Working of Bistable Multivibrator
Bistable Multivibrators operate in a similar fashion to flip-flops producing one of two stable outputs which are the complement of each other.
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.