Flip-Flops and Related Devices
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potential manifests itself at the Q 2 base as voltage across a capacitor cannot change instantaneously.
The output goes to the HIGH state as Q 2 is driven to cut-off. However, C 1 has now started charging
through R 1 and the conducting transistor Q 1 from V CC . The moment the Q 2 base potential exceeds
the cut-in voltage, it is again turned ON, with the result that the output goes to the LOW state. This
process continues and, owing to both the couplings (Q 1 collector to Q 2 base and Q 2 collector to Q 1
base) being capacitive, neither of the states is stable. The circuit produces a square-wave output.
10.2 Integrated Circuit (IC) Multivibrators
In this section, we will discuss monostable and astable multivibrator circuits that can be configured
around some of the popular digital and linear integrated circuits. The bistable multivibrator, which is
functionally the same as a flip-flop, will not be discussed here. Flip-flops are discussed at length from
Section 10.3 onwards.
10.2.1 Digital IC-Based Monostable Multivibrator
Some of the commonly used digital ICs that can be used as monostable multivibrators include 74121
(single monostable multivibrator), 74221 (dual monostable multivibrator), 74122 (single retriggerable
monostable multivibrator) and 74123 (dual retriggerable monostable multivibrator), all belonging to
the TTL family, and 4098B (dual retriggerable monostable multivibrator) belonging to the CMOS
family. Figure 10.7 shows the use of IC 74121 as a monostable multivibrator along with a trigger
input. The IC provides features for triggering on either LOW-to-HIGH or HIGH-to-LOW edges of
the trigger pulses. Figure 10.7(a) shows one of the possible application circuits for HIGH-to-LOW
edge triggering, and Fig. 10.7(b) shows one of the possible application circuits for LOW-to-HIGH
edge triggering. The output pulse width depends on external R and C. The output pulse width can
be computed from T = 07 RC. Recommended ranges of values for R and C are 4–40 K and 10 pf
to 1000 F respectively. The IC provides complementary outputs. That is, we have a stable LOW or
HIGH state and the corresponding quasi-stable HIGH or LOW state available on Q and Q outputs.
Figure 10.8 shows the use of 74123, a retriggerable monostable multivibrator. Like 74121, this IC,
too, provides features for triggering on either LOW-to-HIGH or HIGH-to-LOW edges of the trigger
pulses. The output pulse width depends on external R and C. It can be computed from T = 0.28RC×
[1 + (0.7/R)], where R and C are respectively in kiloohms and picofarads and T is in nanoseconds.
This formula is valid for C > 1000 pF. The recommended range of values for R is 5–50 K . Figures
10.8(a) and (b) give application circuits for HIGH-to-LOW and LOW-to-HIGH triggering respectively.
It may be mentioned here that there can be other triggering circuit options for both LOW-to-HIGH
and HIGH-to-LOW edge triggering of monoshot.
10.2.2 IC Timer-Based Multivibrators
IC timer 555 is one of the most commonly used general-purpose linear integrated circuits. The
simplicity with which monostable and astable multivibrator circuits can be configured around this IC
is one of the main reasons for its wide use. Figure 10.9 shows the internal schematic of timer IC 555.
It comprises two opamp comparators, a flip-flop, a discharge transistor, three identical resistors and
an output stage. The resistors set the reference voltage levels at the noninverting input of the lower
comparator and the inverting input of the upper comparator at (+V CC /3) and (+2V CC /3). The outputs
of the two comparators feed the SET and RESET inputs of the flip-flop and thus decide the logic status
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potential manifests itself at the Q 2 base as voltage across a capacitor cannot change instantaneously.
The output goes to the HIGH state as Q 2 is driven to cut-off. However, C 1 has now started charging
through R 1 and the conducting transistor Q 1 from V CC . The moment the Q 2 base potential exceeds
the cut-in voltage, it is again turned ON, with the result that the output goes to the LOW state. This
process continues and, owing to both the couplings (Q 1 collector to Q 2 base and Q 2 collector to Q 1
base) being capacitive, neither of the states is stable. The circuit produces a square-wave output.
10.2 Integrated Circuit (IC) Multivibrators
In this section, we will discuss monostable and astable multivibrator circuits that can be configured
around some of the popular digital and linear integrated circuits. The bistable multivibrator, which is
functionally the same as a flip-flop, will not be discussed here. Flip-flops are discussed at length from
Section 10.3 onwards.
10.2.1 Digital IC-Based Monostable Multivibrator
Some of the commonly used digital ICs that can be used as monostable multivibrators include 74121
(single monostable multivibrator), 74221 (dual monostable multivibrator), 74122 (single retriggerable
monostable multivibrator) and 74123 (dual retriggerable monostable multivibrator), all belonging to
the TTL family, and 4098B (dual retriggerable monostable multivibrator) belonging to the CMOS
family. Figure 10.7 shows the use of IC 74121 as a monostable multivibrator along with a trigger
input. The IC provides features for triggering on either LOW-to-HIGH or HIGH-to-LOW edges of
the trigger pulses. Figure 10.7(a) shows one of the possible application circuits for HIGH-to-LOW
edge triggering, and Fig. 10.7(b) shows one of the possible application circuits for LOW-to-HIGH
edge triggering. The output pulse width depends on external R and C. The output pulse width can
be computed from T = 07 RC. Recommended ranges of values for R and C are 4–40 K and 10 pf
to 1000 F respectively. The IC provides complementary outputs. That is, we have a stable LOW or
HIGH state and the corresponding quasi-stable HIGH or LOW state available on Q and Q outputs.
Figure 10.8 shows the use of 74123, a retriggerable monostable multivibrator. Like 74121, this IC,
too, provides features for triggering on either LOW-to-HIGH or HIGH-to-LOW edges of the trigger
pulses. The output pulse width depends on external R and C. It can be computed from T = 0.28RC×
[1 + (0.7/R)], where R and C are respectively in kiloohms and picofarads and T is in nanoseconds.
This formula is valid for C > 1000 pF. The recommended range of values for R is 5–50 K . Figures
10.8(a) and (b) give application circuits for HIGH-to-LOW and LOW-to-HIGH triggering respectively.
It may be mentioned here that there can be other triggering circuit options for both LOW-to-HIGH
and HIGH-to-LOW edge triggering of monoshot.
10.2.2 IC Timer-Based Multivibrators
IC timer 555 is one of the most commonly used general-purpose linear integrated circuits. The
simplicity with which monostable and astable multivibrator circuits can be configured around this IC
is one of the main reasons for its wide use. Figure 10.9 shows the internal schematic of timer IC 555.
It comprises two opamp comparators, a flip-flop, a discharge transistor, three identical resistors and
an output stage. The resistors set the reference voltage levels at the noninverting input of the lower
comparator and the inverting input of the upper comparator at (+V CC /3) and (+2V CC /3). The outputs
of the two comparators feed the SET and RESET inputs of the flip-flop and thus decide the logic status