The Bistable Multivibrator is another type of two state device similar to the Monostable Multivibrator we looked at in the previous tutorial but the difference this time is that BOTH states are stable. Bistable Multivibrators have TWO stable states (hence the name: “Bi” meaning two) and maintain a given output state indefinitely unless an external trigger is applied forcing it to change state.
The bistable multivibrator can be switched over from one stable state to the other by the application of an external trigger pulse thus, it requires two external trigger pulses before it returns back to its original state. As bistable multivibrators have two stable states they are more commonly known as Latches and Flip-flops for use in sequential type circuits.
The discrete Bistable Multivibrator is a two state non-regenerative device constructed from two cross-coupled transistors operating as “ON-OFF” transistor switches. In each of the two states, 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 the bistable over from one state to the other, the bistable 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. Consider the circuit below.
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”. Lets suppose that the switch is in the left position, position “A”. The base of transistor TR1 will be grounded and in its cut-off region producing an output at Q. That would mean that transistor TR2 is “ON” as its base is connected to Vcc through the series combination of resistors R1 and R2. As transistor TR2 is “ON” there will be zero output at Q, the opposite or inverse of Q.
If the switch is now move to the right, position “B”, transistor TR2 will switch “OFF” and transistor TR1 will switch “ON” through the combination of resistors R3 and R4 resulting in an output at Q and zero output at Q the reverse of above. Then we can say that one stable state exists when transistor TR1 is “ON” and TR2 is “OFF”, switch position “A”, and another stable state exists when transistor TR1 is “OFF” and TR2 is “ON”, switch position “B”.
Then unlike the monostable multivibrator 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, switch position “A” or position “B”.
So Bistable 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.
Manually switching between the two stable states may produce a bistable multivibrator circuit but is not very practical. One way of toggling between the two states using just one single trigger pulse is shown below.
Switching between the two states is achieved by applying a single trigger pulse which in turn will cause the “ON” transistor to turn “OFF” and the “OFF” transistor to turn “ON” on the negative half of the trigger pulse. 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.
Then on the application of a first negative pulse switches the state of each transistor and the application of a second pulse negative pulse resets the transistors back to their original state acting as a divide-by-two counter. Equally, we could remove the diodes, capacitors and feedback resistors and apply individual negative trigger pulses directly to the transistor bases.
Bistable Multivibrators have many applications producing a set-reset, SR flip-flop circuit for use in counting circuits, or as a one-bit memory storage device in a computer. Other applications of bistable flip-flops include frequency dividers because the output pulses have a frequency that are exactly one half ( ƒ/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).
As well as producing a bistable multivibrator 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.
The circuit above shows us how we can use two NAND gates connected together to form a basic bistable multivibrator. This type of bistable circuit is also known as a “Bistable Flip-flop”. The manually controlled bistable multivibrator is activated by the single-pole double-throw switch (SPDT) to produce a logic “1” or a logic “0” signal at the output.
You may have noticed that this circuit looks a little familiar, and you would be right!. This type of bistable switching circuit is more commonly called a SR NAND Gate Flip-flop being almost identical to the one we looked at back in the sequential logic tutorials. In that particular tutorial we saw that this type of NAND gate bistable makes an excellent “switch debounce” circuit allowing only one switching action to control its output.
In the next tutorial about Multivibrators, we will look at one that has NO stable states because it is continually switching over from one stable state to the other. This type of multivibrator circuit is called an Astable Multivibrator also known by their more common name of “fee-running oscillator”.