LMP7712MM datasheet(15/18 Pages) NSC

15 / 18 page

Application Notes (Continued)

For the time being, ignore C

F. The AC gain of the circuit in

Figure 3 can be calculated as follows:

(1)

This equation is rearranged to find the location of the two

poles:

(2)

As shown in Equation (2), as the values of R

1 and R2 are

increased, 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

1 and R2. Whenever possible, it is best to chose smaller

feedback resistors.

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 5 shows

the frequency response of the schematic presented in Figure

3 with different values of C

F. As can be seen, using a small

value capacitor significantly reduces or eliminates the peak-

ing.

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. Bar-

code scanners, light meters, fiber optic receivers, and indus-

trial sensors are some typical applications utilizing photo-

diodes for current detection. This current needs to be

amplified before it can be further processed. This amplifica-

tion is performed using a current-to-voltage converter con-

figuration 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 amplifier configuration. C

D represents the

photodiode parasitic capacitance and C

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 LMP7711/

LMP7712 are ideal for wideband transimpedance applica-

tions.

20150364

FIGURE 3. Compensating for Input Capacitance

20150359

FIGURE 4. Closed Loop Frequency Response

20150360

FIGURE 5. Closed Loop Frequency Response

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