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:
LMP7715, LMP7716, LMP7716Q
SNOSAV0E –MARCH 2006–REVISED MARCH 2013 www.ti.com
Figure 52. 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 53 shows the frequency response of the
schematic presented in Figure 51 with different values of C
F
. As can be seen, using a small value capacitor
significantly reduces or eliminates the peaking.
Figure 53. 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 54 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.
With a wide gain bandwidth product, low input bias current and low input voltage and current noise, the
LMP7715/LMP7716/LMP7716Q are ideal for wideband transimpedance applications.
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