Monostable multivibrator

In a monostable multivibrator, one state is stable and permanent, while the other is temporary and quasi-stable. When an external trigger pulse is applied at the appropriate moment, the multivibrator transitions from the stable state to the quasi-stable state. It remains in the quasi-stable state for a predetermined duration until another pulse is applied. Consequently, a monostable multivibrator cannot generate a square wave independently, unlike an astable multivibrator; it requires an external pulse to produce a square wave. A monostable multivibrator is characterized by one transistor that is always conducting (ON state) and another that is non-conducting (OFF state). It is also referred to as a single shot, single swing, or one-shot multivibrator, as well as a delay multivibrator or univibrator.

The circuit of a monostable multivibrator using an NPN transistor consists of two similar transistors, Q1 and Q2, with equal collector loads (RL1 = RL2). The values of -VBB and R3 are designed to reverse bias Q1, keeping it in the cutoff region. The collector supply Vcc and R2 forward bias Q2, maintaining it in saturation. A trigger pulse is applied through C2 to generate the square wave. In the absence of a trigger pulse, with switch S closed, the circuit is in its stable state, where Q1 is OFF (cutoff) and Q2 is ON (saturation). If a positive trigger pulse of sufficient amplitude is applied, it overrides the reverse bias of the emitter-base junction of Q1, forward biasing it and causing Q1 to conduct.

As Q1 conducts, its collector voltage drops due to the voltage across RL1, resulting in a decrease in the potential at point A (negative-going signal). This negative voltage is fed to Q2 via C1, which reduces its forward bias. Consequently, the collector current of Q2 decreases, causing the potential at point B to rise (positive-going signal) due to a reduced voltage drop across RL2. Eventually, Q2 ceases conduction. The positive-going signal at point B is fed through R1 to the base of Q1, further increasing its forward bias. As Q1 conducts more, the potential at point A approaches 0V, returning the circuit to its original state, with Q2 conducting in saturation and Q1 in cutoff. This state persists until another trigger pulse is received, repeating the cycle.

The pulse width at the collector or output of either transistor (Q1 or Q2) in the monostable multivibrator is defined by a specific expression. It is noteworthy that there is no feedback coupling network from the collector of transistor Q2 to the base of transistor Q1; instead, regenerative feedback during the transition between states is provided by the common emitter resistor REE. The absence of coupling from the collector of transistor Q2 makes it an excellent output point, ensuring that the monostable period remains unaffected by load variations. Moreover, the voltage drop across the common emitter resistor VE compensates for temperature variations in VBE, stabilizing the time or delay period. Additionally, voltage-controlled delay is achievable by adjusting the collector current of transistor Q1 during the quasi-stable state, which can be modified by altering the forward bias of Q1.

The emitter-coupled monostable multivibrator has a limitation regarding lower input voltages. In the stable state, transistor Q2 operates in saturation while transistor Q1 is OFF. Upon receiving an appropriate trigger pulse, transistor Q2 transitions into the active region, reducing the common emitter voltage and forward biasing transistor Q1. When transistor Q1 begins to conduct, its collector voltage decreases from VCC, resulting in a negative change.In a monostable multivibrator, one of the state is absolutely permanent i. e, stable and the other one is temporary i. e, quasi-stable. When an external trigger pulse is applied to the mono-stable at appropriate point, the mono-stable changes it state from stabe state to quasi-stable state. It stays in the quasi-stable state for a predetermined length of certain interval remains there until another pulse is applied. Thus a mono stable multivibrator can not generate square wave of its own like an astable multivibrator. Only external pulse will cause if to generate the square wave. In other words, a multi vibrator in which one transistor is always conducting (i. e. in the ON state) and the other is non conducting (i. e. in the OFF state) is called mono stable multivibrator. It is also called a single shot or single swing or a one shot multi vibrator. Other names are delay multi-vibrator and univibrator. Figure shows the circuit of a monostable multivibrator using NPN transistor. It consists of two similar transistor Q1 and Q2 with equal collector loads i. e. RL1 = RL2the values of -VBBand R3 are such as to reverse bias Q1 and keep it at cut off. The collector supply Vcc and R2 forward bias Q2 and keep it at saturation. A trigger pulse is given through C2 to obtain the square wave. Let us suppose that in the absense of a trigger pulse and with S closed, initially the circuit is in its stable state i.

e. Q1 is OFF (at cut-off) and Q2 is ON (at saturation). If positive trigger pulse is off sufficient amplitude, it will override the reverse bias of the E/B junction of Q1 and give it a forward bias, Hence Q1 will start conducting. As Q1 conducts, its collector voltage falls due to voltage drop across RL1. It means that potential of pointA falls (negative going signal). This negative going voltage is fed to Q2VIA C1 where it decreases its forward bias. As collector current of Q2 start decreasing, potential of point B increases (positive going signal) due to lesser drop over RL2.

Soon, Q2 comes out of conduction. The positive going signal at B is fed VIA R1 to the base of Q1 where it increases its forward bias further. As Q1 conductors more potential of point A approaches 0V. Hence, the circuit reverts to its original state with Q2 conducting at saturation and Q1 cur-off. It remains in this state till another trigger pulse comes along when the entire cycle repeats itself.

The width of duration of the pulse obtained at the collector or output of either transistor (Q1 or Q2) of the monostable multivibrator is given by the expression It can be observed that the feedback resistive coupling network from the collector of transistor Q2 to the base of transistor Q1 is absent. instead, the regenerative feedback at the change over from one state to other is provided by the common emitter resistor REE.

The absence of any coupling from the collector of the transistor Q2 makes it an excellent output point. This has the further advantage of making the mono stable period independent of any load variation. Further the common emitter resistor voltage drop VE, Swamps the temperature variation in VBE, on with temperature and thus makes time period or delay period stable.

Further it is possible to have the voltage controlled delay, by controlling delay, by controlling the collector current to the transistor Q1 during quasi-stable state. The collector current of transistor Q1 can be varied by changing the forward bias of the transistor Q1.

The emitter coupled mono-stable multi-vibrator has the limitation of lower input voltage. In the normal stable state transistor Q2 is in the saturation region and transistor Q1 is OFF. On application of an appropriate trigger pulse, the transistor Q2starts to work in the active region reducing the common emitter voltage and forward biasing the transistor Q1. When transistor Q1 begins to conduct its collector voltage falls from VCC. This is a negative change 🔗 External reference