The Web This site. 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 Although largely superseded by its equivalent op amp or timer IC versions in many applications, it is still a useful and flexible design for square wave and pulse generation.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.
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A multivibrator is an electronic circuit used to implement a variety of simple two-state    devices such as relaxation oscillators , timers and flip-flops. It consists of two amplifying devices transistors , vacuum tubes or other devices cross-coupled by resistors or capacitors. Multivibrators find applications in a variety of systems where square waves or timed intervals are required.
For example, before the advent of low-cost integrated circuits, chains of multivibrators found use as frequency dividers. A free-running multivibrator with a frequency of one-half to one-tenth of the reference frequency would accurately lock to the reference frequency. This technique was used in early electronic organs, to keep notes of different octaves accurately in tune.
Other applications included early television systems, where the various line and frame frequencies were kept synchronized by pulses included in the video signal.
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. An astable multivibrator consists of two amplifying stages connected in a positive feedback loop by two capacitive-resistive coupling networks.
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. 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. 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. When the voltage of C1 right-hand plate Q2 base voltage becomes positive and reaches 0.
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. 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 The duration of state 1 low output will be related to the time constant R 2 C 1 as it depends on the charging of C1, and the duration of state 2 high output will be related to the time constant R 3 C 2 as it depends on the charging of C2.
Because they do not need to be the same, an asymmetric duty cycle is easily achieved. How long this takes is half our multivibrator switching time the other half comes from C1. In the charging capacitor equation above, substituting:. The output voltage has a shape that approximates a square waveform.
It is considered below for the transistor Q1. During State 1 , Q2 base-emitter junction is reverse-biased and capacitor C1 is "unhooked" from ground. The output voltage of the switched-on transistor Q1 changes rapidly from high to low since this low-resistive output is loaded by a high impedance load the series connected capacitor C1 and the high-resistive base resistor R2.
During State 2 , Q2 base-emitter junction is forward-biased and capacitor C1 is "hooked" to ground. The output voltage of the switched-off transistor Q1 changes exponentially from low to high since this relatively high resistive output is loaded by a low impedance load capacitor C1.
This is the output voltage of R 1 C 1 integrating circuit. To approach the needed square waveform, the collector resistors have to be low in resistance. When the circuit is first powered up, neither transistor will be switched on. 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.
This will quickly put the circuit into one of the above states, and oscillation will ensue. In practice, oscillation always occurs for practical values of R and C. However, if the circuit is temporarily held with both bases high, for longer than it takes for both capacitors to charge fully, then the circuit will remain in this stable state, with both bases at 0. This can occur at startup without external intervention, if R and C are both very small. An astable multivibrator can be synchronized to an external chain of pulses.
A single pair of active devices can be used to divide a reference by a large ratio, however, the stability of the technique is poor owing to the variability of the power supply and the circuit elements. A division ratio of 10, for example, is easy to obtain but not dependable. Chains of bistable flip-flops provide more predictable division, at the cost of more active elements. While not fundamental to circuit operation, diodes connected in series with the base or emitter of the transistors are required to prevent the base-emitter junction being driven into reverse breakdown when the supply voltage is in excess of the V eb breakdown voltage, typically around volts for general purpose silicon transistors.
In the monostable configuration, only one of the transistors requires protection. Assume all the capacitors to be discharged at first. The voltage at inverting terminal will be greater than the voltage at the non-inverting terminal of the op-amp. This is a comparator circuit and hence, the output becomes -V sat. Now the capacitor discharges towards -V sat. The voltage at the non-inverting terminal will be greater than the voltage at the inverting terminal of the op-amp.
This repeats and forms a free-running oscillator or an astable multivibrator. If V C is the voltage across the capacitor and from the graph, the time period of the wave formed at capacitor and the output would match, then the time period could be calculated in this way:. So, the time period of the square wave generated at the output is:. In the monostable multivibrator, one resistive-capacitive network C 2 -R 3 in Figure 1 is replaced by a resistive network just a resistor.
Q2 collector voltage is the output of the circuit in contrast to the astable circuit , it has a perfect square waveform since the output is not loaded by the capacitor. 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. If repeated application of the input pulse maintains the circuit in the unstable state, it is called a retriggerable monostable. If further trigger pulses do not affect the period, the circuit is a non-retriggerable multivibrator.
For the circuit in Figure 2, in the stable state Q1 is turned off and Q2 is turned on. It is triggered by zero or negative input signal applied to Q2 base with the same success it can be triggered by applying a positive input signal through a resistor to Q1 base.
As a result, the circuit goes in State 1 described above. After elapsing the time, it returns to its stable initial state. The circuit is useful for generating single output pulse of adjustable time duration in response to a triggering signal. The width of the output pulse depends only on external components connected to the op-amp. A diode D1 clamps the capacitor voltage to 0. The diode D1 clamps the capacitor to 0.
Now a negative trigger of magnitude V1 is applied to the non-inverting terminal so that the effective signal at this terminal is less than 0. The diode will now get reverse biased and the capacitor starts charging exponentially to -Vsat through R. The capacitor discharges through resistor R and charges again to 0.
The pulse width T of a monostable multivibrator is calculated as follows: The general solution for a low pass RC circuit is. In the bistable multivibrator, both resistive-capacitive networks C 1 -R 2 and C 2 -R 3 in Figure 1 are replaced by resistive networks just resistors or direct coupling. This latch circuit is similar to an astable multivibrator, except that there is no charge or discharge time, due to the absence of capacitors. Hence, when the circuit is switched on, if Q1 is on, its collector is at 0 V.
As a result, Q2 gets switched off. Thus, the circuit remains stable in a single state continuously. Similarly, Q2 remains on continuously, if it happens to get switched on first. Switching of state can be done via Set and Reset terminals connected to the bases. For example, if Q2 is on and Set is grounded momentarily, this switches Q2 off, and makes Q1 on. Thus, Set is used to "set" Q1 on, and Reset is used to "reset" it to off state.
How Astable Multivibrator Circuits Work
A multivibrator is an electronic circuit used to implement a variety of simple two-state    devices such as relaxation oscillators , timers and flip-flops. It consists of two amplifying devices transistors , vacuum tubes or other devices cross-coupled by resistors or capacitors. Multivibrators find applications in a variety of systems where square waves or timed intervals are required. For example, before the advent of low-cost integrated circuits, chains of multivibrators found use as frequency dividers. A free-running multivibrator with a frequency of one-half to one-tenth of the reference frequency would accurately lock to the reference frequency. This technique was used in early electronic organs, to keep notes of different octaves accurately in tune.
An astable multivibrator has no stable states. Once the Multivibrator is ON, it just changes its states on its own after a certain time period which is determined by the R C time constants. A dc power supply or V cc is given to the circuit for its operation. Two transistors named Q 1 and Q 2 are connected in feedback to one another.