ELECTRON TUBE TEKFORMANCE 321 



off the amount S by which the characteristic sags in the middle of the interval 

 ^e (Fig. 2). 



The plate current is then given by the expression 



1 A? 2 , 45 3 /,v 



Single Frequency Input 



When the signal input to an amplifier consists of a single frequency v = P 

 cos pt, the output current is given by 



i = io -\-^P^ + \g + -^pnp cos pi 



Ae L (Ae)2 



The second-order effects consisting of the rectified current and second 

 harmonic are seen to be unaffected by the presence of third-order modulation. 

 However, the first-order effect, the fundamental output, ceases to be linear 

 and a new product, the third harmonic, appears. 



The change in fundamental output is expressible as a loading effect on 

 transconductance. The effective transconductance of the tube is. 



Ge = G+S 



©■ 



Expressed in db's the non-linearity effect is approximately 



When P is large it is convenient to select Ae = 2P and get 



Ge = G-h^, 201og?-' = 2.15|. 



S is positive when the g-e curve is concave upward. 

 The third harmonic content of the output is 



H, = 20 log /^^ = 20 log ^ + 40 log ^'+ 10. 



If we select Ae = 2P the second term drops out 



Hi = 20 log ^ + 22. 



If the curve is concave upward the third harmonic increases the peak value 



