Avionics News April 2020 - 60
THEORY & PRACTICE
Continued from page 59
multivibrators constructed from operational amplifiers or
timer chips can be incredibly easy to construct. I encourage
you to buy a breadboard, some parts, and try some of the
circuits found after an online image search.
There is one last type of multivibrator to discuss, and that
is the bistable multivibrator. This circuit is called bistable
because it has two stable states. Like the astable circuit,
the bistable circuit has two outputs, which will always be
in opposite conditions. Unlike the astable multivibrator,
the bistable version will remain in a particular state until it
is triggered to change. Put a million, or a billion, of these
circuits on a chip, and you have random access memory.
When a square wave of a constant frequency is used as
a trigger, the output frequency will be one-half the input
frequency. Thus, this circuit can be used as a divider, or a
flip-flop circuit. Later, when we discuss flip-flop circuits,
make sure you recall the story this month.
N
EW
Refer to Figure 2. This multivibrator does not rely on
RC time constants for switching, and that is why it can
remain stable in one state. This time, we've abandoned
the oscilloscope display and used logic zeros and ones,
instead. If this were part of a transistor-transistor logic
circuit, then the logic 0 would be zero volts, and the logic
1 would be 5 volts. Because the circuit is stable, a trigger
is required to make the circuit change its state. Like the
other multivibrators, Q1 will always be in the opposite
state of Q2, with one on and the other off. R1 and R4 serve
as load resistors to take the voltage drop when one of the
transistors is turned on. R3, R4 and R5 set the bias voltages
for Q1, while R1, R2 and R6 set the bias voltages for Q2.
The designer used C1 and C2 in parallel with R2 and R3
to speed up the switching action, which makes the output
waveform more square. D1 and D2 are called steering
diodes and will route the triggering pulse to the correct
transistor at the correct time. R7 and R8 bias the diodes.
Let's start our discussion of the bistable multivibrator
by assuming that Q1 is on, as shown in Figure 2(a). When
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AVIONICS NEWS
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APRIL
2020
3/2/2020 2:34:18 PM
Avionics News April 2020
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