Data Sheet
CFB amplifiers can be used in unity gain configurations.
Do not use the traditional voltage follower circuit, where
the output is tied directly to the inverting input. With a
CFB amplifier, a feedback resistor of appropriate value
must be used to prevent unstable behavior. Refer to fig-
ure 5 and Table 1. Although this seems cumbersome, it
does allow a degree of freedom to adjust the passband
characteristics.
Application Information
Basic Operation
Figures 3, 4, and 5 illustrate typical circuit configurations for
non-inverting, inverting, and unity gain topologies for dual
supply applications. They show the recommended bypass
capacitor values and overall closed loop gain equations.
+Vs
6.8μF
Feedback Resistor Selection
One of the key design considerations when using a CFB
0.1μF
amplifier is the selection of the feedback resistor, R . R is
f
f
Input
+
-
used in conjunction with R to set the gain in the tradi-
Output
g
tional non-inverting and inverting circuit configurations.
Refer to figures 3 and 4. As discussed in the Current Feed-
back Technology section, the value of the feedback resis-
tor has a pronounced effect on the frequency response of
the circuit.
RL
0.1μF
6.8μF
Rf
Rg
G = 1 + (Rf/Rg)
-Vs
Table 1, provides recommended R and associated R val-
f
g
ues for various gain settings. These values produce the
optimum frequency response, maximum bandwidth with
minimum peaking. Adjust these values to optimize perfor-
mance for a specific application. The typical performance
characteristics section includes plots that illustrate how
Figure 3. Typical Non-Inverting Gain Circuit
+Vs
6.8μF
the bandwidth is directly affected by the value of R at
various gain settings.
R1
f
0.1μF
+
Output
Rg
Input
-
RL
0.1μF
Rf
±0.1dB BW
(MHz)
-3dB BW
(MHz)
Gain
(V/V
R (Ω)
R (Ω)
f
g
6.8μF
G = - (Rf/Rg)
-Vs
1
2
5
390
270
270
-
136
150
115
1300
1200
750
For optimum input offset
voltage set R1 = Rf || Rg
270
67.5
Figure 4. Typical Inverting Gain Circuit
Table 1: Recommended R vs. Gain
f
+Vs
6.8μF
In general, lowering the value of R from the recom-
f
mended value will extend the bandwidth at the expense
of additional high frequency gain peaking. This will cause
increased overshoot and ringing in the pulse response
0.1μF
Input
+
Output
characteristics. Reducing R too much will eventually
-
f
RL
cause oscillatory behavior.
0.1μF
Rf
Increasing the value of R will lower the bandwidth. Low-
f
6.8μF
G = 1
ering the bandwidth creates a flatter frequency response
and improves 0.1dB bandwidth performance. This is im-
portant in applications such as video. Further increase in
-Vs
Rf is required for CFB amplifiers
R will cause premature gain rolloff and adversely affect
f
Figure 5. Typical Unity Gain (G=1) Circuit
gain flatness.
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www.cadeka.com
13
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