This process will repeat itself over and over again as long as the supply voltage is present. Then we can see that the circuit alternates between one unstable state in which transistor TR 1 is “OFF” and transistor TR 2 is “ON”, and a second unstable in which TR 1 is “ON” and TR 2 is “OFF” at a rate determined by the RC values. This action starts the whole process over again but now with capacitor C2 taking the base of transistor TR 1 to -5.4v while charging up via resistor R2 and entering the second unstable state. However, it never reaches the value of Vcc because as soon as it gets to 0.6 volts positive, transistor TR 2 turns fully “ON” into saturation. Thus the base of transistor TR 2 is now moving upwards in a positive direction towards Vcc with a time constant equal to the C1 x R3 combination. Transistor TR 2 is driven into cut-off so capacitor C1 now begins to charge in the opposite direction via resistor R3 which is also connected to the +6 volts supply rail, Vcc. This rapid fall of voltage on plate “A” causes an equal and instantaneous fall in voltage on plate “B” therefore plate “B” of C1 is pulled down to -5.4v (a reverse charge) and this negative voltage swing is applied the base of TR 2 turning it hard “OFF”. The instant that transistor, TR 1 switches “ON”, plate “A” of the capacitor which was originally at Vcc potential, immediately falls to 0.6 volts.
When the voltage across capacitor C 2 rises to more than 0.6v, it biases transistor TR 1 into conduction and into saturation. Since TR 2 is fully-on, capacitor C 2 starts to charge up through resistor R 2 towards Vcc. Therefore, capacitor C1 has a potential difference of +5.4 volts across its plates, ( 6.0 – 0.6v) from point A to point B.
The other side of capacitor, C1, plate “B”, is connected to the base terminal of transistor TR 2 and at 0.6v because transistor TR 2 is conducting (saturation). Astable oscillators produce a continuous square wave from its output or outputs, (two outputs no inputs) which can then be used to flash lights or produce a sound in a loudspeaker. In electronic circuits, astable multivibrators are also known as Free-running Multivibrator as they do not require any additional inputs or external assistance to oscillate. The astable circuit consists of two switching transistors, a cross-coupled feedback network, and two time delay capacitors which allows oscillation between the two states with no external triggering to produce the change in state. The Astable Multivibrator is another type of cross-coupled transistor switching circuit that has NO stable output states as it changes from one state to the other all the time.
Unlike the Monostable Multivibrator or the Bistable Multivibrator we looked at in the previous tutorials that require an “external” trigger pulse for their operation, the Astable Multivibrator has automatic built in triggering which switches it continuously between its two unstable states both set and reset. Regenerative switching circuits such as the Astable Multivibrator are the most commonly used type of relaxation oscillator because not only are they simple, reliable and ease of construction they also produce a constant square wave output waveform.
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