EE 214 Electronic Circuits Laboratory
Spring 2015
DIODES AND RECTIFIERS
Diodes are devices that allow current to flow in one direction only, from anode to cathode. The anode
is the positive side of the component and the negative side, the cathode, is usually marked with a stripe to
indicate polarity. A usual semiconductor diode package is illustrated in Figure 1.a.
The circuit schematic symbol of a diode forms an arrow that points in the direction of current flow
when the anode is biased positively with respect to the cathode as shown in Figure 1.b.
+
+
(anode)
-
(cathode)
(a)
(b)
Figure 1: (a) A typical wire-lead diode package with identifying mark and (b)
Current-Voltage Characteristics
The 𝐼 vs. 𝑉 characteristics of a typical semiconductor diode with its simple approximation is given in
Figure 2. The current through a diode increases rapidly above the forward threshold voltage, 𝑉𝑜𝑛 . When
reverse biased, a very small reverse saturation current passes through the diode, which is usually neglected
as shown in approximated model. When the reverse bias voltage exceeds a breakdown value, 𝑉𝑏𝑟 , reverse
current also increases rapidly. The breakdown voltage is usually much greater than the forward turn-on
voltage 𝑉𝑜𝑛 . For example, the breakdown voltage for 1N4001 diode is around 50 V and the breakdown voltage
for 1N4007 diode is around 1000 V.
ID
+
ID
ID
VD
-
VD
Vbr
Vbr
VD
Von
(a)
Von
(b)
Figure 2: (a) 𝐼-𝑉 characteristics of a diode and (b) its simple approximation for DC circuit analyses
The 𝐼 vs 𝑉 relation of most semiconductor diodes are modeled reasonably well by (1) where 𝑛 is the
ideality factor, 𝐼𝑠 is the reverse saturation current and the thermal voltage 𝑉𝑇 =
𝑘𝑇
𝑞
is approximately 25 𝑚𝑉 at
room temperature (~300 K).
𝑞𝑉
𝐼 = 𝐼𝑠 [𝑒 𝑛𝑘𝑇 − 1]
(1)
Diode Types
The application in which you want to use diodes may require very different characteristics. For
example, circuits where the signal levels are very small require diodes with a small turn-on voltage; on the
other hand, large forward currents are preferred for power-supply applications. Diodes are produced for a
high variety of applications and some commonly-used are listed below. However, readers are suggested to
make a deeper search on diode types and their properties.
Photodiodes are intended to sense light, in other words, they are used as photodetectors.
Photodiodes are packed in a way that allow light to pass and the current is generated when photons are
absorbed. They may be used in photometry, in solar cells or consumer electronics devices, such as compact
disc players and smoke detectors.
Schottky diodes are semiconductor diodes that have metal to semiconductor contact, which usually
results in lower forward voltage drops and quick responses to changes in voltage. Low forward voltage drops
make Schottky diodes useful for low-loss rectifiers. The high-switching property comes from lower junction
capacitance and provides their use in high-speed circuitry.
Zener diodes are designed to operate in breakdown which occurs at a precisely
defined voltage. Although a zener diode allows current flow in the forward direction as
a pn-junction does, they are mostly used as reverse-biased in order to provide a
reference voltage for voltage regulators. Zener diodes may also be used as voltage
shifters or waveform clippers. The circuit schematic symbol of a zener diode has
representative breakdown curves in addition to the mark that indicates polarity, which
is illustrated in Figure 3.
+
IZ
VZ
-
Figure 3: Schematic
symbol for a zener diode
Light-Emitting Diodes (LEDs) are designed to emit photons when enough current passes through
the diode. The wavelength of the light being emitted depends on the material and wavelengths from infrared
to near ultraviolet may be produced using LEDs. Forward voltage drop of a LED is usually greater than the
other types of pn-junction diodes and is around 2 V. The packaging of LEDs differ from other diodes and has a
variety of sizes. As a consequence, there is no indicator of the cathode as a stripe, rather, the cathode leg of
LED is the shorter one of the two. Another way of determining the cathode is to identify the leg connected to
the bigger metal where the diode chip is present. The detailed structure of a LED is presented in Figure 4.
Figure 4: Structure of a LED
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Diodes in Series and in Parallel
Series combination of diodes are sometimes used to obtain a relatively constant voltage drop. A zener
diode when reverse biased or its series combinations would be more economical for high voltage drops. In
any way, one should note that the constant voltage drop is obtained if there is enough current flow through
diodes.
Consider the circuit given in Figure 4.a. When the load resistance is small, the bias current is shunted
through the load and the current through the diodes are not sufficient to realize a constant voltage drop;
therefore the circuit behaves like a resistive voltage divider. On the contrary, diodes clamp the output at three
times the voltage drop of an individual diode when the load resistance is high.
R
R
RL
Vin
R
Vin
(a)
R
Vin
(b)
(c)
Figure 4: (a) Series clamp with resistive load. Parallel connection of diodes (b) in a wrong way and (c) in a correct way
It may be thought that diodes in parallel as illustrated in Figure 4.b can be used for splitting a large
current among the each individual diode equally; however, it would most probably end up one diode carrying
most of the current. Diode current depends exponentially on the ideality factor and the temperature;
therefore, small differences in diode parameters or environmental conditions results in a large difference in
currents through individual diodes. A possible exception would be within an integrated circuit where diodes
can be regarded as well-matched. Hence, as a general rule, diodes or LEDs in parallel with the same polarity
should have their own bias resistor as shown in Figure 4.c. An exception may be the diodes in parallel with
opposite cathode directions since the current would pass through a different branch for different polarities.
Rectifier Circuits
Rectification is the process of converting alternating current (AC) to direct current (DC) and requires
components to allow one-way current flow, which may be achieved by using diodes.
The simplest way of rectification is to use a half-wave rectifier circuit, which is illustrated in Figure 5.a.
Half-wave rectifier includes a single diode that delivers the one half of an AC waveform to the load as shown
in Figure 5.b.
(a)
(b)
Figure 5: (a) Half-wave rectifier circuit and (b) the corresponding input and output waveforms.
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However, half wave rectification results in the use of half of the power source capacity since power is
supplied to the load one half every full cycle, which is very inefficient. In order to use both half cycles while
rectifying AC power, full-wave rectifier circuits are used. The full-wave bridge rectifier circuit is shown in Figure
6.a and it uses four rectifying diodes connected in a way to configure a closed-loop bridge. In the full-wave
rectifier circuit, the current flows in the same direction through the load regardless of the polarity of the input.
Diodes D1 and D2 conduct in series during the positive half cycle whereas diodes D3 and D4 conduct during the
negative half-cycle of source and convert the negative half cycle to positive at the load.
D1
D4
D3
D2
(b)
(b)
Figure 6: (a) Full-wave rectifier circuit and (b) the corresponding input and output waveforms.
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