STABILIZED FEEDBACK AMPLIFIERS 



♦■E + N +D 



Fig. 1 — Amplifier system with feedback. 



e — Signal input voltage. 



y. — Propagation of amplifier circuit. 



p.e — Signal output voltage without feedback. 



n — Noise output voltage without feedback. 



d{E) — Distortion output voltage without feedback. 



/3 — Propagation of feedback circuit. 



E — Signal output voltage with feedback. 



N — Noise output voltage with feedback. 



D — Distortion output voltage with feedback. 



The output voltage with feedback is E -\- N -\- D and is the sum of fxe -\- n -\- d{E), 

 the value without feedback plus yu/3[£ + N + D] due to feedback. 



E + N + D=iJie + 7i + d{E) + n^lE + N + D^ 



IE + N + Z)](l - M/3) = fxe + n + d{E) 



E + N + D 



fie 



+ 



+ 



d{E) 



1 - M/3 1 - M^ 1 - M/8 



If |ju/3| ^ 1, £ = — -. Under this condition the amplification is independent of 



IX but does depend upon /3. Consequently the over-all characteristic will be con- 

 trolled by the feedback circuit which may include equalizers or other corrective 

 networks. 



General Equation 



In Fig. 1, jS is zero without feedback and a signal voltage, Bq, applied 

 to the input of the /x-circuit produces an output voltage. This is 

 made up of what is wanted, the amplified signal, Eq, and components 

 that are not wanted, namely, noise and distortion designated Nq and 

 Dq and assumed to be generated within the amplifier. It is further 

 assumed that the noise is independent of the signal and the distortion 

 generator or modulation a function only of the signal output. Using the 

 notation of Fig. 1 , the output without feedback may be written as : 



Eq + Nq + Do = ixeo + n + d(Eo), (1) 



where zero subscripts refer to conditions without feedback. 



