (1mA),
E
min
= (10
-
3
* 860) + 0.7 = 1.56V
EE2301: Basic Electronic Circuit
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Rectification: from AC to DC
Supply is AC
DC required
One common application of diodes is rectification. In rectification, an AC
sinusoidal source is converted to a unidirectional output which is further
filtered and regulated to give a steady DC output.
EE2301: Basic Electronic Circuit
EE2301: Block C Unit 1
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Rectifier with regulator diagram
Rectifier
Bi
-
directional
input
Steady DC output
Filter
Regulator
Unidirectional output
We will look at two types of rectifiers and apply the large signal
models in our analysis:
1) Half wave rectifier
2) Full wave rectifier
EE2301: Basic Electronic Circuit
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Half
-
Wave Rectifier
V
S
~
R
L
On the
positive
cycle
Diode is forward biased
Diode conducts
V
L
will follow V
S
V
S
~
R
L
On the
negative
cycle
Diode is reverse biased
Diode does not conduct
V
L
will remain at zero
V
S
V
L
We can see that the
circuit conducts for
only half a cycle
EE2301: Basic Electronic Circuit
EE2301: Block C Unit 1
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Average voltage in a HW Rectifier
1
st
half of
period
2
nd
half
of period
NB: This is equal to the DC term of the Fourier series
EE2301: Basic Electronic Circuit
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Full
-
Wave Rectifier
Also known as BRIDGE rectifier
Comprises 2 sets of diode pairs
Each pair conducts in turn on each half
-
cycle
V
S
~
EE2301: Basic Electronic Circuit
EE2301: Block C Unit 1
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Full
-
Wave Rectifier
EE2301: Basic Electronic Circuit
EE2301: Block C Unit 1
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Full
-
Wave Rectifier
Half a period
T/2
–
time
π
–
phase
Repeats for every half a period:
Integrate through half a period
NB: This is equal to the DC term of the Fourier series
EE2301: Basic Electronic Circuit
EE2301: Block C Unit 1
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Full
-
Wave Rectifier (offset)
V
D
-
on
(only one diode is on)
2V
D
-
on
(two diodes are on)
With offset diodes
With ideal diodes
EE2301: Basic Electronic Circuit
EE2301: Block C Unit 1
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Ripple filter
Charging
Discharging
Anti
-
ripple filter is used to smoothen out the rectifier output
EE2301: Basic Electronic Circuit
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Ripple filter
Approximation: abrupt change in the voltage
From transient analysis: V
M
exp(
-
t/RC)
V
L
Ripple voltage V
r
= V
M
-
V
L min
EE2301: Basic Electronic Circuit
EE2301: Block C Unit 1
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Ripple filter example
Problem 9.40
Find the turns ratio of the transformer and the value of C given that:
I
L
= 60mA, V
L
= 5V, V
r
= 5%, V
line
= 170cos(
ω
t) V,
ω
= 377rad/s
Diodes are fabricated from silicon, V
γ
= 0.7V
EE2301: Basic Electronic Circuit
EE2301: Block C Unit 1
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Ripple filter example solution
a) TURNS RATIO: To find the turns ratio, we need to find V
S1
and V
S2
EE2301: Basic Electronic Circuit
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Ripple filter example solution
But V
M
is not equal to V
S1
due to voltage drop across diodes
So we now apply KVL on the secondary coil side:
V
S1
-
V
D
-
V
M
= 0
V
S1
= 5.825 V (V
D
= 0.7V)
Turns ratio, n = V
line
/ V
S1
~ 29
EE2301: Basic Electronic Circuit
EE2301: Block C Unit 1
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Ripple filter example solution
b) Value of C: Need to find the RC time constant associated with
the ripple
R
L
= V
L
/I
L
= 83.3
Ω
We know it decays by V
M
exp(
-
t/RC), we now just need to know how long this lasts (t
2
)
V
L
-
min
=
-
V
SO
cos(
ω
t
2
)
-
V
D
-
on
v
so
is negative at this point
EE2301: Basic Electronic Circuit
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Ripple filter example solution
V
L
-
min
=
-
V
SO
cos(
ω
t
2
)
-
V
D
-
on
2
nd
half of the sinusoid
t
2
= (1/
ω
) cos
-
1
{
-
(V
L
-
min
+ V
D
-
on
)/V
SO
} = 7.533 ms
Decaying exponential:
V
L
-
min
= V
M
exp(
-
t
2
/R
L
C)
EE2301: Basic Electronic Circuit
Examples of Diode
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EE2301: Basic Electronic Circuit
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EE2301: Basic Electronic Circuit
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Electrical Materials
Insulators
Electrons are bound to the nucleus and are therefore
not free to move
With no free electrons, conduction cannot occur
Conductors
Sea of free electrons not bound to the atoms
Ample availability of free electrons allows for
electrical conduction
Semiconductors
Electrons are bound to the nucleus but vacancies are
created due to thermal excitation
Electrical conduction occurs through positive (called
holes) and negative (electrons) charge carriers
EE2301: Basic Electronic Circuit
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Conduction in Semiconductors
Silicon is the dominant semiconductor material used in the
electronics industry. In a cubic meter of silicon, there are
roughly 10
28
atoms. Among these 10
28
, there will be about
1.5
×
10
16
vacancies at room temperature. This is known as
the intrinsic carrier concentration: n = 1.5
×
10
16
electrons/m
3
.
This corresponds to 1 free electron for every 10
12
atoms.
There will be same number of electrons as holes in intrinsic
silicon since it is overall electrically neutral.
EE2301: Basic Electronic Circuit
Extrinsic semiconductors
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A semiconductor material that has been subjected to the doping process is
called an extrinsic material. Both n
-
type and p
-
type materials are formed by
adding a predetermined number of impurity atoms to a silicon base.
An n
-
type material is created by introducing impurity elements that have five
valence electrons. In an n
-
type material, the electron is called the majority carrier.
An p
-
type material is created by introducing impurity elements that have three
valence electrons. In a p
-
type material, the hole is the majority carrier.
EE2301: Basic Electronic Circuit
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p
-
n Junction
The pn junction forms the basis of the semiconductor diode
Within the depletion region, no free carriers exist since the holes and electrons at
the interface between the p
-
type and n
-
type recombine.
EE2301: Basic Electronic Circuit
EE2301: Block C Unit 1
Response of the depletion region
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Forward biased:
Voltage on the p
-
type side is higher than the
n
-
type side
Depletion width reduces, lowering barrier for
majority carriers to move across the depletion
region
Large conduction current
57
Reverse biased:
Voltage on the p
-
type side is lower than the n
-
type side
Depletion width increases, increasing the
barrier for majority carriers to move across
the depletion region
Very small leakage current
EE2301: Basic Electronic Circuit
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Analogy from tides
Depletion region
Depletion region
Forward Biased
Reverse Biased
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