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1 04
T Y P E
6 0 3
E L E C T R O N I C
M U L T I P L I E R
4
sential to repair the machine and may be skipped
if desired. However, for the benefit of those who
wish a thorough explanation, the complete theory
of opera~ion is discussed.
The basic form of a multivibrator is shown in
Figure 99. By comparing this circuit with a trig-
ger circuit, it will be observed that the multivibra-
tor circuit is derived from the Eccles-Jordan .trig-
ger circuit by removing the grid-to-anode coupling
resistors. In this manner the circuit reverses itself
as fast as the coupling capacitors charge and dis-
charge, instead of depending upon externally ap-
plied ~ulses.
In order to understand the operation of the
multivibrator, assume that on applying power both
tubes start conducting and .that the coupling
capacitors Cc charge. Note that the voltage across
TI
is applied to the Rgz-Ccn combination and tha.t
the voltage across
T 2
is applied to the Rgl-Ccl
combination. Also remember that it takes time for
the voltage across a capacitor to change.
Conse-
quently, once the Cc capacitors are charged, a volt-
age change at the anode of either tube will be
transmitted through the Cc capacitor and be felt
instantaneously at the opposite grid.
With both
TI
and
T:!
conducting, assume there is a momen-
tary increase in anode current through
T1
caused
by a sudden increase in emission. The starting of
oscillation actually depends on the fact that no
two tubes are exactly alike in characteristics. Any
increase in current flow .through RL1 increases the
IR drop across RL1 and thereby reduces the poten-
@ )
Cce Discharging-
TI
Conducting
tial at the anode of
TI.
This reduction in poten-
tial is transmitted through Ccz to the grid of
T 2 .
The reduction in potential at the grid of
T z
lowers
the anode current through T-, which results in a
lower IR drop across RLI and an increased poten-
tial at the anode of
T?.
This increased
is
transmitted via Ccl to the grid of TI, and it results
in a further increase in anode current through
TI.
This continues until
T 2
is completely cut off. The
process is cumulative and with proper design the
action is almost instantaneous.
When
T 2
is cut off, it remains so as long as the
charge on Ccl keeps the grid of
T 2
below cu.toff
value. Once
T 2
is cut off, Cc2 starts discharging
as indicated in Figure IOOA, and as soon as sufi-
cient charge has leaked off through Rgc, the grid
potential on
Ts
rises above cutoff, and anode cur-
rent again starts to flow through
T 2 .
This de-
creases the potential at the anode of TY; and since
the voltage across Ccl cannot change instanta-
neously, this decrease in po.tential at the anode of
T2
is transmitted through Ccl to the grid of TI,
resulting in a decreased anode current through
TI.
The decrease in anode current through
TI
in-
creases its anode potential; .this increase is trans-
mitted through Cc2 to the grid of T2, which in
turn further increases the anode current through
Tz
and lowers the poten~ial at the anode of
T 2
which in turn makes the grid potential of
TI
more
negative.
This cumulative process continues un-
ril
T 1
is cut off;
TI
remains cut off until Cci dis-
charges (Figure 100B), and the cycle repeats i.tself.
@ ccI
Discharging-
TZ Conducting
Figure
100.
Current Flow in Muftivibrator
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