Bistable Multivibrators

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.