Pulse Circuits - Monostable Multivibrator

A monostable multivibrator, as the name imppes, has only one stable state. When the transistor conducts, the other remains in non-conducting state. A stable state is such a state where the transistor remains without being altered, unless disturbed by some external trigger pulse. As Monostable works on the same principle, it has another name called as One-shot Multivibrator.

Construction of Monostable Multivibrator

Two transistors Q₁ and Q₂ are connected in feedback to one another. The collector of transistor Q₁ is connected to the base of transistor Q₂ through the capacitor C₁. The base Q₁ is connected to the collector of Q₂ through the resistor R₂ and capacitor C. Another dc supply voltage –V_BB is given to the base of transistor Q₁ through the resistor R₃. The trigger pulse is given to the base of Q₁ through the capacitor C₂ to change its state. R_L1 and R_L2 are the load resistors of Q₁ and Q₂.

One of the transistors, when gets into a stable state, an external trigger pulse is given to change its state. After changing its state, the transistor remains in this quasi-stable state or Meta-stable state for a specific time period, which is determined by the values of RC time constants and gets back to the previous stable state.

The following figure shows the circuit diagram of a Monostable Multivibrator.

Operation of Monostable Multivibrator

Firstly, when the circuit is switched ON, transistor Q₁ will be in OFF state and Q₂ will be in ON state. This is the stable state. As Q₁ is OFF, the collector voltage will be V_CC at point A and hence C₁ gets charged. A positive trigger pulse appped at the base of the transistor Q₁ turns the transistor ON. This decreases the collector voltage, which turns OFF the transistor Q₂. The capacitor C₁ starts discharging at this point of time. As the positive voltage from the collector of transistor Q₂ gets appped to transistor Q₁, it remains in ON state. This is the quasi-stable state or Meta-stable state.

The transistor Q₂ remains in OFF state, until the capacitor C₁ discharges completely. After this, the transistor Q₂ turns ON with the voltage appped through the capacitor discharge. This turn ON the transistor Q₁, which is the previous stable state.

Output Waveforms

The output waveforms at the collectors of Q₁ and Q₂ along with the trigger input given at the base of Q₁ are shown in the following figures.

The width of this output pulse depends upon the RC time constant. Hence it depends on the values of R₁C₁. The duration of pulse is given by

$$T = 0.69R_1 C_1$$

The trigger input given will be of very short duration, just to initiate the action. This triggers the circuit to change its state from Stable state to Quasi-stable or Meta-stable or Semi-stable state, in which the circuit remains for a short duration. There will be one output pulse for one trigger pulse.

Advantages

The advantages of Monostable Multivibrator are as follows −

One trigger pulse is enough.

Circuit design is simple

Inexpensive

Disadvantages

The major drawback of using a monostable multivibrator is that the time between the apppcations of trigger pulse T has to be greater than the RC time constant of the circuit.

Apppcations

Monostable Multivibrators are used in apppcations such as television circuits and control system circuits.