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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. Although largely superseded by its equivalent op amp or timer IC versions in many applications, it is still a useful and flexible design for square wave and pulse generation. The circuits shown on this page will operate from a DC supply between 3.

Therefore a 5V supply can be recommended for greater reliablity. multifibrators

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. Suppose that at switch on, Multivjbrators is conducting heavily and TR2 is turned off. The collector of TR1 will be almost at zero volts as will the left hand plate of C1.

Multivibrator – Wikipedia

Beause TR2 is turned off at this time, its collector will be at supply voltage and its base will be at almost zero potential, the same as TR1 collector, because C1 is still un-charged and its two plates are at the same potential.


As this plate of the capacitor is also connected to the base of TR2, this transistor will begin to conduct heavily. The rapidly increasing collector current through TR2 now causes a voltage drop across R4, and TR2 collector voltage falls, causing the right hand plate of C2 to fall rapidly in potential.

It is the nature of a capacitor that when the voltage on one plate changes rapidly, the other plate also undergoes a similar rapid change, therefore as the right hand plate of C2 falls rapidly from supply voltage to almost zero, the left hand plate must fall in voltage by a similar amount. With TR1 conducting, its base would have been about 0.

This rapidly turns off TR1 causing a rapid rise in its collector voltage.

BJT Astable Multivibrators

Because a sudden voltage change on one plate of a capacitor causes the other multivibrafors to change by a similar amount, this sudden rise at TR1 collector is transmitted via C1 to TR2 base causing TR2 to rapidly turn on as TR1 turns off. A change of state has occurred at both outputs.

This new state does not last however. This switching action produces the collector and base waveforms shown in Fig.

The circuit keeps on changing state in this manner producing a square wave at each collector. The frequency of oscillation can be calculated, as the time for astabld relevant capacitor to charge sufficiently for a change of state to take place, will be approximately 0.



A problem with the basic astable circuit is that the capacitor action described above 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.

This can be overcome by astabel modified circuit shown in Fig. Each time TR2 collector voltage goes high as the transistor turns off, D2 becomes reverse biased, isolating TR2 from the effect of C2 charging.

The charging current for C2 is now supplied by R5 instead of R6.

Module 4.1

It is useful to be able to vary the frequency of operation, and this may be done as shown in Fig. By varying Multivibrwtors 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.

Hons All rights reserved. Learn about electronics Oscillators.

RF Sine Wave Oscillators 3. AF Sine Wave Oscillators 4. After Studying this section, you should be able to: