. The Bell System technical journal . at the series feedback end of a feedback ampUfier. However, it should be npted that the feedback Fsh involved in (9) is notnow equal to the normal feedback Fn as it was when the terminals 1,1 weretaken as in Fig. 2. The relation between Fat and Fsh may be obtained from(6) by identifying F/f with Fz, and Zi with Z. Hence F^ = (10) From (9) and (10) it follows that even with a very modest amount of normalfeedback the magnification of the impedance may be very large. Forexample, if Zp = 1000 ohms, Zi = 1 megohm and Fsh = —1000, then Zais better than 1000
. The Bell System technical journal . at the series feedback end of a feedback ampUfier. However, it should be npted that the feedback Fsh involved in (9) is notnow equal to the normal feedback Fn as it was when the terminals 1,1 weretaken as in Fig. 2. The relation between Fat and Fsh may be obtained from(6) by identifying F/f with Fz, and Zi with Z. Hence F^ = (10) From (9) and (10) it follows that even with a very modest amount of normalfeedback the magnification of the impedance may be very large. Forexample, if Zp = 1000 ohms, Zi = 1 megohm and Fsh = —1000, then Zais better than 1000 times as large as Zp although Fu is not quite unity inmagnitude. Similarly, the impedance faced by the line impedance Z2 in Fig. 2, asshown in Fig. 4, is 276 where BELL SYSTEM TECHNICAL JOURNAL Z^ = Zp = 1 — (11) Zy, -{- Z^ This shows the now well-known fact that shunt feedback may be used toreduce impedance. p-CIRCUIT f3-NETWORK ■02 Fig. 4—Impedance faced by the line at the shunt feedback end of a feedback Fig. 5—Impedance faced by the hne connected to a bridge feedback amphfier. The relation between the normal feedback F,v and the feedback ¥oj> in-volved in (11) is, by (6) ^^^ (12) Vs - From (11) and (12) it follows that even with a very modest amount of nor-mal feedback the reduction in impedance may be very large. For example, EFFECT OF FEEDBACK ON IMPEDANCE 277 if Zp = 100,000 ohms, Z2 = 100 ohms and Fop = -1000, then Za is lessthan 100 ohms although F^ is not quite unity in The two examples given above illustrate the use of feedback to magnifyor to reduce the impedance of a network. This impedance, however, willbe correspondingly sensitive to changes in the characteristics of the vacuumtubes. A third example of the use of the relationship (3) will show thatfeedback may also be used to make the impedance of a network less sensitiveto changes in the characteristics of the vacuum tubes. In the case of the bridge-type feedback net
Size: 2143px × 1166px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1920, booksubjecttechnology, bookyear1
