The Multivibrator Oscillator

Author: J.B. Hoag

Fig. 14 G. Basic circuit of the multivibrator. Battery voltages omitted for simplicity in explaining its operation

This unusual circuit, shown in Fig. 14 G, contains no inductances; only resistances and capacitances. It really consists of a resistance-capacitance-coupled amplifier with regenerative feedback. In order to understand how this circuit oscillates, let us imagine that the plate of tube 1 momentarily becomes more positive than its static value. This positive potential, acting through C_g, sends electrons on to the grid of tube 2, making the grid more negative and hence decreasing the plate current. Then the voltage drop in the plate resistor R_P2 is reduced. Since this was originally negative at its top, due to the direct current through it, the upper end now becomes less negative, or more positive, than it was under static conditions. This positive impulse, passing through the condenser C, makes the grid of tube 1 more positive than normal. This causes an increased current flow through R_P1. But this makes the top of this resistor more negative than it was. Thus the original excess of positive on plate 1 is reversed. In other words, the second tube serves to reverse the phase of the first tube. The same might be said of the action of tube 1 on tube 2. The frequency of oscillations depends upon the time constants of the R-C combinations.

Inasmuch as this type of oscillator is very unstable, its frequency can be controlled by the introduction into the circuit of a small signal of constant frequency. This is called locking.

Fig. 14 H. One of the many possible, irregular output wave-forms from a multivibrator

As shown in Fig. 14 H, the wave form of the currents generated by a multivibrator circuit is very irregular. This means that the circuit produces many harmonics of considerable strength, along with the fundamental frequency of oscillation. It is not at all uncommon to lock-in this circuit at its tenth harmonic.