Astable Multivibrators

Astable Multivibrators

Regenerative switching circuits such as Astable Multivibrators are the most commonly used type of relaxation oscillator as they produce a constant square wave output waveform as well as their simplicity, reliability and ease of construction. Unlike the Monostable Multivibrators and Bistable Multivibrators we looked at in the previous tutorials that require an "external" trigger pulse for their operation, Astable Multivibrators switch continuously between their two unstable states at a constant repetition rate without the need for any external triggering. Then, Astable Multivibrators have NO stable states and are therefore also known as Free-running Oscillators that produce a continuous square waveform from their output or outputs, (two outputs no inputs).

The basic transistor circuit for Astable Multivibrators produces a square wave output from a pair of grounded Emitter cross-coupled transistors. Both transistors either NPN or PNP, in the multivibrator are biased for linear operation and are operated as Common Emitter Amplifiers with 100% positive feedback and satisfying the condition for oscillation when: ( βA = 1∠ 0^o ). This results in one stage conducting "fully-ON" (Saturation) while the other is switched "fully-OFF" (cut-off) giving a very high level of mutual amplification between the two transistors. Conduction is transferred from one stage to the other by the discharging action of a capacitor through a resistor as shown below.

Basic Astable Multivibrator Circuit

Assume that transistor, TR1 has just switched "OFF" and its Collector voltage is rising towards Vcc, meanwhile transistor TR2 has just turned "ON". Plate "A" of the capacitor C1 is also rising towards the 6 volts supply rail of Vcc as it is connected to the Collector of TR1. The other side of capacitor, C1, plate "B", is connected to the Base terminal of transistor TR2 and is at 0.6v because transistor TR2 is conducting therefore, capacitor C1 has a potential difference of 5.4 volts across it, 6.0 - 0.6v, (its high value of charge). The instant that transistor, TR1 switches "ON" plate "A" of the capacitor immediately falls to 0.6 volts. This fall of voltage on plate "A" causes an equal and instantaneous fall in voltage on plate "B" therefore plate "B" of the capacitor C1 is pulled down to -5.4v (a reverse charge) and this negative voltage turns the transistor TR2 hard "OFF".

Capacitor C1 now begins to charge in the opposite direction via the resistor R3 which is also connected to the 6 volts supply rail, Vcc, thus the Base of transistor TR2 is moving upwards in a positive direction towards Vcc with a time constant equal to the R3.C1 combination. However, it never reaches the value of Vcc because as soon as it gets to 0.6 volts positive, transistor TR2 turns fully "ON" into saturation starting the whole process over again but now with capacitor C2 taking the Base of transistor TR1 to -5.4v while charging up via resistor R2 with the cycle repeating itself over and over again as long as the supply voltage is present.

The amplitude of the output waveform is approximately the same as the supply voltage, Vcc with the time period of each switching state determined by the time constant of the RC networks connected to the Base terminals of the transistors. As the transistors are switching both "ON" and "OFF", the output at either collector will be a square wave with slightly rounded corners because of the current which charges the capacitors. This could be corrected by using more components as we will discuss later.

If the two time constants of R2.C2 and R3.C1 in the Base circuits are the same, the Mark-to-Space ratio ( t1/t2 ) will be ONE-to-ONE making the output waveform symmetrical. By varying the capacitors, C1, C2 or the resistors, R2,R3 the mark-to-space ratio and therefore the frequency can be altered. We saw in the RC Discharging tutorial that the time taken for the voltage across a capacitor to fall to half the supply voltage, 0.5Vcc is equal to 0.69 time constants of the capacitor and resistor combination. Then taking one side of the astable multivibrator, the length of time that transistor TR2 is "OFF" will be equal to 0.69T or 0.69 times the time constant of R3.C1. Likewise, the length of time that transistor TR1 is "OFF" will be equal to 0.69T or 0.69 times the time constant of R2.C2 and is given as.

Astable Multivibrators Periodic Time

Where, R is in Ω's and C in Farads.

By altering the time constant of just one RC network the mark-to-space ratio and frequency of the output waveform can be changed but normally by changing both RC time constants together at the same time, the output frequency will be altered keeping the mark-to-space ratios the same at one-to-one.

If the value of the capacitor C1 equals the value of the capacitor, C2, C1 = C2 and also the value of the resistor R2 equals the value of the resistor, R3, R2 = R3 then the total length of time of the Multivibrators cycle is given below for a symmetrical output waveform.

Frequency of Oscillation

Where, R is in Ω's, C is in Farads, T is in seconds and ƒ is in Hertz.

and this is known as the "Pulse Repetition Frequency". So Astable Multivibrators can produce TWO very short square wave output waveforms from each transistor or a much longer rectangular shaped output either symmetrical or non-symmetrical depending upon the time constant of the RC network as shown below.

Astable Multivibrator Waveforms

Example No1

An Astable Multivibrators circuit is required to produce a series of pulses at a frequency of 500Hz with a mark-to-space ratio of 1:5. If  R2 = R3 = 100kΩ's, calculate the values of the capacitors, C1 and C2 required.

and by rearranging the formula above for the periodic time, the values of the capacitors required to give a mark-to-space ratio of 1:5 are given as:

The values of 4.83nF and 24.1nF respectively, are calculated values, so we would need to choose the nearest preferred values of C1 and C2 allowing for the tolerance. In fact due to the wide range of tolerances associated with the humble capacitor the actual output frequency may differ by as much as ±20%, (400 to 600Hz in our example).

If we require the output waveform to be non-symmetrical for use in timing or gating circuits etc, we can manually calculate the values of R and C for the individual components required as we did in the example above. However, when the two R's and C´s are both equal, we can make our life a little easier by using tables to show the astable multivibrators calculated frequencies for different combinations or values of both R and C. For example,

Calculated Output Frequencies with equal Resistor and Capacitor values

Pre-calculated frequency tables can be very useful in determining the required values of both R and C for a particular symmetrical output frequency without the need to keep recalculating them every time a different frequency is required. By changing the two fixed resistors for a dual-ganged potentiometer and keeping the values of the capacitors the same, the frequency from the Astable Multivibrators output can be more easily "tuned" to give a particular frequency value or to compensate for the tolerances of the components used. For example, using a capacitor value of 10nF from the table above, by using a potentiometer of say, 100kΩ's we would get an output frequency that is adjustable from slightly above 71.4kHz down to 714Hz, some 3 decades of frequency range.

When selecting both the resistance and capacitance values for reliable operation, the Base resistors should have a value that allows the transistor to turn fully "ON" when the other transistor turns "OFF". For example, consider the circuit above. When transistor TR2 is fully "ON", (saturation) nearly the same voltage is dropped across resistors R3 and resistor R4. If the transistor has a current gain, β of 100 and the Collector load resistor, R4 is equal to say 1kΩ the maximum Base resistor value would therefore be 100kΩ. Any higher and the transistor may not turn fully "ON" resulting in the multivibrator giving erratic results or not oscillate at all. Likewise, if the value of the Base resistor is too low the transistor may not switch "OFF" and the multivibrator would again not oscillate.

An output signal can be obtained from the Collector of either transistor in the Astable Multivibrators circuit with each output waveform being a mirror image of itself. We saw above that the leading edge of the output waveform is slightly rounded and not square due to the charging characteristics of the capacitor in the cross-coupled circuit. But we can introduce another transistor into the circuit that will produce an almost perfectly square output pulse and which can also be used to switch higher current loads or low impedance loads such as LED's or loudspeakers, etc without affecting the operation of the actual astable multivibrator. However, the down side to this is that the output waveform is not perfectly symmetrical as the additional transistor produces a very small delay. Consider the two circuits below.

Astable Multivibrators Driving Circuit

An output with a square leading edge is now produced from the third transistor, TR3 connected to the Emitter of transistor, TR2. This third transistor switches "ON" and "OFF" in unison with transistor TR2. We can use this additional transistor to switch Light Emitting Diodes, Relays or to produce a sound from a Sound Transducer such as a speaker or Piezo sounder as shown above.

The load resistor, Rx needs to be suitably chosen to take into account the forward volt drops and to limit the maximum current to about 20mA for the LED circuit or to give a total load impedance of about 100Ω's for the speaker circuit. The speaker can have any impedance less than 100Ω's. By connecting an additional transistor, TR4 to the Emitter circuit of the other transistor, TR1 we can produce a circuit that will flash alternatively two sets of lights or LED's at a rate determined by the time constant of the RC network.