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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Electronic oscillators Digital circuits Analog circuits.

BJT Astable Multivibrators

As the charged capacitor discharges and transfers its energy to the second capacitor, the second capacitor recharges and prepares to discharge the energy back to the input. For example, mulltivibrators Q2 is on and Set is grounded momentarily, this switches Q2 off, and makes Q1 on. AF Sine Wave Oscillators 4.

When triggered by an input pulse, a monostable multivibrator will switch to its unstable position for a period of time, and then return to its stable state. Your email address will not be published. Additionally, astable multivibrators are inexpensive to produce, are relatively simple in design, and can remain functional for extraordinary amounts of time.

Leave this field empty. The period of time during which Q2 remains at saturation and Q1 at cutoff is determined by C2 and R2. Retrieved from ” https: The diode will now get reverse biased and the capacitor starts charging exponentially to -Vsat through R. This works via a transistor or amplifier that amplifies the output signal and forwards the charge to the input.


During State 1Q2 base-emitter junction is reverse-biased and capacitor C1 is “unhooked” from ground. 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.

There are three basic types of multivibrators.

astablf This can be overcome by the modified circuit shown in Fig. Thus C2 restores its charge and prepares for the next State C2 when it will act as a time-setting capacitor.


The circuits shown on this page will operate from a DC supply between 3. Now the capacitor discharges towards -V sat.

A multivibrator is an electronic circuit used to implement a variety of simple two-state [1] [2] [3] devices such as relaxation oscillatorstimers and flip-flops. This is repeated until the circuit drives Q1 to saturation and Q2 to cutoff. Wikimedia Commons has media related to Multivibrators. As this plate of the capacitor is also connected to the base of Asttable, this transistor will begin to conduct heavily. This is the output voltage of R 1 C 1 integrating circuit.

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 rising collector current in Q1 drives its collector more and more positive. Therefore full voltage Vcc appears across Rc1 and voltage across Rc2 will be zero. A diode D1 clamps the capacitor voltage to 0. This feedback is supplied in such a manner that one transistor is astsble to saturation ON state and other to cut-off OFF state.


The action is reversed after a certain time, depending upon the circuit conditions i. The output of transistor Q1 is coupled to the input of Q2 through C1 while the Q2 is fed to the input of Q1 through C2.

Module 4.1

Although largely superseded by its equivalent op amp or timer IC versions in many multivibbrators, it is still a useful and flexible design for square wave and pulse generation.

However, this means that at this stage they will both have high base voltages and therefore a tendency to switch on, and inevitable slight asymmetries will mean that one of the transistors is first to switch on.

multivibratkrs The charging current for C2 is now supplied by R5 instead of R6. A free-running multivibrator with a frequency of one-half to one-tenth of the reference frequency would accurately lock to the reference frequency. During State 2Q2 base-emitter junction is forward-biased and capacitor C1 is “hooked” to ground.