Datasheet

+
R
F
-
C
F
IF R
A
< < R
F
C
F
c =
¨
©
§
1 +
R
B
R
A
¨
©
§
R
A
C
F
c
R
B
OP AMP
OPEN LOOP
GAIN
NOISE GAIN WITH NO C
F
NOISE GAIN WITH C
F
f
Z
f
P
A
0
f
Z
=
1
2S
R
F
C
IN
f
P
=
A
0
2S
R
F
(C
IN
+C
F
)
GAIN
FREQUENCY
LMV791, LMV792
www.ti.com SNOSAG6F –SEPTEMBER 2005–REVISED MARCH 2013
Figure 58. C
F
Selection for Stability
Calculating C
F
from Equation 3 can sometimes return unreasonably small values (<1 pF), especially for high
speed applications. In these cases, its often more practical to use the circuit shown in Figure 59 in order to allow
more reasonable values. In this circuit, the capacitance C
F
′ is (1+ R
B
/R
A
) time the effective feedback capacitance,
C
F
. A larger capacitor can now be used in this circuit to obtain a smaller effective capacitance.
For example, if a C
F
of 0.5 pF is needed, while only a 5 pF capacitor is available, R
B
and R
A
can be selected
such that R
B
/R
A
= 9. This would convert a C
F
′ of 5 pF into a C
F
of 0.5 pF. This relationship holds as long as R
A
<< R
F
.
Figure 59. Obtaining Small C
F
from large C
F
′
LMV791 AS A TRANSIMPEDANCE AMPLIFIER
The LMV791 was used to design a number of amplifiers with varying transimpedance gains and source
capacitances. The gains, bandwidths and feedback capacitances of the circuits created are summarized in
Table 1. The frequency responses are presented in Figure 60 and Figure 61. The feedback capacitances are
slightly different from the formula in Equation 3, since the parasitic capacitance of the board and the feedback
resistor R
F
had to be accounted for.
Table 1.
Transimpedance, A
TI
C
IN
C
F
3 dB Frequency
470000 50 pF 1.5 pF 350 kHz
470000 100 pF 2.0 pF 250 kHz
470000 200 pF 3.0 pF 150 kHz
47000 50 pF 4.5 pF 1.5 MHz
47000 100 pF 6.0 pF 1 MHz
47000 200 pF 9.0 pF 700 kHz
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