SINE WAVE OSCILLATORS 



A operation, which the reader will remember is associated with poor efficiency 

 because of the wasteful steady component of anode current. If now the Q-M 

 gain product is increased — most conveniently by increasing M — the oscil- 

 lations will be larger and the anode current might resemble Figure 14.5b. 



(c) 



This is class B working, in which the steady component of anode current has 

 been eliminated, leading to much better circuit efficiency. Finally, if M is 

 made any larger, the anode current has the form of Figure 14.5c, in which the 

 current flows only in short bursts. This is class C working, in which the 

 valve gives itself a mean bias beyond cut off. This is the most efficient mode 

 of working of all. 



At this point it may be objected that the output of an oscillator having an 

 anode current of the form of Figure 14.5c can surely not be very sinusoidal. 

 The answer is, it all depends on the Q of the tuned circuit. If the Q is 100, 

 then the fundamental component of the anode current will be magnified 

 100 times whereas the harmonics will not, with the result that the voltage 

 wave across the tuned circuit is nevertheless of substantially pure sine form. 

 If the Q is poor, the purity of the output will also be poor. 



Hartley oscillator 



The circuit so far discussed is not in fact much used ; in practice two coils 

 are not necessary, as the same effect may be had by employing a tuned circuit 

 with a tapping point. If the tapping point is on the inductance the circuit is 

 called the Hartley oscillator. The version in Figure 14.6 is called the 'series 

 fed' Hartley oscillator because part of the tuned circuit is in series with the 

 HT supply to the valve. The 'shunt fed' Hartley circuit appears in Figure 14.7. 

 It has the advantage, in some applications, that the tuned circuit is at earth 

 potential so far as direct voltages are concerned. Cg is required to block the 

 HT and exhibit negligible reactance at the oscillation frequency, while the 



210 



