LC OSCILLATORS 



difficult to vary the frequency over a wide range without severely influencing 

 the output. 



When these simple circuits are followed by a buffer valve, the frequency is 

 substantially independent of the load. The output voltage is generally smaller 

 if the buffer valve load is a resistance but not if it is a transformer or another 

 tuned circuit. The output power depends on the buffer valve. There is the 

 cost of the extra valve. The purity of the output waveform is good. 



HT+ 



-Preferably 

 a tuned 

 circuit for 

 l^^^X, good waveform 



Figure 14.16 



With the electron coupled oscillator the frequency is independent of the 

 load and may be made independent of the HT voltage, over a range. The 

 output voltage may be large, and also the output power if the valve is a beam 

 tetrode or power pentode. The purity of the output waveform is poor, and 

 requires resuscitating with a further timed circuit. 



Danger — red hot screens 



In designing Hartley or Colpitts oscillators to deliver appreciable powers — 

 of the order of watts and above — there are good reasons for employing beam 

 tetrodes or power pentodes because of their greater efficiency compared with 

 triodes. If this is done it must be borne in mind that when the anode swings 

 negative, the anode potential will be extremely low — possibly even lower than 

 the cathode — and that this state of affairs coincides with maximum positivity 

 at the grid and hence maximum cathode current. Under these circumstances 

 nearly all the cathode current will flow to the screen, an electrode normally 

 required to pass only a fraction of the cathode current. It is necessary to pay 

 special attention to the maximum aflowable screen dissipation for the valve, 

 and to check the product of average screen current — as measured by a 

 moving coil instrument — and screen potential employed. Failure to remember 

 this, and the use of a screen potential which might be quite suitable when the 

 valve is used as an ampHfier, are liable to lead to oscillators which run with 

 their screens at red heat. This promotes the liberation of adsorbed gases, the 

 bombardment of the cathode by gas ions, and the early demise of the valve. 



Applications of LC oscillators of interest to the electrobiologist are cathode 

 ray tube beam brightening^, tape recorders^, multi-channel oscillography^ and 

 non-lethal high voltage supply*. 



Blocking oscillator 



This is a variety of LC oscillator which is sometimes useful. Though 

 they are not continuous sine wave oscillators this seems an appropriate 



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