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Astable (unstable) multivibrator operating principle, advantages and disadvantages, application

Lecture



An unstable multivibrator has no stable states . When the multivibrator is on, it simply changes its states by itself after a certain period of time, which is determined by the time constants R C. A DC power supply or V cc is applied to the circuit for its operation.

Unstable (Astable) multivibrator design

The two transistors named Q 1 and Q 2 are feedback-coupled to each other. The collector of Q 1 is connected to the base of Q 2 through a capacitor C 1 and vice versa. The emitters of both transistors are connected to ground. The collector load resistors R 1 and R 4 and the bias resistors R 2 and R 3 have the same values. Capacitors C 1 and C 2 have the same value.

The following figure shows a schematic diagram of an unstable multivibrator.

Astable (unstable) multivibrator operating principle, advantages and disadvantages, application

Unstable multivibrator operation

When V cc is applied, the collector current of the transistors increases. Since the collector current depends on the base current,

Astable (unstable) multivibrator operating principle, advantages and disadvantages, application

Since no transistor characteristics are similar, one of the two transistors says that Q 1 has an increase in collector current and therefore conducts. The Q 1 collector is applied to the base from Q 2 to C 1 . This connection allows the increased negative collector voltage of Q 1 to be applied to the base of Q 2 and its collector current decreases. This continuous action causes the collector current of Q 2 to decrease further. This current, when applied to the base Q 1, makes it more negative and with the combined actions of Q 1saturates and Q 2 turns off. Thus, the output voltage of Q 1 will be V CE (sat) and Q 2 will be equal to V CC .

Capacitor C 1 charges through R 1, and when C 1 reaches 0.7 V, that's enough to turn Q 2 to saturation. When this voltage is applied to the base of Q 2 , it saturates, decreasing the collector current. This voltage drop at point B is applied to the base of Q 1 -C 2, which reverse biases Q 1 . This sequence turns off Q 1 and saturates Q 2 . Now point A has a potential V CC . Capacitor C 2 is charged through R 2 . The voltage across this capacitor C 2, when it reaches 0.7 V, turns Q 1 on until it saturates.

Therefore, the output voltage and the output waveform are generated by alternately switching transistors Q 1 and Q 2 . The time period for these ON / OFF states depends on the values ​​of the bias resistors and capacitors used, ie, the values ​​of R C used . Since both transistors operate alternately, the output signal is a square wave with a peak amplitude of V CC .

Wave

The output signals to the Q 1 and Q 2 collectors are shown in the following figures.

Astable (unstable) multivibrator operating principle, advantages and disadvantages, application

Oscillation frequency

The turn- on time of Q 1 or the turn-off time of Q 2 is defined as

t 1 = 0.69R 1 C 1

Similarly, the turn-off time of Q 1 or the turn-on time of Q 2 is defined as

t 2 = 0.69R 2 C 2

Therefore, the total time period of the square wave is

Astable (unstable) multivibrator operating principle, advantages and disadvantages, application

Since R 1 = R 2 = R and C 1 = C 2 = C, the frequency of the square wave will be

Astable (unstable) multivibrator operating principle, advantages and disadvantages, application

advantages

The advantages of using an unstable multivibrator are as follows:

  • No external trigger required.
  • The scheme is simple
  • inexpensive
  • Can function continuously

disadvantages

The disadvantages of using an unstable multivibrator are as follows:

  • Energy absorption is greater within the circuit.
  • The output signal is low energy.
  • A duty cycle less than or equal to 50% cannot be achieved.

Applications

Unstable multivibrators are used in many applications such as hobby radio equipment, Morse code generators, timer circuits, analog circuits, and television systems.

See also


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Electronics, Microelectronics, Element Base

Terms: Electronics, Microelectronics, Element Base