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AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2

CHAPTER 6

Free Running (Astable) Multivibrators

The basic circuit of a multivibrator using triodes is illustrated in Fig 5. The important point to notice is the cross-coupling between the anode of one stage and the grid of the other. This form of coupling ensures that the input of each stage is derived from the output of the other. Thus the output from V1 is applied through an impedance Z1 to the grid of V2. V2 provides an amplified and phase-inverted output which is fed back through an impedance Z2 to the grid of V1. The signal is again amplified and phase-inverted through V1 so that an initial signal at V1 anode reappears at the same point greatly amplified and in the same phase. The circuit therefore tends to oscillate. Whether it does so or not depends upon the nature of the cross-coupling impedances Z1 and Z2. In general, if they are both capacitances the circuit is that of a free-running astable multivibrator. If one is a capacitance and the other a resistance the circuit is monostable. If both are resistances the circuit is bistable. We shall now consider the first of these circuits in more detail. The other two circuits, which require a triggering input, are dealt with in the next chapter.

Free-running Anode-coupled Multivibrator

Fig 6 shows the circuit diagram of an astable multivibrator and also the waveforms which appear at the anodes. The action of the circuit, as of all relaxation oscillators, can be divided into two phases:

a. The periods corresponding to the vertical parts of the waveforms, when both valves are conducting and acting as amplifiers with positive feedback from each anode to the other grid. This condition is unstable and initiates a cumulative action or avalanche which very quickly drives one valve well beyond cut-off and causes the other to conduct heavily.

b. The periods corresponding to the 'fiat' parts of the waveforms during which one valve is conducting heavily and the other is cut off but approaching the point where it will cut on. This is the relaxation period.

Let us imagine that the circuit is approaching the end of a relaxation period and that V2 is conducting heavily (with its grid clamped to zero volts by grid current limiting) and V1 is cut off but rising towards its cut-off point. With V2 conducting, its anode voltage will be at a low working value, depending upon the valve current and the values of RL2 and h.t. Also, since V1 is still cut off, V1 anode will be at the same voltage as h.t.+ (no current through RL1 and no voltage drop across it). This condition before the start of an avalanche, is illustrated in Fig 7a.


 

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