Bistable Multivibrators |
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Bistable Multivibrators
Bistable Multivibrators are another type of two state device similar to the
Monostable Multivibrator we looked at in the last tutorial
but the difference this time is that both states are stable. Bistable Multivibrators have TWO stable states
(hence the name: "Bi"), and they can be switched over from one stable state to the other by the application of a trigger pulse. As Bistable
Multivibrators have two stable states they are more commonly known as
Flip-flops for use in sequential type circuits.
Bistable Multivibrators are two state non-regenerative devices and in each state one of the transistors
is cut-off while the other transistor is in saturation, this means that the bistable circuit is capable of remaining indefinitely in either
stable state. To change over from one state to the other the circuit requires a suitable trigger pulse and to go through a full cycle, two
triggering pulses, one for each stage are required. Its more common name or term of "Flip-flop" relates to the
actual operation of the device, as it "Flips" into one logic state, remains there and then changes or "Flops" back into its first original state.
Bistable Multivibrator Circuit
The Bistable Multivibrator circuit above is stable in both states, either with one transistor "OFF" and the
other "ON" or with the first transistor "ON" and the second "OFF". Switching between the two states is achieved by applying a trigger pulse
which inturn will cause the "ON" transistor to turn "OFF". The circuit will switch sequentially by applying a pulse to each base
in turn and this is achieved from a single input trigger pulse using a biased diodes as a steering circuit. Equally, we could remove the diodes,
capacitors and feedback resistors and apply individual trigger pulses directly to the transistor Bases.
Then unlike Monostable Multivibrators whose output is dependent upon the RC time constant of the
feedback components used, the Bistable Multivibrators output is dependent upon the application of two individual trigger pulses. So
Monostable Multivibrators can produce a very short output pulse or a much longer rectangular shaped output whose leading edge
rises in time with the externally applied trigger pulse and whose trailing edge is dependent upon a second trigger pulse as shown below.
Bistable Multivibrator Waveforms
Bistable Multivibrators have many applications such as part of a counting circuit, or as a one-bit memory
device in a computer, or as frequency dividers because the output pulses have a frequency that are exactly one half (f/2)
that of the trigger input pulse frequency due to them changing state from a single input pulse. In other words the circuit produces
Frequency Division as it now divides the input frequency by a factor of two (an octave).
TTL/CMOS Bistable Multivibrators
As well as producing Bistable Multivibrators from individual discrete components such as transistors, we can also construct
Bistable circuits using commonly available integrated circuits. The following circuit shows how a basic bistable multivibrator circuit can be
constructed using just two 2-input Logic "NAND" Gates.
NAND Gate Bistable
By connecting the two NAND gates together as shown above, we can construct a manually controlled
bistable multivibrator that is activated by the single-pole double-throw switch to produce a logic "1" or a logic "0" output. You may notice that
this circuit looks a little familiar, and you would be right!, as its more commonly called an
SR NAND Gate Flip-flop that we looked at back in the
Sequential Logic tutorials and which we saw that this type of NAND Gate Bistable makes an
excellent switch debouncing circuit.
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