THERMIONIC VACUUM TUBES 



89 



least four or five times as great as r p , and it may advantageously be 

 made 10 to 15 times as great. In these latter cases R is relatively 



Fig. 62 



negligible compared to R in Equation 14 with the result that i P \ can 

 be quite accurately calculated although the exact value of Ro may be 

 unknown. 



50. Types of Oscillating Circuits. There are many different types 

 of oscillating circuits and as they do not lend themselves readily to 

 classification, only a few of the more common types will be described. 



One of the simplest oscillating circuits is that shown in Fig. 63 

 which is characterized by a tuned grid circuit inductively coupled 



Ll M 12 



Fig. 63 



to a coil in the output. This type is satisfactory for low plate voltages 

 and small powers, but is not to be recommended where large amounts 

 of power and high efficiencies are desired. 



The condenser C is inserted to prevent short circuiting of the 

 plate battery or generator and may be made so large as to have no 

 effect on the frequency. 



A circuit of similar properties is shown in Fig. 64, the output being 

 tuned instead of the input. 



51. Colpitis and Hartley Circuits. Two very similar types of circuits 

 which have proved satisfactory for a wide range of frequencies, voltages 

 and powers, and which yield very high efficiencies, are shown in 

 Figs. 64 and 65, the former being known as the Colpitts and the latter 

 as the Hartley circuit. In both circuits, as illustrated, the mean grid 

 potential is secured by a grid leak. The blocking condenser C g 

 should be large enough to offer very little impedance to the flow of 

 the alternating current which causes the variation of the grid potential, 



