Datasheet
1k
10k 100k 1M 10M
FREQUENCY (Hz)
-10
-8
-6
-4
-2
0
2
NORMALIZED GAIN (dB)
V
S
= 5V
C
F
= 0 pF
A
V
= -1
R
1
= R
2
= 100 k:
R
1
= R
2
= 30 k:
R
1
= R
2
= 10 k:
R
1
= R
2
= 1 k:
+
¨
©
§
¨
©
§
-1
2C
IN
P
1,2
=
1
R
1
1
R
2
r
1
R
1
1
R
2
+
2
-
4 A
0
C
IN
R
2
-R
2
/R
1
1 +
s
¨
©
§
¨
©
§
+
s
2
A
0
C
IN
R
2
¨
©
§
¨
©
§
V
OUT
V
IN
(s) =
A
0
R
1
R
1
+
R
2
C
IN
R
1
R
2
V
OUT
+
-
+
-
V
IN
+
-
V
OUT
V
IN
R
2
R
1
A
V
=
-
=
-
C
F
LMP7701, LMP7702, LMP7704
www.ti.com
SNOSAI9H –SEPTEMBER 2005–REVISED MARCH 2013
amplifier, to form a pole. This pole will have little or no effect on the output of the amplifier at low frequencies and
DC conditions, but will play a bigger role as the frequency increases. At higher frequencies, the presence of this
pole will decrease phase margin and will also cause gain peaking. In order to compensate for the input
capacitance, care must be taken in choosing the feedback resistors. In addition to being selective in picking
values for the feedback resistor, a capacitor can be added to the feedback path to increase stability.
The DC gain of the circuit shown in Figure 46 is simply –R
2
/R
1
.
Figure 46. Compensating for Input Capacitance
For the time being, ignore C
F
. The AC gain of the circuit in Figure 46 can be calculated as follows:
This equation is rearranged to find the location of the two poles:
(1)
As shown in Equation 1, as values of R
1
and R
2
are increased, the magnitude of the poles is reduced, which in
turn decreases the bandwidth of the amplifier. Whenever possible, it is best to choose smaller feedback resistors.
Figure 47 shows the effect of the feedback resistor on the bandwidth of the LMP7701/LMP7702/LMP7704.
Figure 47. Closed Loop Gain vs. Frequency
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