THE MULTI-VIBRATOR 



given gain, g^^L^ we see that a figure of merit for pentodes for this and other 

 types of high speed pulse work is merely g„JCg,^. 



Screen-coupled multi-vibrators — Sometimes it is desirable to make a multi- 

 vibrator work into an inductive load such as a relay coil or lightly loaded 

 transformer. If a triode multi-vibrator is used the presence of the inductance 

 in place of pure resistive loads seriously modifies the mode of operation, 

 and renders it highly dependent on the nature of the inductive load. A 

 solution is to use pentodes and obtain the feedback signals from the screens. 

 The anodes are then free for the driving of almost any kind of load, without 

 affecting the operation of the multi-vibrator proper (Figure 15.11). It must 



HT+ 



Figure 15.11 



be borne in mind that so far as speed of transition is concerned this is a 

 triode, not a pentode, multi-vibrator, as there is now considerable Miller 

 capacitance from screens to control grids. 



Cathode coupled multi-vibrators 



Consider a circuit like Figure 15.12. V^ is a cathode follower and V^ a 

 triode amplifier. If K^ grid is made very negative with respect to earth, V^ 

 is cut off and V^ acts as an amplifier valve automatically biased by R^ to 

 some anode current such that the anode potential is, say, V^. 



HT-f 



HT* 



Figure 15.12 



Figure 15.13 



Now let Fi grid be made positive with respect to earth, perhaps 50 V or 

 so. Ki cathode will follow it, taking V^ cathode positive as well, cutting off 

 Kg, so that the anode potential of Kg rises to HT positive. 



231 



