Page 776 - Mechatronics with Experiments

762 MECHATRONICS
7. Compare the input–output voltage relationship for the low pass filter and high pass
filter. Describe the differences.
8. Discuss how you can build a band pass filter using a combination of low pass and
high pass filter.
9. Discuss how you can build a band reject (Notch) filter using a combination of low
pass and high pass filter.
10. (Optional) Complete the same experiment for band pass filter and Notch filter.

11.4 EXPERIMENT 4: ACTIVE FIRST-ORDER LOW PASS
FILTER WITH OP-AMPS

Objectives
1. Understanding the theory of active filter circuits and their applications.
2. Circuit design of an active op-amp low pass filter. Build and test the complete circuit.
3. Measure the input and output voltage signals of the filter circuit and confirm the
expectations with measurements.

Components

Item Quantity Part No. Supplier
LM358 Op-Amp IC 1 23966 Jameco Electronics (www.jameco.com)
Resistors as calculated 2 81832 Jameco Electronics (www.jameco.com)
Capacitor as calculated 1 130232 Jameco Electronics (www.jameco.com)
Breadboard 1 20722 Jameco Electronics (www.jameco.com)
Set of connection wires 1 set 20079 Jameco Electronics (www.jameco.com)

Theory

For our purpose, a filter is designed as an electronic circuit that produces a prescribed
frequency response characteristic, of which the most common objective is to pass a certain
frequency range while rejecting others. Filters may be classified into two major groups:
passive filters and active filters. Passive filters consist of combinations of resistors, capac-
itors, and inductors. Passive RLC filters are capable of achieving relatively good filter
characteristics in applications in the audio frequency range. But at the lower end of the
audio frequency range, a problem occurs due to the high internal loss of inductors at low
frequencies. Active filters consist of combinations of resistance, capacitance, and one or
more active devices, such as op-amps, employing feedback. Since inductances are not
required, the difficulties associated with them at low frequencies are eliminated.
The low pass filter built in this experiment is different from the one discussed in the
Chapter on Electronics in that the DC gain of the low pass filter discussed in Figure 5.32
is equal to +1if R = R , whereas the DC gain of the low pass filter discussed in this
f i
experiment is equal to −1if R = R . Otherwise, they both are first-order (single-pole) low
1
2
pass active filters.
From the point of view of the amplitude of the frequency response, most filters
can be classified as low pass, high pass, band pass and band rejection (notch) filters. In
this experiment we will concentrate on a low pass filter design. Figure 11.10 shows an