HARD VALVES 

 but dV,= -RdI, 



al 



^h 



R 



'+7„ 



The amplification is 



dV, dl^R g„,R 



bV^ bV^ ^ R 



1 + 



^a 



The expression may be left in this form, or, multiplying through by r^ 



gjrJR [xR 



amplification = 



r,^R R + r, 



A typical value for a triode valve would be 40. 



The expression tends to an upper hmit equal to jjl when R ^ r„, hence the 

 term amplification factor. It should, however, be borne in mind that if, in 

 an effort to achieve high amplification, R is made very large, then the anode 

 voltage and current will fall and so will [x, and the expected increase will 

 not be secured. // can only then be restored by increasing the HT voltage. 

 In point of fact there are better methods of getting high gain than squeezing 

 the last drop of amplification out of a triode by the employment of very large 

 high tension. It is best to be content with the gain available from a high 

 tension supply which is convenient. 



Graphical design procedure 



Choose a valve of high jj, and from the makers' catalogue make a tracing 

 of the anode characteristic. Our problem now is to find the 'working region'. 

 This is bounded by: 



(1) A rectangular hyperbola which represents the maximum power which 

 may be dissipated at the anode of the valve (lest it get too hot). 



(2) A vertical line through the voltage which is the maximum which may be 

 appHed to the anode (lest it liberate gas). 



(3) A horizontal line which represents the smallest current below which 

 curvature of the characteristic curves sets in (lest the output cease to be a 

 hnear function of the input). 



(4) The characteristic curve — guess it if it is not there — corresponding to a 

 grid voltage of about — j V (lest there flow grid current). 



(5) A horizontal line which corresponds to the maximum emission current 

 at which the cathode is rated. 



Then the area enclosed by these five lines is the working region. Draw a dot 



in what you guess to be about the middle. Then this point is a first shot at 

 the 'working point' for the valve, and the mean anode current, anode voltage 

 and grid bias may be read off. 



On the voltage axis mark off the proposed value of high tension and draw 

 a straight line through the high tension point and the working point. Then 



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