LMV772 datasheet(15/21 Pages) NSC

15 / 21 page

Application Note (Continued)

HIGH PASS FILTER

In a similar approach, one can derive the transfer function of

a high pass filter. A typical first order high pass filter is shown

below:

Writing the KCL for this circuit :

1 denotes the voltage between C and R1)

(12)

(13)

Solving these two equations to find the transfer function and

using:

(high frequency gain)

and

Which results:

(14)

Looking at the transfer function, it is clear that when f/f

O is

small, the capacitor is open and hence no signal is getting in

to the amplifier. As the frequency increases the amplifier

starts operating. Atf=f

O the capacitor behaves like a short

circuit and the amplifier will have a constant, high frequency,

gain of H

O. The bode plot of the transfer function follows:

BAND PASS FILTER

Combining a low pass filter and a high pass filter will gener-

ate a band pass filter. In this network the input impedance

forms the high pass filter while the feedback impedance

forms the low pass filter. Choosing the corner frequencies so

that f

1 < f2, then all the frequencies in between, f1

≤ f ≤ f

2, will

pass through the filter while frequencies below f

1 and above

2 will be cut off.

The transfer function can be easily calculated using the

same methodology as before.

(15)

Where

The transfer function is presented in the following figure.

20039654

FIGURE 5.

20039658

FIGURE 6.

20039660

FIGURE 7.

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