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Bistable multivibrator

Bistable Multivibrator
Bistable multivibrator with transistors

The bistable multivibrator has two stable states:

1. Q1 conducting and Q2 cut off.

2. Q2 conducting and Q1 cut off.

It is also called a flip-flop or an Eccles-Jordan multivibrator. Figure above shows a transistorized flip-flop circuit:

1. R1, R3, and R5 bias Q2.

2. R2 and R4 bias Q1.

3. R1 is a collector resistor for the input of transistor Q2.

4. R2 is a collector resistor for the output of transistor Q2.

Circuit operation
Assume that Q1 is conducting. The collector voltage of Q1 decreases, which places a negatively changed voltage on the base of Q2 through R3, holding Q2 cut off. The collector of Q2 is positive, placing a positive voltage on the base of Q1, keeping it in conduction. A negative trigger onto the base of Q1 will cause Q1's collector to go positive. This charge is quick-coupled to the base of Q2, which causes Q2 to conduct, causing its collector to go in a negative direction. This negative change is coupled to the base of Q1, causing Q1 to decrease conduction further. This action continues until Q1 goes into cutoff and Q2 goes into saturation. The multivibrator will stay in this state until the eventual input trigger onto the base of Q2.