LC OSCILLATORS 



'choke' inductance is required to exhibit a high reactance at the oscillation 

 frequency so that the a.c. load seen by the valve is still substantially the 

 tuned circuit. 



In both series and shunt-fed Hartley circuits the position of the tap deter- 

 mines the amount of feedback to the grid and hence the level of oscillation and 



HT+ 



Figure 14.6 Figure 14.7 



the class of valve operation. Starting at the bottom, moving the tap up the 

 coil increases the feedback, and the loop gain up to a maximum; thereafter 

 the loop gain falls again because the grid leak, in parallel with the valve input 

 resistance, is reflected across the tuned circuit by autotransformer action as a 

 progressively lower resistance, thus damping it and reducing the Q. The 

 best position for the tap is usually found by experiment; about 1/5 of the way 

 up is reasonable for a first guess. 



Colpitis oscillator 



In the Colpitts circuit {Figure 14.8) which must necessarily be shunt-fed, 

 the tuned circuit capacitance is tapped instead of the inductance. If the total 

 tuning capacitance required is C, then the capacitances C„ and Q must be 

 related to C by C = (C„Q)/(C„ + Q), and if the equivalent distance up the 

 inductance for the tapping point is to be l/«th, then /i = Q + CJC^. 



HT* 



Choke 



-^♦'i/i- = JT 



Ci 



I 





y^ 



X c, 



^ 



Figure 14.8 



Figure 14.9 



The Colpitt circuit requires one extra component and may be difficult to 

 arrange if the frequency is to be adjusted by varying the total tuning capaci- 

 tance. Its exponents claim for it an advantage over the Hartley oscillator 

 which runs roughly as follows : In a Hartley oscillator there are in effect two 



211 



