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7 Basic diode circuits The discrete solid-state diode is the most fundamental element used
in modern electronics. It is available in a variety of forms, includ-
ing those of signal detector, rectier, zener voltage reference,
noise-generator, varicap variable capacitor, light-sensitive diode,
and light-emitting diode (LED). This chapter looks at the basic
characteristics of these devices, and shows various ways of using
standard diodes and rectiers.
Basic diode characteristics
Most modern diodes are of the junction type, and use the basic
structure (and symbol) shown in Figure 7.1. They are made from a
single p-n junction; the pterminal is the anode, and the ntermi-
nal the cathode.
Figure 7.2 illustrates the basic characteristics of the diode. When
forward biased (with the anode positive to the cathode) it has a low
resistance and readily passes current, but when reverse biased it has
a high resistance and blocks current: this action is implied by the
diode symbol, which resembles an arrow pointing in the direction
of easy current conduction.
Most junction diodes are made from either germanium or silicon
materials. Figure 7.3 compares the basic characteristics of the two
types of device when operated at a normal room temperature of
20
0
C; note the following important points.
7 Basic diode circuits
Figure 7.1.
Symbol (a) and structure (b) of solid-state diode.
Figure 7.2.
Diode conduction when (a) forward and (b) reverse
biased. (1). A forward biased diode passes little current (I
f
) until the applied
voltage (V
f
) exceeds a certain kneevalue (typically 150 to 200mV
in germanium diodes, 550 to 600mV in silicon types). When V
f
exceeds the knee value, small increases in V
f
cause large increases
in I
f
, and the diodes forward dynamic impedance (Z
f
) is inversely
proportional to applied voltage.
(2) The Z
f
of a silicon diode is typically 25/I, where I is in mA; i.e.,
Z
f
= 25R at 1mA, or 0R25 at 100mA. The Z
f
of a germanium diode
is greater than that of a silicon type, and its V
f
usually exceeds that
of a silicon type at I
f
values above a few tens of mA.
(3). When a diode is reverse biased by more than 1V or so it passes a
r everse leakage current (I
r
) that is proportional to the reverse vo l t a g e
( V
r
) value. At normal room temperatures I
r
values are measured in
microamps in germanium diodes, and in nanoamps in silicon ones. I
r
t y p i c a l ly doubles with each 8
0
C increase in junction temperature.
Because of their low knee voltage values, germanium diodes are
used almost exclusively in low-level signal detectionapplications.
Silicon types can be used in many general-purpose applications.
Diodes that have high voltage and current ratings are, by conven-
tion, usually called rectiers.
Special diode characteristics
Ordinary silicon diodes have several special characteristics addi-
tional to those already described; the most important of these are
shown in Figures 7.4 to 7.7.
If a silicon diode is increasingly reverse biased a point is eve n t u a l ly
reached where its reverse current suddenly starts to increase, and any
f u rther increase in V
r
causes a sharp rise in I
r
, as shown in Fi g u re 7.4.
The voltage at which this action occurs is known as the avalanche or
z e n e r value of the device. Zener diodes are specially made to
exploit this effect, and are widely used as reference vo l t a g e g e n e r a-
tors. Note, howeve r, that when zener diodes are operated at low cur-
rents their impedances uctuate in a rapid and random manner, and
t h ey can thus be used as excellent wh i t e-n o i s e g e n e r a t o r s .
149
Figure 7.3.
Basic characteristics of germanium (Ge) and silicon
(Si) junction diodes (at 20
O
C). If a silicon diode is forward biased at a constant current, its V
f
value
varies with junction temperature at a typical rate of 2mV/
0
C, as
shown in Figure 7.5. Thus, if V
f
= 600mV at +20
0
C, it falls to
440mV at 100
0
C or rises to 740mV at 50
0
C. Silicon diodes can
thus be used as temperature-to-voltage converters.
If a silicon diode is reverse biased from a high-impedance source
(as shown in Figure 7.6), its junction capacitance decreases (from
perhaps 17pF at 1 volt to maybe 10pF at 8 volts) as the reverse
bias is increased. Varicap(or Varactor) diodes are specially made
to exploit this voltage-variable-capacitoreffect; they use the cir-
cuit symbol shown in the diagram.
When p -n junctions are reverse biased their leakage currents and
impedances are inherently opto-sensitive; they act as very high
impedances under dark conditions and as low impedances under
bright ones. Normal diodes are shrouded in opaque material to stop
this unwanted effect, but photo-diodes are specially made to
exploit it; they use the Figure 7.7(a) symbol. Some photo-diodes
are designed to respond to visible light, and some to infra-red (IR)
light.
150
Figure 7.4.
Zener diode symbol and characteristics .
Figure 7.5.
Thermal characteristics of a silicon diode at I
f
= Another useful junction diodedevice is the LED, or light emitting
diode, which is made from an exotic material such as gallium phos-
phide or gallium arsenide, etc., and may be designed to emit either
red, green, yellow, or infra-red light when forward biased. They use
the Figure 7.7(b) symbol. Note that LEDs and photo-diodes are
optoelectronic devices, and are described in greater detail in
Chapter 13 of this Volume.
Finally, one other important diode is the Schottky type, which uses
the standard diode symbol but has a very fast switching action and
develops only half as much forward voltage as a conventional sili-
con diode. They can be used to replace germanium diodes in many
signal detectorapplications, and can operate at frequencies up to
tens or hundreds of Gigahertz.
Half-wave rectier circuits
The simplest applications of a diode is as a half-wave rectier, and
Figure 7.8 shows a transformer-driven circuit of this type (with the
diodes V
in
value specied in volts r.m.s.), together with relevant
output waveforms. If this circuit has a purely capacitive load it acts
as a peak voltage detector, and the output (V
pk
) equals 1.41
¥
V
in
;
if it has a purely resistive load it acts as a simple rectier and gives
an r.m.s output of 0.5
¥
V
in
; if it has a resistively-loaded capacitive
load (as in most power supply units) the output is rippledand has
an r.m.s. value somewhere between these two extremes. In capaci-
tively-loaded circuits D1 needs a peak reverse-voltage rating of at
least 2.82
¥
V
in
; if purely resistive loading is used, the rating can be
reduced to 1.41
¥
V
in
.
151
Figure 7.6.
Varactor (varicap) diode symbol and typical charac-
teristics.
Figure 7.7.
Photodiode (a) and LED (b) symbols. If a half-wave rectier circuit is used to power purely resistive loads,
they consume only a quarter of maximum power, since power is
proportional to the square of applied r.m.s. voltage. Some loads are
not purely resistive, and Figures 7.9 to 7.11 show how the basic
half-wave rectier circuit can be adapted to give 2-level power con-
trol of lamps, electric drills, and soldering irons that are operated
from the AC power lines. Note in each of these circuits that the
r.m.s. voltage fed to the load equals V
in
when S1 is in position-3, or
0.5
¥
V
in
when S1 is in position 2.
The Figure 7.9 circuit uses a lamp load, which has a resistance
roughly proportional to its lament temperature; when it is operat-
ed at half of maximum voltage its resistance is only half of maxi-
mum, so the lamp operates at about half of maximum power and
thus burns at half-brilliance with S1 in the DIM position.
152
Figure 7.8.
Circuit and wave fo rms of tra n s fo rm e r-d ri ve n
half-wave rectier.
Figure 7.9.
Lamp burns at half brightness in DIM position.
Figure 7.10.
Drill motor runs at 70% of maximum speed in PART
position. The Figure 7.10 circuit uses the uni versal motor of an electric drill
(etc.) as its load. Such motors have an inherent self-regulating speed
control capacity, and because of this the motor operates (when light-
ly loaded) at about 70 percent of maximum speed when S1 is in the
PART position.
The Figure 7.11 circuit uses a soldering iron element as its load, and
these have a resistance that increases moderately with temperature;
thus, when the iron is operated at half voltage its resistance is slight-
ly reduced, the net effect being that the iron operates at about one
third of maximum power when S1 is in the SIMMER position, thus
keeping the iron heated but not to such a degree that its bit deterio-
rates.
Full-wave rectier circuits
Figure 7.12 shows four diodes connected in bridgeform and used
to give full-wave rectication from a single-ended input signal; the
output frequency is double that of the input. The best known appli-
cation of full-wave rectifying techniques is in DC power supply cir-
cuits, which provide DC power out