SQUARE WAVE OSCILLATORS 



Let the least negative potential at which V^ is cut off, and the least positive 

 potential at which V^ is cut off, be called Vj^ and Vq respectively. Then when 

 Ki grid is between K^ and Vq the circuit will behave as an approximately 

 linear amplifier, and if a feedback path be established between Fg anode and 

 Ki grid, the feedback is clearly positive. 



In the cathode coupled multi-vibrator the feedback is via a capacitance C, 

 a leak R being provided from Fj grid to earth (Figure 15.13). Circuits of 

 this kind can operate under either of two regimes, according to whether or 

 not Fi passes significant amounts of grid current. The frequency is deter- 

 mined largely by the product CR; the mark-space ratio by the regime. 



Regime 1 — no grid current — In this case the circuit performs oscillations 

 with a mark-space ratio of approximately unity, the waveforms having the 

 form of Figure 15.14. The amplitude of the output is (HT+) — F^, that is, 



l^ anode 



V\ grid 



Figure 15.14 



the voltage drop across the anode load when Fg is conducting hard. We 



have for F 



2» 



Anode current oc r-^- approximately 



1 



and so 



oc 



anode current oc 



anode current X /?o 



1 



{R,fl^ 



Multi-vibrator output = Voltage drop in R^ == Anode current X R^ 



n 



cc ^^^2 roughly 



If we try — by reducing R^ — to increase the amplitude of the output beyond a 

 certain point, the circuit moves into the other regime and the mark-space 

 ratio changes rapidly. The output has the appearance o^ Figure 15.15. The 

 explanation is as follows: for satisfactory cathode follower action to occur 

 in Fi, Fi cathode must always be able to supply enough current to keep its 

 own potential positive with respect to F^ grid. When R^ is reduced the 

 current which the cathode has to be able to emit to follow a given positive 

 grid potential has also to increase. As we reduce R^ a point is reached at 



232 



