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F ig . l ·l B. The opftallona l c haracteristics of • closed s witch .
The model oprerate as follows :
When the diode is forward
biased , a current will flow , Is in
figure 3-2. The quantity of Is is
determined by the resistance
value of As and Ohm's Law. The
voltage dropped across the diode
is te rmed VsE or voltage base to
emitter. The characteristics of
VsE are identical to those of VF in
lesson two. When a current flows
in the base circuit , a correspond -
ing current wi ll flow i n the
coll ector circuit . The collector
current will be beta times larger
than the base current [lc = beta x
Is] . The voltage drops across Rc
is the Ohm 's Law value of Rc and
I c.
The Ideal Switch
As was the case with the diode
in lesson two , we will first discuss
an ideal switch before explaining
a transistor switch . The purpose
of a switch is to transfer a voltage
from a source to a load . The
circuit in figure 3-3A and the
diagram of its operating point
indicates that the open circuit
characteristics of S are such that
the entire battery voltage (6V) is
present across the open contacts
of the switch and that there is zero
volt across RL, and there is no
current flow .
·
Figure 3-38 shows the trans i-
tions which occur to the
operating points when the switch
is c losed . The voltage across the
switch contacts falls to zero while
the voltage drop across RL
increases to the battery voltage .
Also , the current through the
circuit has risen to the Ohm 's Law
value of the battery voltage and
RL. Thus far, the characteristics
of an open and closed switch
have been described .
The dynamic characteristics
of the switch as it closes are
shown in figure 3-3C . As the
voltage across the switch starts
falling to zero , the voltage across
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F ig. 3 ·3C . Tht dynam ic charac ltfittlct of a s witched circul i .
The load li nt re preaen la lht tr•naltr of voltage from lhe source
to the toad .
F ig. l--4 . The opefl tl onal characterlsllcs o t a lest·lhan ·l deal
t wItch . The voltage lransferred to the load Ia leu than that
ava ila ble al the source .
RL begins to rise . The voltage
across RL is t he difference
between the voltage across the
switch contacts and the battery
voltage . If we could stop the
transition at V' across the switch
contacts , we would find the
current had risen to a value less
than maximum . If the voltage and
current scales are projected , they
would intercept at point A on
figure 3-3C . If all the intercepts of
the current and voltage values
were plotted , a line would be
drawn between the battery
voltage and the static current (1) .
Th is line is commonly referred to
as the load line .
A Less -than -ideal switch
The switch described above
was assumed to have an infinite
resistance when open and zero
ohms when closed ; hence , the
zero voltage drop . Electro-
mechanical dev ices , such as
relays and switches , can - for
practical purposes - be called
ideal. However, as we shall soon
see in the case of resistors , this is
not so . The following paragraph
will deal directly with a less-than-
ideal switch in general before
discussing a transistor switch .
The switch whose characteris-
tics are shown in figure 3-4 is less
than ideal in the sense that when
open , its resistance is such that
enough current will flow to cause
a voltage drop across RL
(remember: VAL = VsATT
- VSWITCH) and when closed, the
voltage across the contacts is
greater than zero .
The condition of not enough
resistance when open and too
much resistance when closed
lim its the change in voltage [delta
V] and current [delta I) to less
than the ideal. Note in figure 3-4
that the voltage across the switch
never reaches 6V and the current
through the contacts does not in-
crease to the maximum permitted
by RL.
The Transistor Switch
We have made no mention
above as to how the switches
were activated and de-activated .
Switches are familiar devices , and
they are usually operated by an
external force applied to a shaft or
lever (rotary or toggle switches) .
The transistor switch is activated
by the base current . Because
base current will flow only when
the base-emitter diode is forward
biased , it can then be stated that
the transistor switch is controlled
by the base -emitter diode .
The circuit in figure 3-5 is a
basic switching circuit and
operates as did the model of the
fi rst paragraph in this lesson . The
switch (S) from base to ground is
a representation of a switching
device, which in reality may be
another transistor. When S is
open, the base-emitter diode is
forward biased and base current
flows. This operating condition is
represented at point A on the
curves of figure 3-6A and 3-68 .
The vertical section f the base
current in figure 3-6A is termed
the saturation region .
Figure 3-68 is an expanded
curve of the saturation region of
figure 3-6A. It should be noted
that while in the saturation region
the transistor has a small voltage
drop collector to emitter, there-
fore exhibiting a characteristic of
the less-than-ideal switch . This
voltage drop is termed VCEcsat 1
on the manufacturers ' data
sheets.
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When S is closed , the base is
shorted to the ground , as is the
emitter , removing the emitter
base diode from the forward
biased state. The transistor is ,
with S closed , operating at point
11