Amateur Radio Report Card

'7:1
UA.r:.A.7UJI:
Fig. 4
LOAD
SWITCHING
DIOD E
HIGH
IMPEDANCE
TO AC
the "conductive bands" where it can migrate
as an electric current when only a small voltage
is applied across the semiconductor crystal.
Group V impurities prod uce a region of "freeable"
electrons which is commonly called an
"N" region. On the other hand, Group III impurities
in a tetrahedral lattice produce a region
of "electron deficiency" known as a " P"
region. It is possible for a P-region as well as
an N-region to conduct electric current because
both freeable electrons and electron deficiencies
(called "holes") can migrate through
a semiconductor crystal lattice if the proper
impurity density has been established.
A semiconductor diode is manufactured by
forming a junction between a piece of N-type
and P-type semicond uctor crystal material.
Because diode switch circuits, as we will see
later, operate by virtue of changing bias polarity
and voltage across the switching diode, it
might be helpful to quickly examine the electronic
operation of reverse and forward biased
diodes.
Fig. 2 shows a reversed biased diode. The
"holes" in the P-region are attracted by electrostatic
forces toward the negative terminal
of the applied voltage, a direction away from
the P-N junction. At the same time the "free
electrons" in the N-region migrate away from
junction towards the diode electrode
which is connected to the positive battery
terminal. Remembering that the basic units
for voltage ("volts") are joules per elementary
charge, it follows that as the reverse bias voltage
is raised, the electrostatic forces acting on
the carriers (ie. freeable electrons and holes)
increase. Thus as the bias increases, the concentration
of carriers further decreases in the
t
BIAS
AMPL ITUDE
i
~---'"
. , ,
lNSTANTANEOUS VALUES
OF BIAS
regions of the N and P crystals near the p .;-.I
junction. Under normal conditions current cannot
flow through the reverse biased diode because
a "depletion zone" of carriers exists at
the P-N junction and there is no available
mechanism for a transfer of charge. Under abnormal
conditions (for most purposes ), the
reversed biased diode may be made to conduct
by raising the bias voltage to the point
where "break down" occurs in the crystal with
chemical decomposition and the formation of
movable ions.
In a forward biased diode, the situation is
reversed. See Fig. 3. Due to the bias voltage
polarity difference in the forward biased d i­
ode, the carriers migrate across the P-N junction
instead of away from it. This permits
current to flow through the diode.
So far, we have looked at forward and reverse
biased diode circuits where the bias
voltage or current has remained constant with
time. In practical diode switching circuits
where ac voltages and currents are being
switched, however, the effective values of
"bias" current and voltage vary with time.
Fig. 4 shows a general diagram for a diode
switch circuit where the diode is being used
to switch an ac signa l. The switching state of
the diode depends upon both the value 01
its steady dc bias and the instantaneous values
of ac current and voltage. If the value of
dc bias is zero, we recognize the circuit to be
identical to that of a half-wave rectifier. In
this case, rectified ac in the form of pulsating
dc appears across the load. We are interested ,
however, in switching the ac signal without
clipping or rectifying it. With proper adjustment
of the value of dc bias and polarity,
it is possible to make the diode function as a
switch without rectifying. The graphs in Fig.
5 show how this is accomplished. In graph A,
the value of fixed de bias is periodicall y exceeded
by peaks in the waveform of the signal
which is being switched. As a result clipping
takes place whenever the instantaneous bias
amplitude changes polarity. The ordinate axis,
t
BIAS
AM PL IT UOE
,
VALUES
OF BIAS
f-INSTANTANEOUS
.
~ - - -
, , '
, ,
-,
....... AC SIGNAL
F;g. 5A F;g. 58