Bistable Multivibrator | Binary

Bistable Multivibrator generates a square wave but has two stable states. The circuit stays in one stable say state one indefinitely. Only on application of a suitable trigger pulse, circuit changes to the other stable state i.e. state two. Now again the circuit continues to stay in stable state two indefinitely unless a suitable trigger pulse is applied causing the circuit to revert to the stable state one.

Bistable multivibrators are basically of two types:

(i) Fixed bias bistable multivibrator

(ii) Self bias bistable multivibrator

We here take up the study of a simple self-biased stable multivibrator

Self-Bias Bistable Multivibrator using BJTs

Figure 1 gives the basic circuit using a common resistor R₁ from emitter to ground. It is also referred to as Eccles-Jordon bistable multivibrator. The voltage drop across Ri provides the self-bias for keeping one of the transistors in the OFF state.

Operation of the Bistable Multivibrator Circuit

Since T₁ is nonconducting the base current of T₂ is . If R₃ is small enough, this base current is large enough to saturate T₂. Then the saturation voltage of T₂ is smaller than the turn-on voltage of T₁. Thus, essentially no base current flows through R₁ and T₁ is cutoff as assumed earlier. The multivibrator remains in this stable state until a suitable switching pulse is applied at some point in the circuit to turn T₁ ON. Then T₁ will saturate and its saturation voltage will be insufficient to provide forward bias to T₂ with the result that T₂ will be turned OFF.

The multivibrator will remain in this second state until another suitable switching pulse is applied at a suitable point in the circuit to turn T₂ ON and T₁ OFF. Thus, this multivibrator has two stable states and is, therefore, called bistable multivibrator. It is also known as slip-flop because a trigger pulse will make it flip or flop from one state to the other. The trigger pulse is needed only to start conduction of the OFF transistor and the circuit regeneration or positive feedback will carry out the change of state.

This may be seen from figure 2. Thus, if T₁ begins to conduct, its collector voltage decreases and T₂ is pulled out of saturation. The collector voltage of T₂ then rises and forward bias is applied through R₁ to reinforce the initial trigger. This effect is cumulative and the change of state is completed. It may be noted that the change of state can be initiated with a very small pulse if T₂ is just barely saturated. However, if T₂ is heavily saturated then the trigger pulse must be large in magnitude and must remain long enough to permit T₁ to pull T₂ out of saturation.

Bistable Multivibrator with Symmetrical Collector Triggering:

Fig. 2 gives the circuit of binary (flip-flop) using symmetrical collector triggering where trigger tries to turn OFF a Saturated or ON transistor. The negative trigger is applied through coupling capacitor C_T to point P which is connected to V_CC through resistor R_X. The R_X-C_T network forms the differentiating circuit. The input pulses after differentiation appear at point P to which cathodes of two steering diodes D₁ and D₂ are connected. Anodes of the diodes D₁ and D₂ are connected to the collectors of the transistors T₁ and T₂ respectively.

Diodes serve to permit binary to respond to only one polarity of pulse. Thus, with transistor T₁ ON, diode D₁ is reverse biased by an amount equal to the Voltage drop across R_c1. Diode D₁ then does not transmit any triggering signal unless the signal is negative and has amplitude exceeding the voltage drop across R_c1. However, with T₁ OFF, voltage drop across diode D₁ is zero. But the diode still does not transmit a positive trigger signal but does transmit a negative step or pulse to the input of ON transistor T₂. However, resistor R_X must be sufficiently large So as not to load the trigger source.

With transistor T₁ OFF, there is no collector current and hence Zero Voltage drop across resistor R_c1. The collector voltage of T₁ then equals the supply Voltage VCC. The anode and the cathode of diode D₁ are at the same potential namely V_CC. Diode D₁ is forward biased. The voltage drops across the collector circuit resistor R_c2 of conducting transistor T₂ makes the collector voltage of T₂ very low equal to V_c2, ON. Thus, anode of diode D₂ is at a low potential while the cathode is at V_CC. Hence diode D₂ is reverse biased.

The differentiated pulse appears at point P. Polarities of steering diodes are such that they block the transmission of positive pulses. Only the negative pulses follow the low impedance path offered by diode D₁ and Coupling network R_3-cm2 to the basic of ON transistor T₂. This negative pulse assisted by the regenerative feedback of binary pulls the transistor T₂ from saturation to OFF state and simultaneously bring T₁ into saturation.

Now diode D₁ is reverse biased while D₂ is forwarded biased. Next negative trigger gets transmitted through D₂ and appears at the collector of OFF transistor T₂. This negative trigger pulse passes through the coupling network R_1-cm1 and appears at the base of the ON transistor T₁ and turns it OFF.