After Studying this section, you should be able to: • Recognise BJT astable multivibrator circuits. • Understand the operation of astable multivibrators. • Calculate. An Astable Multivibrator or a Free Running Multivibrator is the multivibrator which has no stable states,oscillates continuously between two unstable states. A multivibrator is a device that switches between two states. It is a type of oscillator and can be used as a trigger, converter, moderator, or divider. It.

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The basic bipolar transistor BJT version of an astable multivibrator as shown in Fig.

A diode D1 clamps the capacitor voltage to 0. The width of the output pulse depends astagle on external components connected to the op-amp. Astable multivibrators are also used in morse code generators, timers, and systems that require a square wave, including television broadcasts and analog circuits. Q2 is on and connects the right-hand positive plate of C2 to ground. An astable multivibrator consists of two amplifying stages connected in a positive feedback loop by two capacitive-resistive coupling networks.

It is implemented by the coupling capacitors that instantly transfer voltage changes because the voltage across a capacitor multivibrtors suddenly change. The action is reversed after a certain time, depending upon the circuit conditions i. When the multivibratods of C1 right-hand plate Q2 base voltage becomes positive and reaches 0. The period of time during which Q2 remains at saturation and Q1 at cutoff is determined by C2 and R2.


As C1 discharges, the base bias at Q2 becomes less positive and at a time determined by R1 and C1, forward biased is re-established at Q2. The circuit produces two anti-phase square wave signals, with an amplitude almost equal to its supply voltage, at its two transistor collectors as shown in Fig 4.

What is an Astable Multivibrator?

When the circuit is first powered up, neither transistor will be switched on. Astable multivibrators are used in amateur radio equipment to receive and transmit radio signals.

Thus, Set is used to “set” Q1 on, and Reset is used to “reset” it to off state. The general solution for multivibraators low pass RC circuit is.

A problem with the basic astable circuit is that the capacitor action described multivibrarors slows down the rise in voltage as each transistor turns off, producing the curved rising edges to the square wave as can be seen in Fig. From Wikipedia, the free encyclopedia.

Module 4.1

The circuit essentially consists of two symmetrical CE amplifier stages, each providing a feedback to another. 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.

The circuit has two astable unstable states that change alternatively with maximum transition rate because of the “accelerating” positive feedback. Mail this article Print this article. It is also called free-running relaxation oscillator. Assume all the capacitors to be discharged at first.

The capacitor discharges through resistor R and charges again to 0. Views Read Edit View history. As this plate of the capacitor is also connected to the base of TR2, this transistor will begin to conduct heavily. The circuits shown on this page will operate from a DC supply between 3. Your email address will not be published. During State 1Q2 base-emitter junction is reverse-biased asstable capacitor C1 is “unhooked” from ground. Multivibrators find applications in a variety of systems where square waves or timed intervals are required.


There are three basic types of multivibrators. Hence the base of Q1 will become more positive than Q2. By varying VR1 the voltage at the top of both R3 and R4 is varied so that whatever mark to space ratio is used, only the frequency alters, whilst the mark to space ratio is maintained.


This feedback is supplied in such a manner that one transistor is driven to saturation ON state and other to cut-off OFF state.

The diode will now get reverse biased and the capacitor starts charging exponentially to -Vsat through R. 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.

The rapidly increasing collector current through TR2 now causes a voltage drop across R4, and TR2 collector voltage falls, causing the right hand plate multivubrators C2 to fall rapidly in potential.