Wednesday, 4 July 2012


Bistable multivibrator circuit is nothing but a cross coupling of two transistor switches. It has two stable states. An external higher switches this circuit from one stable state to the other. Another trigger is needed to switch this circuit back to the old state. Bistable multivibrator is also called a flip-flop or binary circuit. It is nothing but two inverters connectde back to back.
It is the circuit bistable multivibrator using single bias supply. As soon as the circuit is powerf, one transistor goes to OFF state and other to ON state due to the imbalance of circuit parameters. Both the transistors cannot remain in the same state at same time. The collector voltage of the ON transistor will be (Vce)sat +Vre and base voltage will be (Vbe)sat + Vre.
Reverse will be the case for the other transistor. The collector voltage of the OFF transistor will be Vcc. Its base voltage is a samll positive voltgae but less than the voltage at its emitter, to keep it indeed in OFF state.
Suppose transistor Q1 is turned ON and transistor Q2 is turned OFF as soon as the Vcc supply is switche ON. When a negative going trigger appears at the collector of the transistor Q2, it goes to ON state.
Due to the regenerative action, transistor Q1 goes to OFF state. The bistable multivibrator will continue to remain in this state until  a negative going trigger appears at the colletor of Q1.
A speed up capacitor is connected in parallel with the resistor R2 to couple the transition of the voltage at the collector of one transistor to the base of the other transistor.
Components for wiring thia circuit
Q1=Q2= BC107
D1=D2= 1N4001
Rc1=Rc2= 4.7K
R2 = 39K
R1 = 18K
Re = 470Ω
C1 = 4.7pF
C = 220pF
Vcc = 10V
trigger- square waves
Vc1,Vc2 are outputs

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