428 REGULATORY CIRCUITS 



third derivative of the transconductance. In designing an amplifier it is not 

 convenient to obtain an infinite series expressing the transfer characteristic. 

 The published transconductance curves can, however, be employed to 

 obtain a reasonable estimate of the distortion. The transconductance 

 corresponding to an infinitesimal signal applied to the tube at a grid voltage 

 El is determined, and then the transconductance at the positive and 

 negative peak values of the IF signal is determined. From these three 

 values the distortion is computed from 



m' _ 1 

 tn 2 



1+^— + ^" 



2g. 



(8-20) 



When yf and Ei are so large that the negative excursion of the signal 

 extends beyond cutoff, Equation 8-20 is not sufficiently accurate. However, 

 it serves as an estimate of the distortion provided that the tube is cut oflF. 

 Sharp cutoff tubes do not always cut off at the voltages indicated by the 

 tube characteristics. The tubes are only required to exhibit less than a 

 specified maximum value of plate current at cutoff bias. As a result of 

 inadequately controlled cutoff characteristics ^m.mrn does not go to zero for 

 large Ei and J, and the distortion in an actual amplifier is sometimes 

 observed to be much greater than estimated by Equation 8-20. In the 

 design of a gain-controlled amplifier employing sharp cutoff tubes the AGC 

 voltage applied to the stages is therefore restricted so that the peak negative 

 voltage E -\- A does not exceed cutoff. The number of stages in the IF 

 amplifier must then greatly exceed the minimum number determined by 

 gain, bandwidth, and stability requirements. 



A more suitable arrangement in the radar receiver involves the use of two 

 or three remote cutoff pentodes in the early stages of the amplifier. Gain 

 reductions of 35 per stage with negligible distortion can be obtained with 

 some of the available semiremote cutoff tubes having reasonably good gain 

 bandwidth products. Very little or no AGC is then applied to the remaining 

 stages of the IF amplifier. 



It is desirable to limit AGC loop gain variations with the input signal 

 level in order to maintain loop stability and the required dynamic perform- 

 ance. AGC loop gain variations of 2 : 1 or 6 db represent a practical goal 

 employed in the design of the IF amplifier. 



As was noted in Paragraph 8-16, the AGC loop gain is proportional to 

 the derivative of the logarithm of the IF amplifier gain with respect to the 

 AGC bias. The contribution of an individual stage is thus proportional to 

 the derivative of the logarithm of the transconductance curve. Unfor- 

 tunately, this quantity, like the third derivative of the transconductance 

 whose importance was noted above, is not normally specified or controlled 

 in tube manufacture, and large variations can occur in the cutoff region. 

 The use of a greater number of remote or semiremote cutoff tubes, limiting 



