Datasheet

10k
100k 1M 10M
FREQUENCY (Hz)
-40
-30
-20
-10
0
10
20
GAIN (dB)
C
F
= 0 pF
C
F
= 5 pF
C
F
= 2 pF
R
1
, R
2
= 30 k:
A
V
= -1
10k
100k 1M 10M 100M
FREQUENCY (Hz)
-25
-20
-15
-10
-5
0
5
10
15
GAIN (dB)
R
1,
R
2
= 30 k:
A
V
= -1
R
1,
R
2
= 10 k:
R
1,
R
2
= 1 k:
LMP7711
SNOSAP4F –SEPTEMBER 2005–REVISED MAY 2013 www.ti.com
As shown in Equation 2, as the values of R
1
and R
2
are increased, the magnitude of the poles are reduced,
which in turn decreases the bandwidth of the amplifier. Figure 57 shows the frequency response with different
value resistors for R
1
and R
2
. Whenever possible, it is best to chose smaller feedback resistors.
Figure 57. Closed Loop Frequency Response
As mentioned before, adding a capacitor to the feedback path will decrease the peaking. This is because C
F
will
form yet another pole in the system and will prevent pairs of poles, or complex conjugates from forming. It is the
presence of pairs of poles that cause the peaking of gain. Figure 58 shows the frequency response of the
schematic presented in Figure 56 with different values of C
F
. As can be seen, using a small value capacitor
significantly reduces or eliminates the peaking.
Figure 58. Closed Loop Frequency Response
TRANSIMPEDANCE AMPLIFIER
In many applications, the signal of interest is a very small amount of current that needs to be detected. Current
that is transmitted through a photodiode is a good example. Barcode scanners, light meters, fiber optic receivers,
and industrial sensors are some typical applications utilizing photodiodes for current detection. This current
needs to be amplified before it can be further processed. This amplification is performed using a current-to-
voltage converter configuration or transimpedance amplifier. The signal of interest is fed to the inverting input of
an op amp with a feedback resistor in the current path. The voltage at the output of this amplifier will be equal to
the negative of the input current times the value of the feedback resistor. Figure 59 shows a transimpedance
amplifier configuration. C
D
represents the photodiode parasitic capacitance and C
CM
denotes the common-mode
capacitance of the amplifier. The presence of all of these capacitances at higher frequencies might lead to less
stable topologies at higher frequencies. Care must be taken when designing a transimpedance amplifier to
prevent the circuit from oscillating.
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