Bistable Multivibrator – Working and Types:

A bistable multivibrator, as its name implies, has two stable states. It can stay in either of the two states indefinitely (as long as dc power supply remains on). Only an application of a suitable trigger pulse makes the circuit to change to the other stable state. The circuit can be brought back to its original stable state only by applying another suitable trigger pulse. Thus one trigger pulse causes the multivibrator to flip from one state to another and the next pulse causes it to flop back to its original state. This is the very reason that it is called the flip-flop multivibrator. The bistable multivibrator is employed in many digital operations such as counting and storing of binary information, as a frequency divider in timing circuits and can also be used for generation of pulses.

Types of Bistable Multivibrator:

The bistable multivibrator circuits considered here are of two types viz. fixed-bias bistable and self-bias bistable multivibrators. Fixed-bias bistable multivibrator makes use of two separate sources for biasing the devices whereas self-bias multivibrator uses one self-bias to derive the biasing voltage.

Beside these two circuits, there is a third variation of bistable multivibrators, namely, the Schmitt trigger. This circuit, in addition to being used as a bistable, may also be employed for other applications like waveshaping, comparators etc.

Fixed Bias Bistable Multivibrator:

The circuit of a fixed bias bistable multivibrator is shown in Fig. 31.30 (a). It consists of two identical transistors Q₁ and Q₂ with equal collector resistors R_C1 and R_C2 and with output of one supplied to the other. The feedback is coupled through resistors R₁ and R₂. The output may be taken from either of the two transistors Q₁ and Q₂.

Working:

When power is first applied to the circuit, both transistors start conducting. Because of small differences in their operating characteristics, one of the transistors will conduct slightly more than the other. This starts a series of events. Assume arbitrarily that transistor Q₁ initially conducts more than transistor Q₂. This causes the collector voltage of transistor Q₁ to drop more rapidly than that of transistor Q₂. The resulting negative signal is fed to the base of transistor Q₂. The falling voltage makes the conduction of Q₂ less. Thus, its collector current starts falling with corresponding increase of voltage at terminal C₂. This increasing voltage is applied to the base of transistor Q₁ which makes it more forward biased. Because of this cumulative feedback within no time transistor Q₁ goes to saturation and Q₂ to cut-off. The output of saturated transistor Q₁ is approximately zero and that of cut-off transistor Q₂ approximately V_CC. This is the first stable state of the multivibrator. The multivibrator can be driven from the first stable state (Q₁ on and Q₂ off) to the other stable state (Q₁ off and Q₂ on) by applying either a negative trigger pulse to the base of transistor Q₁ or a positive trigger pulse to the base of transistor Q₂.

Let a negative pulse of short duration and sufficient magnitude be applied to the base of transistor Q₁ through capacitor C₁. This reduces the forward bias on transistor Q₁ and causes a reduction in collector current and, thereby, increase in potential of its collector terminal C₁. The rising collector voltage appears across the emitter-base junction of transistor Q₂ as it is connected to collector terminal C₁ via resistor R₂. As a result collector current of transistor Q₂ increases and, therefore, its collector voltage falls. The decreasing collector voltage appears across the emitter-base junction of transistor Q₁ where it further reverse biases the emitter-base junction of transistor Q₁ to make collector current to fall. After few cycles, the transistor Q₂ is driven into saturation and transistor Q₁ to cut-off. This is the second stable state of the multivibrator. The circuit will now remain in this second stable state (Q₁ off and Q₂ on) until a positive pulse is applied to the base of transistor Q₁ or a negative pulse to the base of transistor Q₂.

The noteworthy points are :

1. Under cut-off condition nearly the full supply voltage V_CC appears across the transistor and this restricts the supply voltage V_CC to the order of several tens of volts (smaller than the transistor breakdown voltage V_CE).

2. Under saturation condition, the collector current is maximum. Hence R_C1 and R_C2 must be chosen so as to restrict the collector current to the permissible value. The values of resistors R₁, R₂, R₃ and R₄ and V_BB must be so selected that in one state the base current is large enough to drive the transistor into saturation while in second state the emitter-base junction must be below cut-off value.

3. The values of R₁, R₂, R₃ and R₄ are selected so that they are much larger than collector resistances R_C1 and R_C2 and consequently have no loading effects on the amplifier circuit.

Analysis: If transistor Q₁ is in conducting state and transistor Q₂ is in non-conducting state, the voltages V_B1 between point B₁ and ground (points E₁ and E₂) and V_C1 between point C₁ and ground are both small. The base-bias voltage V_B2 on transistor Q₂ is then

which being negative, keeps transistor Q₂ in its cut-off state. Since R_C2 is much smaller than R₁ and the collector current of transis­tor Q₂ is assumed zero, the collector voltage of transistor Q₂ is + V_CC. The base current I_B1 is then

and if V_B1 can be considered negligibly small,

For keeping transistor Q₁ conducting, the value of I_B1, must be sufficiently positive to ensure continued current i.e.,

The values of V_CC, V_BB, R₁, R₂, R_C1 and R_C2 are so selected that Eq. (31.27) is satisfied and consequently a stable state exists with Q₂ cut-off and Q₁ in saturation. For the symmetry of circuit (R_C1 = R_C2; R₁ = R₂ and R₃ = R₄), it is evident that a second stable state exists with Q₁ cut-off and Q₂ in saturation.

Two points should be noted about the switching process. First the pulse used for switching need only be applied for a time sufficient for the changeover; it can then be removed. Second, if a negative pulse is used to affect switching, a positive pulse is required for the next switching, a negative pulse for the one fol­lowing, etc.

Idealized output waveforms for the bistable multivibrator are shown in Fig. 31.30 as solid curves. The dotted waveforms are those of an actual circuit, the deviations from the idealized result from switching transients.

Loading: The bistable multivibrator may be employed to drive other circuits and therefore at one or both the collectors there are shunting loads which are not indi­cated in Fig. 31.30(a). Such loads reduce the magnitude of the collec­tor voltage V_C1 of the cut-off tran­sistor. This will cause reduction in the output voltage swing. A re­duced V_C1 will result in the reduc­tion of I_B, and it is possible that transistor Q₂ may not be driven into saturation. Hence, the components of the multivibrator must be chosen so that under the heaviest load, which the multivibrator drives, one transistor remains in saturation while the other is in cut-off.

For some applications, the loading varies during the operation. In such cases, the extent to which a transistor is driven into saturation varies. A constant output swing V_w = V, and a constant base saturation current I_B, can be had by clamping the collectors to an auxiliary voltage V < V_CC through the diodes D₁ and D₂ as shown in Fig. 31.32. As transistor Q₁ cuts off, its collector voltage increases and when it becomes V, the collector “catching diode” D₁ conducts and clamps the output to V.

Self Bias Bistable Multivibrator:

The circuit of a self-biased bistable multivibrator is shown in Fig. 31.33.

Here the emitter of both the transistors are common and a resistance R_E is connected from the common emitter to ground. The voltage drop across R_E provides the self-bias to keep one of the transistors in the off state. The two stable states of the bistable multivibrator (binary) are:

1. Transistor Q₁ off and transistor Q₂ on.
2. Transistor Q₂ off and transistor Q₁ on.

The working and operation of self-bias binary is the same as that of fixed-bias binary.