Pulse Circuits - Astable Multivibrator

An astable multivibrator has no stable states. Once the Multivibrator is ON, it just changes its states on its own after a certain time period which is determined by the R_C time constants. A dc power supply or V_cc is given to the circuit for its operation.

Construction of Astable Multivibrator

Two transistors named 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 and vice versa. The emitters of both the transistors are connected to the ground. The collector load resistors R₁ and R₄ and the biasing resistors R₂ and R₃ are of equal values. The capacitors C₁ and C₂ are of equal values.

The following figure shows the circuit diagram for Astable Multivibrator.

Operation of Astable Multivibrator

When V_cc is applied, the collector current of the transistors increase. As the collector current depends upon the base current,

$$I_c = \beta I_B$$

As no transistor characteristics are alike, one of the two transistors say Q₁ has its collector current increase and thus conducts. The collector of Q₁ is applied to the base of Q₂ through C₁. This connection lets the increased negative voltage at the collector of Q₁ to get applied at the base of Q₂ and its collector current decreases. This continuous action makes the collector current of Q₂ to decrease further. This current when applied to the base of Q₁ makes it more negative and with the cumulative actions Q₁ gets into saturation and Q₂ to cut off. Thus the output voltage of Q₁ will be V_{CE (sat)} and Q₂ will be equal to V_CC.

The capacitor C₁ charges through R₁ and when the voltage across C₁ reaches 0.7v, this is enough to turn the transistor Q₂ to saturation. As this voltage is applied to the base of Q₂, it gets into saturation, decreasing its collector current. This reduction of voltage at point B is applied to the base of transistor Q₁ through C₂ which makes the Q₁ reverse bias. A series of these actions turn the transistor Q₁ to cut off and transistor Q₂ to saturation. Now point A has the potential V_CC. The capacitor C₂ charges through R₂. The voltage across this capacitor C₂ when gets to 0.7v, turns on the transistor Q₁ to saturation.

Hence the output voltage and the output waveform are formed by the alternate switching of the transistors Q₁ and Q₂. The time period of these ON/OFF states depends upon the values of biasing resistors and capacitors used, i.e., on the R_C values used. As both the transistors are operated alternately, the output is a square waveform, with the peak amplitude of V_CC.

Waveforms

The output waveforms at the collectors of Q₁ and Q₂ are shown in the following figures.

Frequency of Oscillations

The ON time of transistor Q₁ or the OFF time of transistor Q₂ is given by

t₁ = 0.69R₁C₁

Similarly, the OFF time of transistor Q₁ or ON time of transistor Q₂ is given by

t₂ = 0.69R₂C₂

Hence, total time period of square wave

t = t₁ + t₂ = 0.69(R₁C₁ + R₂C₂)

As R₁ = R₂ = R and C₁ = C₂ = C, the frequency of square wave will be

$$f = \frac{1}{t} = \frac{1}{1.38 R C} = \frac{0.7}{RC}$$

Advantages

The advantages of using an astable multivibrator are as follows −

• No external triggering required.
• Circuit design is simple
• Inexpensive
• Can function continuously

Disadvantages

The drawbacks of using an astable multivibrator are as follows −

• Energy absorption is more within the circuit.
• Output signal is of low energy.
• Duty cycle less than or equal to 50% can’t be achieved.

Applications

Astable Multivibrators are used in many applications such as amateur radio equipment, Morse code generators, timer circuits, analog circuits, and TV systems.