Chapter Three: DIODES
3.1
Introduction
A diode is an electronic component that allows current to flow in only one direction.
Cathode
Anode
Example
Circuit Symbol
Diodes are essentially pn junctions: made by joining two pieces of semiconductor, one doped n type and
the other p type. We will examine this in detail later on. For now though, it is important to note that a
diode has two terminals: the positive terminal (anode) and the negative terminal (cathode). The order of
the terminals is important to note because it affects diode operation when we apply a voltage across it,
a process we call biasing. Generally, when the cathode is at a lower potential (negative) with respect to
the anode, we say the diode is Forward Biased (FB) and current flows through it in the direction shown
in the schematic by the direction of the triangle. The diode is then said to be ON and acts as a short
circuit. The reverse is known as Reverse Bias (RB) and negligible current flows. The diode is then OFF
and acts like an open circuit. However, all diodes have a maximum reverse voltage (usually 50V or
more) and if this is exceeded the diode will fail and pass a large current in the reverse direction. This is
called Breakdown. Ordinary diodes can be split into two types: Signal diodes which pass small currents
of 100mA or less and Rectifier diodes which can pass large currents. In addition there are LEDs,
Switching, Varactor, Schottky and Zener diodes.
The most common type of semiconductor diode is the silicon diode. Semiconductor pn junction diodes
can be formed from materials other than silicon, such as germanium, selenium, and a host of
semiconductor materials used in light emitting diodes (LEDs).
3.2
I-V Characteristics
3.2.1
The Ideal Diode
I
Diode OFF
1
Slope = =0, R=∞
𝑅
Diode ON
1
Slope = =∞, R=0
𝑅
V
3.2.2 The Real Diode
Diodes exhibit a voltage drop across them known as the forward voltage drop, Vf, an intrinsic property of
the semiconductor material used to make the pn junction (and related to the band gap). Vf is thus the
same for all silicon diodes, about 0.7V. The diode will conduct only when the forward bias exceeds the
forward voltage drop of the diode. We would then expect the following characteristics:
Cosmas Mwikirize
CMP 1101 Chapter 3: Diodes
I
0.7 V
V
However, diodes are non-linear devices and exhibit a dynamic resistance within them. They do not obey
Ohm’s law!! In fact, the real characteristics of a diode are as shown below with three regions of interest:



Forward bias, v> 0
Reverse bias, v<0
The break down region, v< -Vzk
𝑣
Under FB, the current through the diode is approximated by the equation 𝐼 = 𝐼𝑠 (𝑒 𝑉𝑇 − 1)
IS is known as saturation current, 𝑉𝑇 is the thermal voltage (hope you recall this from chapter two
𝑘𝑇
𝑞
≈ 26𝑚𝑉 ) at room temperature while 𝑣 is the applied voltage.
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CMP 1101 Chapter 3: Diodes
(1)
Under RB, 𝑣 is negative and the exponential term negligibly small compared to 1. The expression for the
current is 𝐼 = −𝐼𝑠 (constant). In real diodes, reverse current increases a bit with increase in magnitude
of the reverse voltage, junction area and temperature.
The breakdown region is entered when the reverse voltage exceeds a certain threshold called the
breakdown voltage, the voltage at the knee of the i-v curve, denoted by Vzk. The reverse current
increases somewhat rapidly with little increase in voltage drop.
For your consideration:




3.3
Signal diodes are used to process information (electrical signals) in circuits, so they are only
required to pass small currents of up to 100mA.
General purpose signal diodes such as the 1N4148 are made from silicon and have a forward
voltage drop of 0.7V.
Germanium diodes such as the OA90 have a lower forward voltage drop of 0.2V and this makes
them suitable to use in radio circuits as detectors which extract the audio signal from the weak
radio signal.
For general use, where the size of the forward voltage drop is less important, silicon diodes are
better because they are less easily damaged by heat when soldering, they have a lower
resistance when conducting, and they have very low leakage currents when a reverse voltage is
applied.
Diode Models
Read Section 3.3 of Microelectronic Circuits, Sedra and Smith
3.4
3.4.1
Diode Applications
Rectification Circuits
Rectification is the process of converting ac signal into dc signal. It is done in almost all electrical
appliances and any other applications that require a constant dc level derived from the mains
ac voltage supply. For example the rectifier together with a transformer in a power supply
enables you to plug items such as mobile phones and VCRs into an ac supply source.
Electronic rectifiers are components that convert an ac input of voltage and current into a
unidirectional or dc output. They utilize diodes, which allow only unidirectional flow of current
(when forward biased). The output of the circuit may not be perfect dc (may contain an ac
ripple).Rectifier circuits fall in two categories namely, half wave and full wave.
Half Wave Rectifier
A half wave rectifier circuit can be constructed from a single diode in series with a resistor.
During the positive half cycle of the ac, the diode is forward biased and current is supplied to
the load. During the alternate negative half cycle, the diode is reverse biased and the load
current is blocked. The dc output ripple has the same frequency as the ac.
Cosmas Mwikirize
CMP 1101 Chapter 3: Diodes
Vs
Vrms
RL
-Vs
Vdc
The average dc output voltage of a half wave rectifier can be calculated with the following two ideal
equations (assuming zero voltage drop across the diode).
𝑉𝑠 = 𝑉𝑟𝑚𝑠 √2 and 𝑉𝑑𝑐 =
𝑉𝑠
𝜋
Dc Current through the load
.
𝐼𝑑𝑐 =
𝑉𝑑𝑐
𝑅
Peak Loss
There is a loss from peak input voltage to the peak output voltage, caused by the built-in voltage drop
across the diodes (around 0.7 V for ordinary silicon p-n-junction diodes). Half-wave rectification has a
peak voltage loss of one diode drop. The diode will not conduct below this voltage, so the circuit is only
passing current through for a portion of each half-cycle, causing short segments of zero voltage to
appear between each “hump".
Hence 𝑉 ≈ 𝑉𝑆 − 0.7 𝑉
Peak Inverse Voltage (PIV)
This is the maximum reverse voltage that is expected to appear across the diode. For a half wave
rectifier, 𝑃𝐼𝑉 = 𝑉𝑆 .
Question
For the half wave rectifier circuit shown below, the input is an ac voltage with an rms value of 10V at
60Hz frequency.
Cosmas Mwikirize
CMP 1101 Chapter 3: Diodes
𝑅𝐿 = 10𝑘Ω
i)
Sketch the input and output waveforms on the same scale
ii)
Derive the expression for the average dc voltage Vdc given above. (Hint: Average
𝜋
value of the output function is
1 2
∫ 𝑓(𝜃)
2𝜋 0
𝑑𝜃-Area under the curve over the full cycle
divided by the period). Take into account the voltage drop across the diode. (𝑽𝒅𝒄 =
iii)
iv)
𝑽𝑺
𝝅
−
𝟎.𝟕
)
𝟐
Calculate Vdc (Ans: 4.15 V)
Find the dc current flowing Idc and hence the output power. (Ans: 0.415 mA, 1.722 mW)
While the output of the half-wave rectifier is DC , it would not be suitable as a power supply for a
circuit. Firstly, the output voltage continually varies between 0V and Vs-0.7V, where Vs is the peak of the
input voltage, and secondly, for half the time there is no output at all.
Full Wave Rectifier
A full-wave rectifier converts the whole of the input waveform to one of constant polarity (positive or
negative) at its output. Full-wave rectification converts both polarities of the input waveform to DC and
is more efficient.
Examples of full wave rectifiers are:
i.
Bridge Rectifier
It uses four diodes connected as shown below.
A
C
ac
B
RL
D
Cosmas Mwikirize
CMP 1101 Chapter 3: Diodes
Vs
Vs
During the positive half cycle diodes A and B are forward biased while diodes C and D are
reverse-biased. Current flows around the circuit formed by these diodes, the load and the
transformer winding. During the next (negative) half cycle, diodes C and D are
are forward biased. Current again flows in the same direction through the load and produces
another pulse of voltage. The dc output ripple has twice the frequency of the ac. The Bridge
rectification has a peak loss of two diode drops.
Why isn’t the peak loss 2.8V since we have 4 diodes? Prove that the PIV= Vs - 0.7 V and that
the average value of the dc is given by
𝑽𝒅𝒄 =
𝟐𝑽𝑺
𝝅
− 𝟎. 𝟕
While the full-wave rectifier is an improvement on the half-wave rectifier, its output still isn't
suitable as a power supply for most circuits since the output voltage is not steady since it varies
between 0 V and Vs -1.4 V.
ii.
Centre Tapped Transformer
Two diodes are required. During the positive half cycle, D1 is forward biased and current flows
through it, the load and to the centre tap. During the negative half cycle, D2 is forward biased
and current flows through it, the load and to the centre tap. In both cycles, current flows in the
same direction hence full wave rectification. However, twice as many windings are required on
the transformer secondary to obtain the same output voltage compared to the bridge rectifier
above. The full-wave rectification with a centre tapped transformer has a peak voltage loss of
one diode drop.
Cosmas Mwikirize
CMP 1101 Chapter 3: Diodes
Question: Prove that PIV = 2Vs - 0.7 V
+
Vs
+ Vs
-
What are the advantages of a bridge rectifier over the full wave rectifier with a centre tapped
transformer?
Rectifier Output Smoothing
The above rectification forms do not produce a constant dc voltage. To produce a steady dc output, a
smoothing circuit is required. The simplest smoothing circuit consists of a reservoir capacitor across the
load. However, some ripple still remains. Proper sizing of the capacitor is a trade-off.
Smoothing for a Half Wave Rectifier
Current pulses flow through the load resistance (RL) each time a diode conducts. For the halfwave rectifier, when a capacitor is connected across the output, the average value of output
voltage is increased due to the filtering action of relatively large (several microfarads) capacitor
C.
When the pulsating voltage is first applied to the circuit, the capacitor charges rapidly and
reaches the peak value of the rectified voltage. Beyond the peak, the diode is turned off and
the capacitor discharges through the load resistance for the next half-cycle, until the output
again exceeds the voltage across it. The diode turns on again and charges the capacitor again
to the peak value.
Cosmas Mwikirize
CMP 1101 Chapter 3: Diodes
Why should the value of the capacitor be large and what is the significance of the RC
product? What are the disadvantages of using a very large capacitor for smoothing?
Smoothing for a Full Wave Rectifier
The best technique of eliminating the ripple is putting a voltage regulator circuit after the capacitor. We
will look at voltage regulators in more detail when we tackle Zener diodes.
Applications of Rectifiers




In power supplies, to derive DC power from an AC supply.
DC to DC conversion. One method of DC-to-DC conversion first converts power to AC (using an
inverter), then use a transformer to change the voltage, and finally rectifies power back to DC.
Read about this!!
Detection of amplitude modulated radio signals at the demodulation stage.
Supplying polarized voltage for welding. Control of the output current is required and this is
sometimes achieved by replacing some of the diodes in bridge rectifier with thyristors, whose
voltage output can be regulated by means of phase fired controllers.(Read about this!!)
Cosmas Mwikirize
CMP 1101 Chapter 3: Diodes