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E L E C T R I C A L
P R I N C I P L E S
t
105
-
.O
+
-
5~
0
p o d
oz
- - -
--
-
-
.-
L
(3
Cutoff
l b t e n t i a l
c. 1
: I
Discharaes
Figure 101. Voltage and Current Curves for Multivibrator
-
0
I
.-
-
5-
0
-
0
8 g
Time-
The time interval between the triggering oper-
fore i t is important to have the value of the RL
-
I
ations, that is, the frequency of oscillation, is deter-
resistors much smaller .than the R g resistors if a
0
+
- - - - -
-
- - - - - - - - -- - -
-
- -
- - - - -
L
Cutoff
w
Potential
mined primarily by the value of R g and CC and to
rectangular wave form is desired.
a small exxent by the value of RL and the supply
voltage.
The time interval that
TI
conducts can
be made different from the time interval that
Tz
conducts by varying the values of Ccl and Ccz. In
this applica~ion, the two time intervals must be the
same; consequently, the two sides of the multi-
vibrator must be symmetrical.
The voltage across RL1 rises and falls exponent-
ially because of the charge and discharge of Ccz.
If RL1 is sufficiently small in comparison with Rgz
so thar the capacitor current does not greatly af-
fect the voltage drop across RL1, the voltage pulse
across RL1 is approximately rectangular.
There-
Figure 101 shows the theoretical grid potential
and anode current curves for a typical multivibra-
tor. The graphs assume that on starting b o ~ h tubes
are conducting; at time a, the first triggering
takes place because of an increase in current
through tube
1
and continues at regular intervals.
Note that the anode current is not constant dur-
ing conduction.
This is due t o the discharging
current of the capaci~ors. From Figure 100 it will
be observed that the direction of capacitor dis-
charge current flow through the RL resistors is the
reverse of the direction of tube currenT flow.
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