ASTABLE MULTIVIBRATORS-AN INTRODUCTION

EDITOR: B. SOMANATHAN NAIR

1. INTRODUCTION

Multivibrators belong to the class of non-sinusoidal oscillators that produce square waveforms. The Fourier-series expansion of a square wave reveals that it can be treated as the sum of an infinite number of multiple sinusoidal (or harmonic) vibrations and hence the name multivibrator. It may be remembered in this context that any non-sinusoidal waveform can be analyzed to contain an infinite number of harmonics; hence all such non-sinusoidal waveform generators are candidates of being called as multivibrators. However, only those non-sinusoidal oscillators that produce square waves have been designated as multivibrators; other non-sinusoidal oscillators have different names.

Multivibrators are classified as astable, bistable, and monostable, respectively. Of these, the astable multivibrators are the square-wave generators (or square-wave oscillators); only these circuits produce waveforms on their own. Bistable circuits are not self-generating circuits; they produce square waves only as outputs on the application of trigger input pulses. Monostable multivibrator is a half-astable, half-bistable circuit; it self-generates one (and only one) square pulse upon getting triggered.

2. GENERATION OF SQUARE WAVE FROM SINE WAVE

Consider a multistage amplifier having a gain of, say 10⁶, as shown in Fig. 1. Let the supply voltage of the amplifier be fixed at ±10 V. We assume that the Q-point is fixed in the middle of the active region of V­-I characteristic of the amplifier. The amplifier will then deliver a maximum of ±10 V as undistorted output voltage. Let us now apply a sinusoidal voltage V_i having a peak-to-peak amplitude of 10 mV at the input of the amplifier. Since the gain of the amplifier is 10⁶, the corresponding undistorted output voltage V_o is the maximum of ±10 V. This undistorted output sine wave is shown in Fig. 2.

Assume that the input is now increased to 100 mV. With the gain of 10⁶, the amplifier should amplify this into a 100-V output sine wave. However, since the amplifier can deliver a maximum output of only ±10 V (since the power supply is only ±10 V), it is not possible to get a 100-volt sinusoidal output from this amplifier; instead, what we would get at the output would be an approximate square wave with its peak amplitude limited to ±10 V. This is shown in Fig. 3 with blue-colored lines, which also shows the sine wave of 100-V amplitude expected from the amplifier drawn in red-colored lines. This is explained further below.

We find that the amplifier amplifies the input sine wave into an output sine-wave of amplitude ±100 V. However, when the output amplitude exceeds its supply-voltage maximum limit of ±10 volts, the amplifier keeps the output constant at ±10 V, as it cannot deliver an output more than this value. The output will hence remain fixed at ±10 V till such time within which the amplitude would have risen to ±100 V and returned to ±10 V in sinusoidal fashion. We thus find that the output is a sine wave clipped at ±10 V. A clipped sine wave can be considered as an approximate square wave. This is clear from Fig. 3.

Now, suppose the input is increased to 1 mV. Then, for the same gain of 10⁶, the output must be ±1000 V. But, as discussed above, the output is forced to remain constant at ±10 V. Therefore, in this case also, we will get a square wave output. This is shown in Fig. 4 in blue lines. However, in this case, the leading (trailing) edge of the wave can be seen to rise (fall) more sharply than before. This is because the wave in this case has to rise to an amplitude of ten times more than that of the wave in the previous case; this makes it more steeply rising than before. 

It may then be noted that ideally a perfect square wave with sharp vertical edges can be generated only with an amplifier of infinite gain! However, in practical cases because of the very large gain, the transistors will be operating in the saturation and cut-off modes. In moving from cut-off to saturation and vice versa, the transistors will have to cross through their active region of operation; which requires some time. This results in making the generated wave always to have some kind of sloping edges with finite rise time; the higher the gain, the shorter this crossing time and hence the smaller the rise time.

3. ASTABLE MULTIVIBRATOR AS A TWO-STAGE AMPLIFER CONNECTED IN THE BACK-TO-BACK MODE

The operation of the astable [to be pronounced as (h)ay-stable] multivibrator may be explained in a variety of ways. Here, we explain its operation by considering it as a two-stage positive-feedback amplifier having very high gain. For this, consider an astable multivibrator constructed using two RC-coupled amplifiers, connected in the back-to-back mode. In this mode, the output of one amplifier is connected to the input of the other, as shown in Fig. 5. This configuration forms a closed-loop positive-feedback system, and hence will produce sinusoidal oscillations as per the general theory of oscillations. However, since the loop gain is very high, the resulting oscillations will be in the form of square waves, as described in Section 2. This statement also implies that this circuit can be designed to produce sinusoidal oscillations, if we reduce the gain to unity; unity gain can be achieved by adding a potentiometer of value enough to reduce the overall loop gain to unity in the feedback path, and adjusting it.

            In the forthcoming blogs, we will be discussing more on multivibrators.