"Chapter 1 - The Op Amp's Place in the World" - HTL Wien 10

Selection of the Feedback Resistor
8-10
Z
ZF1 ZB1 ZF1 ZG1
Z
ZFN ZB1 ZFN ZGN Z FN Z F1 Z B1 Z F1
Z G1 1 Z FN
Z GN
(8–26)
(8–27)
Equation 8–27 leads one to believe that a new value for Z F can easily be chosen for each
new gain. This is not the case in the real world; the assumptions don’t hold up well enough
to rely on them. When you change to a new gain not specified on the data sheet, Equation
8–27, at best, supplies a starting point for R F, but you must test to determine the final value
of R F.
When the R F value recommended on the data sheet can’t be used, an alternate method
of selecting a starting value for R F is to use graphical techniques. The graph shown in Figure
8–7 is a plot of the typical 300-MHz CFA data given in Table 8–1.
Gain
Figure 8–7. Plot of CFA R F, G, and BW
10
9
8
7
6
5
4
3
2
GAIN and BANDWIDTH
vs
FEEDBACK RESISTOR
Gain
vs.
Feedback
Resistance
1
0 100 200 300 400 500 600 700 800 900
Feedback Resistor – Ω
Bandwidth
vs.
Feedback
Resistance
130
120
110
100
90
80
70
60
50
40
1k
Bandwidth – MHz