EFFECT OF FEEDBACK ON IMPEDANCE 



273 



terminals 1, 1' so that either Fsh or Fop is zero. If Fop — and Fsh — Fa 

 where Fn is the normal feedback, then 



F^^ \-^ 



On the other hand, if Fsh = and Fop = F^ then 



Zp 



F.M ^ 1 - 



Z, 



(4) 



(5) 



Fig. 2 shows a feedback amplifier in which the ju-circuit and the /3-network 

 are connected in series at one end and in parallel at the other end. At 

 terminals 1, 1' in this figure the conditions for formula (4) are obviously 

 fulfilled. Hence, if the impedance measurements are made at these ter- 



Zi 



Z2 



Fig. 2 — Feedback amplifier with series feedback at one end and shunt feedback at the 



other end. 



minals, the feedback is given by formula (4). On the other hand, at ter- 

 minals 2, 2' in Fig. 2 the conditions for formula (5) are obviously fulfilled. 

 Hence, if the impedance measurements are made at these terminals, the 

 feedback is given by formula (5). 



If the grid-plate parasitic admittance of a tube in a feedback amplifier 

 is not negligible it is not possible to open any physical mesh in the amplifier 

 so that Fop = for that tube. In such a case, therefore, (4) is not ap- 

 plicable. However, if the impedance measurements are made between the 

 grid and the cathode of that tube the conditions for formula (5) are ob- 

 viously fulfilled, and the feedback is given by formula (5). Hence, of the 

 two particular forms (4) and (5) of the general relationship (3), only (5) 

 enjoys complete generality in the determination of feedback by impedance 

 measurements. 



