Astable multivibrator

An astable multivibrator , also known as an astable multivibrator , is an electronic circuit that only knows two states as an output signal, between which it switches periodically. For this purpose, it contains two RC elements as time-determining circuit parts . These flip-flops are often constructed symmetrically so that they have complementary outputs.

Terms and way of working

There are three variants of the tilting stages:

Bistable flip-flop : your state changes due to an external stimulus. It remains in this until a further suggestion is made, regardless of time.
Monostable multivibrator : Their state changes due to an external stimulus. It remains in this for a period of time specified by the circuit, then it tilts back to its initial or basic state. It remains in this until another suggestion.
Astable flip-flop: Without an external stimulus, its state changes. It remains in this for a time specified by the circuit, then it tips over again. Here, too, it remains for a set time until it tips over again.

Astable multivibrators basically consist of two electronic switches that are mutually connected in such a way that positive feedback is created, which brings the two switches into opposite states (one closed, the other open). The respective electrical voltage that caused the positive feedback is reduced by timing elements ; after a delay, the initial state changes. If this also tilts after a while, a periodic behavior occurs. This means that the circuit also belongs to the relaxation oscillators . The frequency results from the two delay times.

Henri Abraham and Eugène Bloch are considered to be the inventors of the astable multivibrator (square-wave oscillator) .

Circuits and properties

Typical circuits with discrete components

Multivibrator with electromechanical switches

R ₁ and R ₂ are resistors, K ₁ and K ₂ are the coupling elements with a delay function, they can be resistors or capacitors . The two states of the multivibrator are:

S ₁ switched on and S ₂ switched off,
S ₁ switched off and S ₂ switched on.

Both coupling elements are capacitors that reload faster or slower depending on the capacitance and the resistance circuit. Thus, either of the two states is not stable and the circuit alternately toggles between the two states. The times that the circuit spends in each of the two states can be changed by the size of R ₁ , K ₁ and R ₂ , K ₂ . If R ₁ · K ₁ = R ₂ · K ₂ , then it is the same length in both.

The resulting almost ideal square waves have a high proportion of harmonics . With this circuit, the generated fundamental frequency depends heavily on the operating voltage used.

In this representation, the multivibrator can be understood as an interconnection of two monostable multivibrators.

Astable multivibrator with transistors

Multivibrator with bipolar transistors; instead of + V , the article speaks of U _B

A similar multivibrator circuit was implemented with the “ Lectron ” electronics experiment system from the 1960s

In discrete circuit technology , electronic circuits are implemented using individual transistors . Here the functionality of an astable multivibrator is explained using an example with bipolar npn transistors .

In the currentless circuit, the transistors Q1 and Q2 are blocking, so their resistance value (from collector to emitter) is almost infinite. The capacitors C1 and C2 are initially discharged. R2 and R3 are chosen so that the bases of the transistors get enough current to be able to control. R1 and R4 limit the working current. The switching frequency of this multivibrator is determined by the values of R2 , C1 and R3 , C2 . The resistance values of R2 and R3 are considerably larger than R1 and R4 . The digital signal is picked up at one of the two collectors. The operating voltage of the circuit shown here is limited to the maximum permissible negative base-emitter voltage of the transistors used, i.e. it must not be more than 5 ... 6 V. In order to be able to use the circuit with a higher supply voltage, an additional, correspondingly voltage-proof diode can be provided in series with each base connection without any fundamental changes in the functional principle.

Switch-on behavior

When the operating voltage U _B is applied, current first flows via R1, C1 in parallel to R2 via Q2 and, secondly, from R4, C2 parallel to R3 via Q1. From a certain base current one of the transistors first becomes conductive and draws the base of the other transistor towards 0 V via its collector-side connected (and not yet significantly charged) capacitor , via whose base no more current flows and thus into the non-conductive one Condition device.

Which transistor becomes conductive first depends on the specific component values, above all on the transistors, some of which can have considerable characteristic value tolerances.

While a transistor is now conducting, its base receives current via the corresponding capacitor until it has been charged, and at the same time via R2 or R3. These resistors are there to deliver quiescent current to the transistors in the switched-on state, regardless of the state of charge of the capacitor, so that they still switch easily through when the capacitor is full. This capacitor then has approximately the voltage U _B - 0.7 V between its poles. The 0.7V is the base-emitter forward voltage .

The other capacitor is charged during this time via R2 or R3 , so that the voltage at the base of the blocking transistor increases slowly until its base-emitter threshold voltage is reached at approx. 0.6 V. This is the voltage from which the blocking transistor begins to turn on.

This phase, as described so far, occurs only once after each switch-on and is of considerably shorter duration than the two states described below. After switching on, the circuit begins its periodic behavior. It alternately toggles between two time-limited states, here arbitrarily called state A and state B , with transistor Q1 conducting in state A and transistor Q2 conducting in state B.

Condition a

Q1 is conductive here and thus its collector-emitter voltage drops from U _B to approx. +0.2 V ( collector-emitter saturation voltage ). This also pulls the collector-side plate of C1 from U _{B down} to +0.2 V, i.e. by U _B - 0.2 V, the other plate by the same difference. But since the side of the plate towards the base Q2 has a potential U _B - 0.7 V lower than the other side, suddenly 0.2 V - ( U _B - 0.7 V), i.e. - U _B + 0 , are present on it , 9 V. This is well below zero, and therefore Q2 locked until C1 via R2 has reloaded again slowly and at the base of Q2 lie approximately 0.65 V, which begins therefore by heading and the circuit in state B tilt leaves. In the meantime, C2 charges itself via R4 to a plate voltage of U _B - 0.7 V (collector Q2 has U _B , base Q1 0.7 V).

The jumps in potential of the capacitors during the tilting process cause positive feedback and thereby shorten the tilting processes, i.e. H. increase the switching speed.

The duration of state A is determined by C1 and R2 , since C1 has to charge from about −U _B + 0.9 V to about 0.65 V via R2 so that Q2 can flip the circuit.

Condition B

C1 is charged via R2 until the voltage at the base of Q2 exceeds the base-emitter threshold voltage of approx. +0.6 V and Q2 therefore switches to the controlled state. This pulls the right side of C2 down from U _B to about 0.2V. Due to the potential difference between the plates (the left plate had about U _B - 0.7 V less than the right one) the left plate of C2 now has about 0.2 V - (U _B - 0.7 V), i.e. - U _B + 0.9 V. this is well below zero, thereby disabling Q1 now until this plate side via R3 again exceeds approximately +0.65 V and in Q1 the circuit in state A, tilts. By turning Q1 into the blocking state, C1 charges itself via R1 and the base of Q2 to a plate potential of U _B - 0.7 V (left side has U _B , the right side +0.7 V). At the same time, the holding current flows via R2 in order to keep Q2 open even if there is no longer sufficient current flowing via C1 and the time until Q1 has to be bridged.

Here, too, the capacitors shorten the tilting process through positive feedback.

The duration of state B depends on the values of C2 and R3 and lasts until C2 has been reloaded via R3 from −U _B + 0.9 V to approx. +0.65 V.

Calculation of the time periods

The left side of C2 is at the beginning of state B at about −U _B and should be reloaded to + U _B ; the state tilts at around 0 V (more precisely 0.7 V), i.e. around half of this recharging process. The up / unloading or reloading of a capacitor through a resistor takes place after an exponential rate law : . The duration for half of the reloading corresponds exactly to the half-life , see also time constant . ${\ displaystyle U (t) = U _ {\ text {end}} + \ left (U _ {\ text {beginning}} - U _ {\ text {end}} \ right) \ mathrm {e} ^ {- {\ frac {t} {R \ cdot C}}}}$ ${\ displaystyle T _ {\ mathrm {H}} = \ ln \ left (2 \ right) \ cdot R \ cdot C}$

Period duration and switching frequency of the astable multivibrator

The period of an astable multivibrator results from the time periods and the two individual switching states: ${\ displaystyle T}$ ${\ displaystyle T_ {1}}$ ${\ displaystyle T_ {2}}$

{\ displaystyle T_ {1} \ approx \ ln (2) \ cdot R_ {2} C_ {1}}

or.

{\ displaystyle T_ {2} \ approx \ ln (2) \ cdot R_ {3} C_ {2}}

.

{\ displaystyle T = T_ {1} + T_ {2} \ approx \ ln (2) \ cdot (R_ {2} C_ {1} + R_ {3} C_ {2})}

.

With symmetrical switching, i.e. as well as , this simplifies to: ${\ displaystyle R = R_ {2} = R_ {3}}$ ${\ displaystyle C = C_ {1} = C_ {2}}$

{\ displaystyle T \ approx 2 \ times \ ln (2) \ times RC}

The frequency results from: ${\ displaystyle f}$

{\ displaystyle f = {\ frac {1} {T}} \ approx {\ frac {1} {2 \ cdot \ ln (2) \ cdot RC}} \ approx {\ frac {1} {1 {,} 386 \ cdot RC}}}

.

Circuits with integrated components

Astable multivibrator with NE555

Circuit with NE555

The following circuit also generates a square-wave voltage, but has a simpler structure than the multivibrator shown above and has the advantage that the frequency is almost independent of the operating voltage. This can be in the range 0.1 Hz to 500 kHz and can be varied very widely with just a single potentiometer . The function of the NE555 module used can be described as follows: As long as the voltage on the capacitor C is less than 66% of the operating voltage, it is charged via R (series connection of potentiometer and 1 kΩ resistor). The output voltage at pin 3 is approximately the operating voltage during this time. If this 66% value is exceeded, an internal flip-flop tips over, the output voltage drops to 0 volts and the capacitor is discharged via R. As soon as the operating voltage falls below 33%, the flip-flop flips back into its original position and the game starts all over again. The voltage across the capacitor has approximately the shape of a triangle, but can only be loaded slightly.

With a 20 kΩ potentiometer, the generated frequency can be changed in a ratio of about 1:20. Doubling the capacity halves the generated frequency. By changing the voltage at pin 5 (nominal value: 66% of the operating voltage), the frequency can be changed electronically ( voltage controlled oscillator ). A frequency modulation can be achieved using an alternating voltage at this connection ("Kojak siren").

Astable multivibrator with logic gates

Astable multivibrator with Schmitt trigger gate

An astable multivibrator can also be built with logic gates. It may NAND , NOR gridset or inverters are used.

The adjacent circuit generates square waves with a frequency dependent on R and C as well as the switching threshold difference if the current through R is large compared to the input bias current of the circuit. With CMOS circuits (e.g. 1/6 of the CD40106) this is also the case with a large R (e.g. 100 kΩ).

Applications

As a signal generator for generating square wave oscillations: By changing the time-determining elements or the switching thresholds, the frequency and / or the duty cycle can be changed.

In sensor technology and telemetry : If constructions are used for the frequency-determining resistors and capacitors, the value of which depends on a physical quantity, pulse sequences can be generated in this way, the pulse length or pause length of which depends on this quantity. Such pulse signals can be evaluated by an evaluation circuit (e.g. a microcontroller) with regard to the pulse parameters, and conclusions can be drawn about the physical quantities (e.g. temperature, air pressure). The pulse can be transmitted on long cables or modulated onto an electromagnetic wave ( radio signal or fiber optic cable ). The advantage over analog sensor signals is the lower-interference transmission.

As a blink generator in signal lamps or as a tone generator in signal horns (e.g. piezoelectric sounders)

Special forms

Astable multivibrators are also available in special forms in which three or more active components are involved (multi-phase multivibrators).

Web links

Commons : Multivibrators - Collection of pictures, videos and audio files

elektronik-kompendium.de: Flashing circuit - print and wiring tester (astable multivibrator, volume with PWM, 741) - sample circuits
An explanation (Java required)

Individual evidence

↑ Erwin Böhmer, Dietmar Ehrhardt, Wolfgang Oberschelp: Elements of applied electronics. 16th ed., Vieweg + Teubner, 2010, p. 218 ff.
↑ Hans-Jürgen Gevatter (Ed.): Automation technology 2: devices. Springer, 2000, p. 191
↑ Bodo Morgenstern: Electronics 3: Digital circuits and systems. 2nd ed., Vieweg, 1997, pp. 62 and 78 f.
^ Henri Abraham, Eugène Bloch: Mesure en valeur absolue des périodes des oscillations électriques de haute fréquence. In: J. Phys. Theor. Appl. Vol. 9, 1919, pp. 211–222 ( journaldephysique.org ; PDF, accessed on March 24, 2018).
↑ https://www.onsemi.com/pub/Collateral/P2N2222A-D.PDF Emitter − Base Breakdown Voltage in the data sheet of the 2N2222
^ Ulrich Tietze, Christoph Schenk: Semiconductor circuit technology, 12th edition, p. 604f

[1] Erwin Böhmer, Dietmar Ehrhardt, Wolfgang Oberschelp: Elements of applied electronics. 16th ed., Vieweg + Teubner, 2010, p. 218 ff.

[2] Hans-Jürgen Gevatter (Ed.): Automation technology 2: devices. Springer, 2000, p. 191

[3] Bodo Morgenstern: Electronics 3: Digital circuits and systems. 2nd ed., Vieweg, 1997, pp. 62 and 78 f.

[4] Henri Abraham, Eugène Bloch: Mesure en valeur absolue des périodes des oscillations électriques de haute fréquence. In: J. Phys. Theor. Appl. Vol. 9, 1919, pp. 211–222 ( journaldephysique.org ; PDF, accessed on March 24, 2018).

[5] ttps://www.onsemi.com/pub/Collateral/P2N2222A-D.PDF Emitter − Base Breakdown Voltage in the data sheet of the 2N2222

[6] Ulrich Tietze, Christoph Schenk: Semiconductor circuit technology, 12th edition, p. 604f