Monostable Multivibrator – Operation, Types and Application:

Monostable multivibrator is a two-stage amplifier with two states—one stable state and another quasi-stable state. The circuit has two transistors Q₁ and Q₂, one being in the offstate, say Q₁, and the other, Q₂, in the on state preferably in saturation. These transistors remain in this state for ever and only on the application of trigger, the multivibrator goes into the quasi-stable state, i.e., Q₁ is on and Q₂ is off. The circuit remains in the quasi-stable state for a period determined by the circuit components and then returns to its initial stable state, i.e., Q₁ is off and Q₂ is on. Thus, this circuit generates a pulse of duration T.

Types of Monostable Multivibrators:

1. Collector-coupled monostable multivibrator
2. Emitter-coupled monostable multivibrator.

1. Collector Coupled Monostable Multivibrator:

The circuit arrangement of a collector-coupled monostable multivibrator is shown in Fig. 31.24.

It consists of two similar transistors Q₁ and Q₂ with equal collector resistances R₃ and R₄ (i.e., R₃ = R₄). The circuit is de­signed for operation with transistor Q₂ normally conducting and transistor Q₁ cut off. The output of transistor Q₁ is coupled to the base of transistor Q₂ via capacitor C₁. However, the other coupling is direct one through base resistor R₁. Transistor Q₂ is forward biased by power supply V_CC and base resistor R₂ and remains saturated in stable state. Operation in this stable state is ensured if the biasing resistor R₂ is selected to permit sufficient base current in transistor Q₂ for operation in saturation region. Transistor Q₁ is reverse biased by power supply V_BB and resis­tor R₅ remains cut off in the stable state. The input pulse is ap­plied through capacitor C₂. The output may be taken from either of the transistors Q₁ or Q₂.

Operation:

When the power supply is switched on but with no input pulse applied, transistor Q₂ is on and is operating in its satura­tion region. It is being forward biased by supply V_CC and base resistor R₂. The collector of transistor Q₂ is virtually at ground potential. The base of transistor Q₂ is 0.7 V above ground po­tential because of the forward-bias of base-emitter junction. This is illustrated in Figs. 31.25(a) and 31.25(b).

Transistor Q₁ is cut off being reverse biased by supply V_BB and resistor R₅. Its base resistor R₁ is connected to the collector of transistor Q₂, which is at zero potential. Thus collector resistor R₃ is completely disconnected from the grounded emitter of transistor Q₁ and is free to carry current to charge capacitor C₁. Since capacitor C₁ is connected to the base of transistor Q₂, which is close to ground potential, it will charge up almost to the supply voltage V_CC. The polarity of the charge on capacitor C₁ is plus (+) on the left and minus (–) on the right, as shown.

When a positive trigger pulse of short duration and sufficient magnitude is applied to the base of transistor Q₁ through capacitor C₂, it (input pulse) overrides the reverse bias of emitter-base junction of transistor Q₁ and gives it a forward bias. Thus transistor Q₁ starts conducting and the potential of collector of transistor Q₁ comes down to ground. Since charge on capacitor C₁ cannot disappear instantly, the voltage across the capacitor plates is maintained. With the positive side of the capacitor C₁ pulled down to zero volt by transistor Q₁, the negative side goes to a voltage far below ground potential. Thus a negative bias is applied to the base of transistor Q₂ and transistor Q₂ is cut off. The collector of transistor Q₂ rises toward V_CC and is now capable of supplying base current to transistor Q₁ through its base resistor R₁. Thus transistor Q₁ remains turned on even after the positive spike from the transistor Q₁ is removed.

As time passes charging current flows onto the plates of capacitor C₁. The flow path is down through R₂, through capacitor C₁ and through collector to emitter of Q₁ into ground. As can be seen, this path seeks to charge capacitor C₁ to the opposite polarity, what happens is that the voltage across capacitor C₁ gets reduced. When the voltage across the capacitor crosses through zero and attains 0.7 V in the opposite polarity, as shown in Fig. 31.25 (b), it bleeds a small amount of current into the base of transistor Q₂. This small base current causes collector current to flow in transistor Q₂, lowering its collector voltage. The reduced collector voltage causes a reduction in base current to transistor Q₁. This in turn causes a reduction in collector current of Q₁. The Q₁ collector voltage rises slightly, thereby raising the base of Q₂ higher yet. This action is regenerative; once it begins, it avalanches. In the end, transistor Q₂ is saturated once again and transistor Q₁ is cut off. Thus the circuit reverts back to its original stable state. The circuit remains in this state until another triggering pulse causes the circuit to switch over the state and the other cycle repeats itself.

The width of the output pulse is determined by the time constant of C₁R₂. The multivibrator generates one output pulse for every input trigger pulse and that is why it is sometimes called the one shot multivibrator. The width or duration of the pulse is given as T = 0.693 C₁R₂.

Applications:

1. The falling part of the monostable multivibrator output is often used for triggering another pulse generator circuit, thus producing a pulse delayed by a time T with respect to the input pulse.
2. Monostable multivibrator is also employed for regener­ating or rejuvenating old and worn-out pulses. Various pulses used in computers and telecommunication systems become some­what distorted during use. A monostable multivibrator can be employed to generate new, clean and sharp pulses from these distorted and used pulses.

2. Emitter Coupled Monostable Multivibrator:

The circuit of an emitter-coupled monostable multivibrator is shown in Fig. 31.26. This circuit is also designed for operation with transistor Q₂ normally conducting and transistor Q₁ non­conducting.

Operation:

Let initially MMV is in stable state with Q₁ non-conducting and Q₂ conducting. As transistor Q₂ is conducting, voltage drop across R_E is large enough to provide a negative bias at base of transistor Q₁. Thus the output states are stable.

Now voltage at base of transistor Q₁ is increased with the help of potentiometer R so as to forward bias the transistor Q₁ and make it conducting. When Q₁ starts conducting its collector potential falls. This reduced potential is applied to the base of transistor Q₂ through capacitor C and current through transistor^. Q₂ falls and so the voltage drops across R_E. This in turn increases the Q₁ collector current by which the collector voltage reduces further and finally transistor Q₂ is cut off. As time passes, charging current flows onto the plates of capacitor C. The flow path is down through R_B, capacitor C and through collector to emitter of transistor Q₁ into ground. As the capacitor C gets charged, the base potential of transistor Q₂ rises. Thus charging continues until the voltage at base of transistor Q₂ becomes suf­ficiently large to make it conducting. When the transistor Q₂ starts conducting, voltage drop across R_E increases. This reduces the bias applied to Q₁ and so its collector current increases its collector voltage. Thus base voltage of Q₂ is further increased, Q₂ conducts more and more and so voltage drop across R_E rises and reduces the bias applied to the base of transistor Q₁. The action is regenerative, once it begins, it avalanches. In the end transistor Q₂ is saturated once again and transistor Q₁ is cut off. Thus the circuit reverts back to its original stable state.

Emitter-coupled monostable multivibrator (MMV) has got the follow­ing advantages over collector-coupled MMV.

1. Since in such an MMV, collector of Q₂ is not connected to base of Q₁, signal at collector of Q₂ is not directly involved in regenerative loop and hence terminal C₂ can be used freely to provide output.
2. Separate negative supply is not required as emitter junction of transistors is self-biased through R_E.
3. Since base of Q₁ is free from coupling in the circuit, trigger source cannot load the circuit, if it is connected at B₁.
4. Due to presence of R_E, circuit has good stability. The pulse width depends on the magnitude of input voltage at base of Q₁, which can be varied by potentiometer rotation.