SINE WAVE OSCILLATORS 



tapping points: one, the intentional one on the inductance, and another 

 unintentional one of the Colpitts type in virtue of the two stray capacitances 

 between the ends of the tuned circuit and earth (Figure 14.9). Since the two 

 taps are unUkely to be at the same effective distance up the tuned circuit, 

 the oscillator is liable to peculiar behaviour due to, as it were, not knowing 

 which tap it is supposed to be using. The author has never experienced this 

 difficulty and has so far always used the Hartley circuit with satisfactory 

 results. Probably the Colpitts circuit becomes necessary only at frequencies 

 much higher than those usually of interest to electrobiologists. 



The Hartley and Colpitts oscillators are the important LC oscillators and 

 are the only ones with which it is proposed to deal. We have now to attend to 

 how to lead out the oscillations the circuit is generating. 



HT+ 



HT+ 



Output 



(a) 



(b) 



Figure 14.10 



If the requirements of the oscillator in terms of frequency and amplitude 

 stability are not too stringent, energy may be removed from the tuned circuit 

 by an extra inductively coupled winding, or possibly even a lead taken — via a 

 blocking capacitance — direct from the anode {Figure 14.10). A typical 

 example is the erasing oscillator of magnetic tape recorders, where the 



HT+ 



Figure 14.11 



amplitude of the erase signal can be any size provided it is sufficient to 

 eradicate previous recordings, and where any frequency will do provided it is 

 several times the highest frequency that can be recorded. If the output is 

 taken from the anode it is not difficult to get a peak-to-peak swing of nearly 



212 



