Data Sheet
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.
+V
s
6.8μF
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.
Feedback Resistor Selection
One of the key design considerations when using a CFB
amplifier is the selection of the feedback resistor, R
f
. R
f
is
used in conjunction with R
g
to set the gain in the tradi-
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.
Table 1, provides recommended R
f
and associated R
g
val-
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
the bandwidth is directly affected by the value of R
f
at
various gain settings.
Comlinear CLC2600, CLC3600, CLC4600
Dual, Triple, and Quad 300MHz Amplifiers
Input
+
-
0.1μF
Output
0.1μF
R
L
R
f
G = 1 + (R
f
/R
g
)
R
g
-V
s
6.8μF
Figure 3. Typical Non-Inverting Gain Circuit
+V
s
6.8μF
R
1
Input
R
g
+
-
0.1μF
Output
0.1μF
6.8μF
-V
s
R
L
R
f
G = - (R
f
/R
g
)
For optimum input offset
voltage set R
1
= R
f
|| R
g
Gain
(V/V
1
2
5
R
f
(Ω)
1240
510
200
R
g
(Ω)
-
510
50
±0.1dB BW
(MHz)
129
140
18
-3dB BW
(MHz)
300
230
111
Figure 4. Typical Inverting Gain Circuit
Table 1: Recommended R
f
vs. Gain
+V
s
6.8μF
Input
+
-
0.1μF
Output
0.1μF
6.8μF
-V
s
R
L
R
f
G=1
R
f
is required for CFB amplifiers
In general, lowering the value of R
f
from the recom-
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
characteristics. Reducing R
f
too much will eventually
cause oscillatory behavior.
Increasing the value of Rf will lower the bandwidth. Low-
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
Rf will cause premature gain rolloff and adversely affect
gain flatness.
www.cadeka.com
Rev 1A
Figure 5. Typical Unity Gain (G=1) Circuit
©2004-2008 CADEKA Microcircuits LLC
10
CADEKA [ CADEKA MICROCIRCUITS LLC. ]
