Principles of operation.
The multivibrator has latterly attained great importance, and much discussion has taken place as to how its principle of action should be presented to students and even as to how it operates, especially as the oscillograms given in several of the most reputable textbooks are misleading and some are incorrect.
Fig. 1. Circuit of a multivibrator, with additional resistances R_{5} and R_{6} inserted.
It is suggested that the principle of action of the multivibrator should be dealt with after the students have seen the cathoderay oscillograms referred to below and that the explanation and the derivation of the waveforms should be based upon the waveforms of anode currents in the asymmetrical multivibrator. These waveforms are easily obtained by inserting additional resistances R_{5} and R_{6}, as shown in Fig. 1. That these resistances have little effect upon the waveforms can be checked by making R_{5} and R_{6} small and using an amplifier stage on the oscillograph. In practice, it is simpler to use values such as those suggested in Fig. 1, thereby avoiding having to switch over from amplifier to 'direct' or vice versa when shifting the oscillograph tappings to the circuit.
Fig. 2. Typical waveforms as developed by the circuit of Fig. 1.
The oscillograms in Fig. 2 are typical waveforms of the PDbs across R_{5}, etc. It is extremely important that the zero axis should be inserted for each oscillogram. This axis can be obtained by merely disconnecting the HT supply.
Let us first assume that the switch S in the anode circuit of V_{2} is open. Anode current flows through V_{1}; C_{2} is charged to the PD existing across R_{5} and V_{1}, and the PD across C_{1} becomes equal to the voltage of the HT supply. The various PDbs are as represented at instant a in Fig. 2.
Suppose the switch to be closed at instant b. The PD across R_{2} due to the anode current of V_{2} reduces the PD across R_{6} and V_{2} so that C_{1} discharges through V_{2} and R_{3}, making the grid end of R_{3} negative relatively to the cathode end. Consequently, the anode current of V_{1} and the PD across R_{1} are reduced. The increased PD across R_{5} and V_{1} sends a charging current to C_{2}, making the grid of V_{2} positive relatively to its cathode, thereby accentuating the growth of anode current in V_{2} responsible for starting the cycle of reactions. The latter, though taking long to describe, occur at such a rate that the anode current of V_{2} increases to its maximum and that of V_{1} falls to zero simultaneously and practically instantaneously.
The anode current of V_{1} remains zero as long as the grid of V_{1} remains sufficiently negative. Owing, however, to C_{1} being small, it discharges quickly, as indicated by curve L. Also, the PD across PE increases instantly at b, and would be equal to the HT voltage had it not been for the voltage drop in R_{1} due to the charging current of C_{2}.
At instant c, the combination of anode and grid voltages on V_{1} is such as to allow anode current to flow, and the reactions, described earlier, follow one another but in the reverse direction. Hence, the anode current of V_{1} rises instantly to its maximum while that of V_{2} falls to zero equally suddenly. Capacitor C_{2} now discharges through V_{1} and R_{4}; but owing to the time constant of circuit C_{2}  R_{4} being far greater than that of C_{1}  R_{3}, C_{2} discharges comparatively slowly. Consequently, the grid of V2 is maintained negative for a correspondingly long time, as shown by curve M in Fig. 2. The shape of M is not quite exponential owing to the variation of the PD across R_{5} and V_{1}.
At instant c, the PD across SE increases almost instantly to that of the HT supply, any delay being due to the charging current of C_{1} flowing through R_{2}. But since C_{1} is relatively small, this delay is also small. That the PD across SE during interval cd is equal to the HT voltage can easily be demonstrated by opening switch S.
Let us next consider what is happening at V_{1} during the interval cd. From the oscillogram for the PD across R_{5}, we find that at c, the anode current suddenly grows to its maximum, as explained above, and then falls to a steady value corresponding to X. This transient effect is due to the corresponding positive potential on the grid of V_{1} caused by the charging current of C_{1}. As the anode current of V_{1} falls to a steady value, the PD across PE increases, as shown by curve N, to a steady value Y. That X and Y represent the corresponding values under static conditions can be checked by opening switch S and noting the deflections when the oscillograph is connected across R_{5} and PE respectively.
At c, the positive potential on the grid of V_{1} is accompanied by grid current of such a value that most of the charging current of C_{1} flows via the grid. Consequently, the corresponding PD across R_{3} is very small; and owing to the smallness of C_{1} the latter is quickly charged and the gridcathode voltage falls to zero accordingly.
At instant d, the negative grid voltage on V_{2} has decreased to such an extent that anode current begins to flow in V_{2}, thereby producing reactions similar to those which occurred when switch S was closed.
During interval bc, most of the charging current of C_{2} flows as grid current in V_{2}, so that the grid potential of the latter becomes only slightly positive. Also, during this interval, the PD across SE is comparatively small owing to the large voltage drop in R_{2} and R_{6}.
It will now be evident that the duration of intervals bc and cd depends upon the rate of discharge of C_{1} and C_{2} respectively. Hence, the frequency of the oscillations is determined mainly by the time constants of circuits C_{1}  R_{3} and C_{2}  R_{4}; While the asymmetry of the oscillations depends upon the relative values of these time constants. Also, it is seen from Fig. 2 that during the latter part of interval cd, valve V_{1} is operating under static conditions and is just waiting for C_{2} to discharge until the grid potential of V_{2} has fallen sufficiently for anode current to commence.
We may summarise the above treatment thus:
 The anode currents of an asymmetric multivibrator can be drawn roughly as complementary rectangles of unequal widths.
 When the anode current is zero, the grid potential is negative but decreasing at a rate depending upon the time constant of the grid capacitor and leak, and the anodecathode voltage tends to rise to the HT value.
 When anode current is flowing, the grid voltage may be slightly positive and decreasing, and the anodecathode voltage tends to rise to its static value.
