THERMIONIC VACUUM TUBES 65 



stages at fault may be increased and lower grid leaks may be desir- 

 able. The volume of input to each stage should also be considered. 



33. Distortion. Distortion in an amplifier circuit may result either 

 from a failure to amplify all frequencies by the same amount or from 

 the generation of overtones of the fundamental frequencies in the 

 input. 



The unequal amplification of various frequencies arises from the 

 presence of resonant characteristics in the circuit. This may take 

 the form of a feed-back which discriminates in favor of certain fre- 

 quencies, the feed-back not being pronounced enough to cause singing. 

 A negative feed-back may also occur, causing a loss of efficiency over 

 some particular frequency range. 



The distortion which arises from the generation of overtones is due 

 to non-linear voltage-current characteristics in one or more branches 

 of the circuit. The usual sources of this trouble are curvature of the 

 plate and grid characteristics (See Equation 4) and the variable 

 permeability of the iron used as cores. With properly chosen coils, 

 practically distortionless amplification can be secured by the method 

 indicated in Sec. 12. In general, to accomplish this, the output 

 impedance need not be more than two or three times r p . In case it 

 is necessary to use a low output impedance in the final stage, distortion 

 may be reduced by using the push-pull circuit of Sec. 23. 



In using an amplifier under circumstances such that distortionless 

 output is desired, care should be taken that no tube by itself is over- 

 loaded or caused to work in such fashion that its dynamic character- 

 istic is curved. Distortion which arises from curvature of this char- 

 acteristic can be detected by inserting an ammeter in the plate circuit 

 of each tube. When each characteristic is straight, or nearly so, 

 there should be no change in ammeter reading as the source of input 

 voltage is thrown on and off, and in the case of a variable input such 

 as that arising from speech, the ammeter readings should remain 

 constant while the amplifier is in operation. This test will not detect 

 distortion which arises from selective amplification with respect to 

 frequency. 



34. Calculation and Measurement of Amplification. Provided all 

 parts of an amplifier circuit are functioning properly and its constants 

 are known, its amplification can be calculated quite accurately. The 

 following example will illustrate the procedure to be followed in any 

 case. Referring to the transformer coupled amplifier of Fig. 26, as- 

 sume that, the ratio of the first input transformer is z\Z\ and that the 

 ratio of the second input transformer is Z2'.z 3 ; assume also that z 2 is 

 numerically equal to r p , the plate circuit resistance of the first tube. 



