THE MULTI-VIBRATOR 



In the symmetrical multi-vibrator the circuit alters very rapidly between 

 the states Vi conducting, Fg cut off, and Kg conducting, V^ cut off. The 

 times for which it dwells in each of these states depend on the time constants 

 C^Ri and C2R2 respectively. Let us suppose V-^ has just been switched on 

 and V2 off. Vi anode potential has fallen to its low 'conducting hard' value 

 and has taken, via Q, Kg grid to a potential far below cut off (A in Figure 

 15.7). Current will flow into the right-hand plate of Q, via R^, causing it 



Earth 

 cut-off ~ 



anode 



Earth 

 cut-off 



Figure 15.7 



and Kg grid to return exponentially towards earth. When V^ grid has reached 

 cut off, anode current begins to flow in Kg and V^ anode begins to fall in 

 potential. This carries V^ grid negative, reducing Kj anode current and 

 begins to raise V^ anode. This rise is communicated to F2 grid so that a 

 cumulative action is initiated as a result of which K^ is completely cut off 

 and Kg is switched to hard conduction {B in Figure 15.7). Notice that V^ 

 grid is carried shghtly positive at this instant. Grid current then flows into 

 Kg from Q, re-estabhshing the potential difference across it at its value just 

 before A. 



The other half of the cycle takes place in similar fashion, the roles of Fj 

 and Kg being reversed {B to A' in Figure 15.7). 



Now the length of the period AB depends on the time it takes V^ grid to 

 rise to cut-off bias, which in turn depends on the time constant R^C^ and on 

 how far negative V^, was originally carried. The latter quantity depends on 

 the magnitude of the negative swing at Fj anode, which is strongly dependent 

 on the HT and the parameters of F^. Similarly BA' depends on R^ and Cg, 

 the HT and the parameters of V^. From this it should be clear that whilst 

 with this circuit the mark-space ratio can easily be varied by differential 

 variation of the products R^C^ and R^C^, the frequency of oscillation is not a 



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