What are Timing Circuits and Multivibrator? and 3 of its Gigantic types

Hello everyone I hope that you all are doing good in your lives. The topic for today is Timing circuits. Most digital systems require some kind of timing waveform; for example, a source of trigger pulses is necessary for all clocked sequential systems. However, in digital systems, a rectangular pulse is of desire. The generator of a rectangular waveform is known as a multivibrator.

Certainly, in this blog, we are going to focus on three types of multivibrators;

  1. Astable
  2. Monostable
  3. and lastly Bistable

Types of Multivibrator

An astable multivibrator is nothing but an oscillator that generates a rectangular waveform. It has two quasi-stable states and does not require any triggering, hence it also goes by the name free-running multivibrator. This acts as a source of clock pulses in circuits.

A monostable multivibrator has one stable state i.e. under steady-state conditions its output is either in the HIGH or LOW state. While triggering the circuit with an external pulse it goes to the other state, i.e. it alters in the two states HIGH and LOW if it is at one stage then it’ll move to the other and vice versa (HIGH -> LOW, LOW -> HIGH). The circuit remains in this state for a time duration depending upon the values of the elements in the circuit.

This state of the circuit is refer as a quasi-state since it recovers back to the stable state without any external trigger. The width of the trigger pulse is very small and the width of the output pulse depends on the time for which the circuit remains in the quasi-stable state. This circuit also goes by the name of one-shot as one trigger pulse produces only one pulse but of different widths.

A bistable multivibrator circuit in which both the states are refer to be as a bistable flip flop. This circuit makes the transition from one stable state to another only when a triggering pulse is present. These often act as a memory element in the digital system.

Multivibrator circuits using ICs

Until a few years ago, multivibrators were designed in such a way that they use discrete devices like vacuum triode, BJTs, FETs, etc. However, their production is certainly out of date due to the availability of various ICs. Therefore, we will be only focusing on the circuit consisting ICs for the development of multivibrators. The ICs are;

  1. Logic gates
  2. Op-Amps
  3. Monostable Multivibrators
  4. And lastly timers

Applications of logic gates in timing circuits

Logic gates can be useful for generating the pulse. These circuits are easy to design and analyze but due to a lack of precision, their application is limited.

Free Running Multivibrator

A simple free-running multivibrator is;Free Running Multivibrator Using Inverter

The working principle of the Rc phase shift oscillator and free-running multivibrators are one and the same. In this circuit, the phase shift is provided by the propagation delay of the inverters.

Suppose, for example, the input V1 to inverter I1 makes a transition from 0 to logic 1 at t=0. At t1 its output changes from logic 1 to logic 0. Due to this, the output of inverter I2 will move from logic 0 to logic 1 at t2. At t3, the voltageV0 and V1 will change from logic 1 to logic 0. However, this process will go on indefinitely. T is the time period of the square waveform mathematically which is equal to;

T = 6Δt

In this circuit, we have no control over the frequency of the square wave and it is difficult to determine the exact propagation delay time of the logic gate and hence the frequency of the square wave. Therefore, it is not applicable in systems calling for precise and stable operation frequencies. However, because of its simplicity, it is useful whenever it is necessary to get a high-frequency trigger pulse at a very low price.

Op-Amp and its application in timing circuits

An operational amplifier popular known as Op-Amp is a very high gain d.c. amplifier. Originally, it was design using a vacuum tube to perform mathematical operations such as addition, subtraction, multiplication by a constant, differentiation, etc. And it was a basic building block of an analog computer. Nowadays, it is available as a linear IC, and because of its price, and reliability, has become very popular and finds applications in the generation of waveforms like square, triangular, pulse, sweep, staircase, etc.

The figure of Op-Amp is;Block diagram of Op-Amp

It basically has two inputs, label as – and +, and referred to as inverting and non-inverting input respectively. Apply V1 at the inverting terminal and the ground, and apply V2 to the non-inverting terminal and the ground. The ground terminal is not there in the circuit but is the ground terminal of the supplies + V and – V. The output Vo is between the output terminal and the ground. The input is also known as differential input and the output is dingle end. The output voltage depends on the input difference.

Vo = Av . V1

Where Av is the voltage gain of the amplifier.

Op-Amp Comparator

The op-amp comparator is used as an analog comparator to compare two analog signals. The analog signal is to be compared are applied at the two inputs and the output voltage indicates the comparison. The magnitude of the output voltage is Vsat. This is the basic building block that requires a non-sinusoidal waveform generator.

In some practical applications, the input voltage at the comparator may approach the reference voltage very slowly and may oscillate around Vref. In such situations, Vo either would not switch quickly from one saturation voltage to another or would oscillate between + Vsat and – Vsat. This oscillation may appear due to the ringing that occurs because of the fast voltage transition or due to the presence of noise on wires leading to the Op-Amp’s input terminal.

74121 Monostable Multivibrator

The functional diagram and the function table of the most popular and common one-shot TTL IC 74121 are;Functional Block Diagram Of Monostable Multivibrator IC 74121

Functional Table of IC 74121In order to trigger the one-shot, there must be a rising pulse edge at point Z. This is possible in one of the following two ways;

  1. One or both of the A inputs are at logic 0 and the B input makes a transition from 0 to 1
  2. The B input is at logic 1 and either one of the A inputs makes a transition from logic 1 to 0 while the other A input remains at logic 1 or both A inputs go from logiic 1 to 0 simultaneous.

The duration of the output pulse is dependent upon the values of the resistor (REXT or RINT) and capacitor (CEXT) which are use. A timing capacitor (CEXT) is externally connected between the two terminals REXT / CEXT and CEXT. In the case of electrolytic capacitor, + terminal should be in connection with REXT / CEXT. The maximum allowable value of the external capacitor is 1,000 micro F. If the external timing capacitor is not in use, the stray capacitance between the pins of the IC will result in a very low pulse width output.

For timing resistor there are two options;

  1. Firstly internal timing resistor (RINT) of 2KΩ becomes efective if the RINT terminal is in connection with VCC.
  2. External timing resistor (REXT) is to be connected between REXT / CEXT terminal and VCC. The range of REXT is from 1.4 KΩ to 40KΩ.

In any case, both the REXT and RINT must not be used simultaneously. The duration of the output pulse is;

TON = 0.7 R C


However here we are at the end of the blog. I hope that you like it and are satisfied with the content given. If you do like the log then, please share it with others and do mention below the part which you liked the most. I hope that all your doubts are clear but if still have any then feel free to ask them down below in the comments or you can get in touch with us through our contact us page. And will love to have your suggestion on the topic.


Have a nice day 🙂

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