11 



NEGATIVE VOLTAGE FEEDBACK 

 AND THE STABILIZED GAIN AMPLIFIER 



Suppose we have an amplifier with a gain of A times, and we take a fraction 

 B of the output, feed it back and subtract it from the input, then this is called 

 'negative feedback'. If the input to the amplifier proper is 1 unit, the output 

 is A, the fed-back quantity is AB, so the input to the whole device must have 

 been 1 + AB. The overall gain is thus Al(\ + AB), and if AB ^ 1 the overall 

 gain is approximately l/B, independent of A (Figure 11.1). 



Figure 11.1 



Now if B is derived by a potential divider, it is both linear and independent 

 of frequency. Thus an amplifier with negative feedback for which it is possible 

 to say that AB^ 1 ought, at a cost only of reduced overall amplification, 



.With feedback 



nstantaneous 

 input 



c 

 '5 

 o 



Witiiout 

 feedbacl< 



r 



^ 



With 

 feedback 



1 



(a) 



Frequency 



(b) 



Figure 11.2 



to exhibit very level frequency response, constant gain and absence of dis- 

 tortion, and in point of fact this is exactly what does happen with feedback 

 properly applied {Figure 11.2). In a straightforward electronic amplifier 



164 



