420 



REGULATORY CIRCUITS 



Small deviations in the output can be related to small deviations in d and eg 

 in the following manner: 



oei deo 



(8-6) 



= KiAei + K2Aeg. 



The gain factors Ki and K2 represent the incremental gain of the IF 

 amplifier to the input and the incremental AGC loop gain, respectively. 

 Deviations of the bias will be simply related to output deviations through 

 the AGC filter: 



Ae^ = -G2{s)Aeo. (8-7) 



The approximate linear feedback loop corresponding to Equations 8-6 

 and 8-7 is pictured in Fig. 8-1 6b. The output-input ratio for this linear 

 regulating loop will have the following form : 



Aen 1 



KiAei 1 + K2G2(sy 



(8-8) 



8-16 STATIC REGULATION REQUIREMENTS OF AGC 

 LOOPS 



The transfer function represented by Equation 8-8 gives the small-signal 

 modulation transmission characteristics of the loop. If the zero frequency 



gain of the AGC filter is assumed 

 unity [G2(0)- = 1], the static gain 

 around the loop is K2 as is indicated 

 in Fig. 8-1 6b. The static regulation 

 performance is directly related to the 

 loop gain K2. In order to display this 

 relation, though, the gain control 

 characteristic of the IF amplifier indi- 

 cated in Equation 8-4 must be ex- 

 amined in detail. Typically, the 

 logarithm of the IF amplifier gain is 

 approximately a linear function of the 

 AGC bias voltage. That is, the slope 

 of the gain-bias curve in decibels per 

 volt is a constant. Fig. 8-17 shows a 

 typical IF amplifier gain control 

 Gain characteristic. Such a linear relation 

 can be generally expressed in the 

 following form: 



AGC BIAS VOLTAGE 



Fig. 8-17 Typical IF Amplifi 

 Control Characteristics, 



20 logio Gi = ^ + Beg 



(8-9) 



