Mumbai University > COMPS > Sem 3 > Digital Logic Design and Analysis

Marks: 5 M

Year: June 2014

• The Bistable Multivibrator is another type of two state device similar to the Monostable Multivibrator but the difference this time is that BOTH states are stable. Bistable Multivibrator 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”.