SM73304 Datasheet, PDF(16/22 Page) Texas Instruments – Dual and Single Precision, 17 MHz, Low Noise, CMOS Input Amplifiers with Enable

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SM73304 Datasheet, PDF (16/22 Pages) Texas Instruments – Dual and Single Precision, 17 MHz, Low Noise, CMOS Input Amplifiers with Enable

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(2)

As shown in Equation 2, as the values of R1 and R2 are in-

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 R1

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

back resistors.

30159475

FIGURE 2. Input Common Mode Capacitance

This input capacitance will interact with other impedances

such as gain and feedback resistors, which are seen on the

inputs of the amplifier to form a pole. This pole will have little

or no effect on the output of the amplifier at low frequencies

and under 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 also causes gain

peaking. In order to compensate for the input capacitance,

care must be taken in choosing 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 3 is simply âR2/

R1.

30159459

FIGURE 4. Closed Loop Frequency Response

As mentioned before, adding a capacitor to the feedback path

will decrease the peaking. This is because CF 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 CF. As can be seen, using a small val-

ue capacitor significantly reduces or eliminates the peaking.

30159464

FIGURE 3. Compensating for Input Capacitance

For the time being, ignore CF. 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:

30159460

FIGURE 5. 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

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