lmp7716q National Semiconductor Corporation, lmp7716q Datasheet - Page 15

• Manufacturer Part Number: lmp7716q
• Description: Single And Dual Precision, 17 Mhz, Low Noise, Cmos Inpu Amplifiers
• Manufacturer: National Semiconductor Corporation

For the time being, ignore C
Figure 3 can be calculated as follows:
This equation is rearranged to find the location of the two
poles:
As shown in Equation 2, as the values of R
creased, the magnitude of the poles are reduced, which in
turn decreases the bandwidth of the amplifier. Figure 4 shows
the frequency response with different value resistors for R
and R
back resistors.
As mentioned before, adding a capacitor to the feedback path
will decrease the peaking. This is because C
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 5 shows the
frequency response of the schematic presented in Figure 3
with different values of C
ue capacitor significantly reduces or eliminates the peaking.
FIGURE 4. Closed Loop Frequency Response
2
. Whenever possible, it is best to chose smaller feed-
F
. As can be seen, using a small val-
F
. The AC gain of the circuit in
1
and R
F
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will form yet
2
are in-
(1)
(2)
1
15
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 be-
fore it can be further processed. This amplification is per-
formed 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 6 shows a transimpedance am-
plifier configuration. C
capacitance and C
tance of the amplifier. The presence of all of these capaci-
tances 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.
FIGURE 5. Closed Loop Frequency Response
FIGURE 6. Transimpedance Amplifier
CM
D
denotes the common-mode capaci-
represents the photodiode parasitic
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