It consists of two similar transistor Q1 and Q2 with equal collector loads i. The collector supply Vcc and R2 forward bias Q2 and keep it at saturation. A trigger pulse is given through C2 to obtain the square wave. When Trigger Pulse is applied Let us see as what happens when the trigger is applied. As collector current of Q2 start decreasing, potential of point B increases positive going signal due to lesser drop over RL2.
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History[ edit ] A vacuum tube Abraham-Bloch multivibrator oscillator, France, small box, left. Its harmonics are being used to calibrate a wavemeter center. Since it produced a square wave , in contrast to the sine wave generated by most other oscillator circuits of the time, its output contained many harmonics above the fundamental frequency, which could be used for calibrating high frequency radio circuits.
For this reason Abraham and Bloch called it a multivibrateur. It is a predecessor of the Eccles-Jordan trigger  which was derived from the circuit a year later. Historically, the terminology of multivibrators has been somewhat variable: — multivibrator implies astable: "The multivibrator circuit Fig. The earliest and best known of these circuits was the multivibrator. Figure 1, below right, shows bipolar junction transistors.
The circuit is usually drawn in a symmetric form as a cross-coupled pair. The two output terminals can be defined at the active devices and have complementary states. One has high voltage while the other has low voltage, except during the brief transitions from one state to the other. Operation[ edit ] The circuit has two astable unstable states that change alternatively with maximum transition rate because of the "accelerating" positive feedback.
It is implemented by the coupling capacitors that instantly transfer voltage changes because the voltage across a capacitor cannot suddenly change. In each state, one transistor is switched on and the other is switched off. Accordingly, one fully charged capacitor discharges reverse charges slowly thus converting the time into an exponentially changing voltage. At the same time, the other empty capacitor quickly charges thus restoring its charge the first capacitor acts as a time-setting capacitor and the second prepares to play this role in the next state.
The circuit operation is based on the fact that the forward-biased base-emitter junction of the switched-on bipolar transistor can provide a path for the capacitor restoration. State 1 Q1 is switched on, Q2 is switched off In the beginning, the capacitor C1 is fully charged in the previous State 2 to the power supply voltage V with the polarity shown in Figure 1.
Q1 is on and connects the left-hand positive plate of C1 to ground. As its right-hand negative plate is connected to Q2 base, a maximum negative voltage -V is applied to Q2 base that keeps Q2 firmly off. As Q2 base-emitter junction is reverse-biased, it does not conduct, so all the current from R2 goes into C1.
Simultaneously, C2 that is fully discharged and even slightly charged to 0. Thus C2 restores its charge and prepares for the next State C2 when it will act as a time-setting capacitor. Q1 is firmly saturated in the beginning by the "forcing" C2 charging current added to R3 current.
In the end, only R3 provides the needed input base current. The resistance R3 is chosen small enough to keep Q1 not deeply saturated after C2 is fully charged.
Q2 begins conducting and this starts the avalanche-like positive feedback process as follows. Q2 collector voltage begins falling; this change transfers through the fully charged C2 to Q1 base and Q1 begins cutting off.
Its collector voltage begins rising; this change transfers back through the almost empty C1 to Q2 base and makes Q2 conduct more thus sustaining the initial input impact on Q2 base. Thus the initial input change circulates along the feedback loop and grows in an avalanche-like manner until finally Q1 switches off and Q2 switches on.
The forward-biased Q2 base-emitter junction fixes the voltage of C1 right-hand plate at 0. State 2 Q1 is switched off, Q2 is switched on Now, the capacitor C2 is fully charged in the previous State 1 to the power supply voltage V with the polarity shown in Figure 1.
Q2 is on and connects the right-hand positive plate of C2 to ground. As its left-hand negative plate is connected to Q1 base, a maximum negative voltage -V is applied to Q1 base that keeps Q1 firmly off.
Simultaneously, C1 that is fully discharged and even slightly charged to 0. Thus C1 restores its charge and prepares for the next State 1 when it will act again as a time-setting capacitor
Multivibrator Astable Multi vibrator A multi vibrator which generates square waves of its own i. It is also called free ramming multivibrator. It has no stable state but only two quasi-stables half-stable makes oscillating continuously between these states. Thus it is just an oscillator since it requires no external pulse for its operation of course it does require D. C power.
Pulse and Digital Circuits by Venkata Rao K., Rama Sudha K., Manmadha Rao G.
Schmitt in while he was a graduate student,  later described in his doctoral dissertation as a "thermionic trigger". It is a system with positive feedback in which the output signal fed back into the input causes the amplifier A to switch rapidly from one saturated state to the other when the input crosses a threshold. The positive feedback is introduced by adding a part of the output voltage to the input voltage. There are three specific techniques for implementing this general idea. The first two of them are dual versions series and parallel of the general positive feedback system. In the third technique , the threshold and memory properties are separated.