THE MULTI-VIBRATOR 



specifically for the production of square waves*. This circuit is the multi- 

 vibrator; it has the additional advantage that the mark-space ratio is variable, 

 whereas in the case of the squared-up sine wave it is necessarily about unity. 



THE MULTI-VIBRATOR 



The multi-vibrator exists in two forms, the symmetrical and the cathode 

 coupled. 



Symmetrical multi-vibrators 



The symmetrical version is shown in Figure 15.4. The circuit oscillates 

 because the feedback is clearly positive, and the degree of feedback is much 

 higher than that required just to maintain oscillations. Thus the level of 



-^r 



HT+ 



* ^ri 



Figure 15.4 



oscillation is limited only by the maximum excursions of which the valve 

 anodes are capable, i.e. from HT positive (valve cut off) to a level of anode 

 potential corresponding to zero, or slightly positive, grid bias, We shall call 

 this latter state 'conducting hard'. 



This circuit and those which we shall consider in this and the next chapter 

 are all non-linear ones in which the anode potential of a valve is not a 

 replica of the grid potential, as it has been in most of the valve circuits we 

 have dealt with up to now. With the non-linear or 'pulse' circuits we often 

 find valves being used in the rather simple role of switches. They are either 

 'off' (or 'cut off') or 'on' (or 'conducting hard'). 



Design procedure for pulse circuits may be carried out along the same 

 general lines as for amplifier circuits. That is to say, a working region is 

 constructed on the valve anode characteristic and a load line drawn. Because 

 non-linear operation is intended the working region extends to the anode 

 voltage axis and somewhat outside the characteristic for zero grid bias. 

 'Off' and 'on' are represented by the blobs on the load line {Figure 15.5). 



High speed of operation on transients in pulse circuits is secured by, 

 amongst other things, the use of anode loads of rather low resistance, of 

 the order 5-10 kQ instead of 50-100 kQ.. This has the effect of making the 

 load line steep, and in order to secure a swing of anode voltage comparable 

 with that of Figure 15.5 a high anode current and anode dissipation become 



* Except in cases where the frequency has to be very stable, when it is best to square up 

 the output of a crystal oscillator. 



227 



