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BELL SYSTEM TECHNICAL JOURNAL 



the capacitance Co is first reduced and finally eliminated, leaving the 

 inter-electrode capacitance as the only capacitance in the oscillating 

 circuit. The tube leads then form a part of the main oscillating 

 circuit, in which large circulating currents must exist for stable opera- 

 tion. For a further increase in frequency the external inductance Lq 



lg 



Lp 



Fig. 2 (left) — A standard Colpitts oscillator circuit. The heavj- lines indicate 

 the main oscillating circuit. The dotted portions represent the inter-electrode 

 capacitances and lead inductances which play a minor role at low frequencies. 



Fig. 3 (right) — The limiting circuit with the external tuning capacitance eli minated 

 and the external inductance reduced to a short circuiting bar between the grid and 

 plate leads. The main oscillating circuit, indicated by the heavy lines, is seen to 

 include the inter-electrode capacitances and lead inductances. 



must be reduced and, in the limit, it becomes the shortest possible 

 connection between the grid and plate terminals. The oscillating 

 frequency for this limiting circuit, shown in Fig. 3, is determined by 

 the product of the lead inductance L, and the inter-electrode capaci- 

 tance Ct] that is, 



/o = ,■ , 



27rVL,Ct 



where Lt is the sum of the grid and plate lead inductance and Ct is 

 the total grid-plate capacitance. Even before this frequency limit is 

 reached the output power and plate efficiency are seriously reduced by 

 the lack of full control over the relative amplitude and phase of the 

 alternating grid and plate potentials. Whereas the ratio of these 

 amplitudes is controlled in the circuit shown in Fig. 2 by the con- 

 densers Ci and C2, it is determined in the limiting case primarily by 



