LC OSCILLATORS 



supply {Figure 14.2). On first switching on, C^ is uncharged, the valve 

 receives no mean bias and the loop gain is arranged to be amply large for 

 oscillations to begin and to grow. When the point A swings positive the grid 

 will also be carried positive and grid current will flow into Q and charge it. 



HT + 



HT + 



Figure 14.1 



Figure 14.2 



The effect of this charging will be such as to cause the grid to become less 

 positive, as shown in Figure 14.3. The time constant C2R2 is chosen so that it 

 would take much longer than the time of one cycle of oscillation for the charge 

 on C2 to leak away via R^*. Thus when A swings negative again Cg remains 



flf-© 



Anode 1 

 current I 



Figure 14.3 



Figure 14.4 



charged to approximately the peak voltage developed across Lg. Thus the 

 voltage to which Cg charges represents a steady bias on the valve of magnitude 

 proportional to the amplitude of oscillation. 



As the oscillations grow a point is reached at which, on the extreme negative 

 excursions of grid voltage, the valve is cut off altogether {Figure 14.4). It is 

 now clear that the form of the anode current is of the form of a fundamental 

 wave of frequency co plus harmonics. More important, as a result of the 

 clipping of the waveform, a given proportional increase in the valve input can 

 no longer produce a commensurate increase in anode current swing, that is, 

 the effective valve gain is reduced. The amplitude of oscillations is therefore 

 automatically stabilized, for any tendency to increase further is offset by a 

 further reduction in effective valve gain, and vice versa. 



The equilibrium level of oscillation and the form of the anode current wave 

 depend on the magnitude of Q, M and the valve gain. If this product is 

 barely sufficient to maintain oscillation the output will be small and the anode 

 current might have the form of Figure 14.5a. This is clearly substantially class 



But see under Blocking Oscillators 



209 



