UNIVERSITI TEKNIKAL MALAYSIA MELAKA
FAKULTI KEJURUTERAAN ELEKTRONIK DAN
KEJURUTERAAN KOMPUTER
BENG 3211
ELECTRONIC ENGINEERING LAB 3
LAB 7: ACTIVE FILTERS
BENG3211
1.0
Objective:
To design, construct and test active filer circuit using LM741CN.
2.0
No.
1
2
3
4
Material
Equipment
DC power supply
Digital Multimeter
Proto-board
Oscilloscope
3.0
Theory
Qty
1
l
1
1
No.
1
2
3
Component
Resistor 1kΩ , 1.5kΩ
Capacitor l.0nF, l00nF
IC Op-Amp LM741CN
Qty
2
An electronics filter is a circuit that is used to pass signals which are within a selected band or
frequencies while attenuating all other signals which are beyond this band. Filter networks can be
classified as active or passive filters. Passive filter network contains only passive components
such as resistors, inductors and capacitors. Active filters, on the other hand, provide amplification
to the pass-band signals with the use of transistor or op-amps together with a few frequencyselective passive components.
There are four basic type of filters, namely low-pass, high-pass, band-pass and band-stop filters.
A low pass-pass filter allows low-frequency signals to be passed forward to its output terminals
with little or no attenuation. Any signal above cutoff frequency will be attenuated, and the
attenuation increases with frequency, which is a measure of how selective the filter.
The higher the order of the filter, the higher the slope of the diagonal asymptote or the roll-off
rate. The roll-off rate for the first order is 20 dB/decade and it will be 40 dB/decade for the
second-order.
First-order Low pass filter
A first-order low pass active filter passes frequencies below the filter cutoff frequency. Figure 1
shows the connection of an op-amp unit as a first-order low pass filter. The cut-off frequency of
the first-order low pass filter is determined by:
fL =
1
Hz
2π R2C2
The output drops off at 6 dB/octave or 20 dB/decade above the cutoff frequency.
(1.0)
BENG3211
R1
1.5kΩ
C2
1.0nF
R2
-
1kΩ
Vo
+
Vin
Figure 1 : First-order low pass filter
First-order High pass filter
A first-order high pass filter, as shown in Figure 2, maintains the output amplitude at frequencies
above a high cutoff frequency which is determined by:
fH =
1
Hz
2π R2C2
(1.1)
The output drops off at 6 dB/octave or 20 dB/decade below the cutoff frequency.
R1
C2
100nF
1.5kΩ
R2
1kΩ
+
Vin
Figure 2 : First-order high pass filter
Vo
BENG3211
Second-order Low pass filter
The second-order low pass filter configuration shown in Figure 3 is commonly used in many
practical electronic systems.
0.01uF
3kΩ
3kΩ
R1
R2
C1
Vin
C2
0.01uF
RF
10kΩ
RS
15kΩ
Figure 3 : Second-order low pass filter
The critical frequency fo, of the second-order low pass filter is determined by:
fo =
1
2πRC
(1.2)
and pass-band gain APB,
A PB =
RF
+1
Rs
(1.3)
The roll-off rate is -40 dB/decade.
Safety Guide
1.
Before making any connections to your circuit, always make sure that the power supply is
off.
2. Ensure that the circuit 'Ground' and 'Power' connections are correct before proceeding to
making the other connections.
3. Keep your working area clean and organized. Store away all unused components or
equipment in their proper places.
Notes:
All calculation must use actual value of the component (measure the value of the
components)
BENG3211
4.0
Procedures
Part 1: First-order low pass active filter
1. Construct the first-order low pass filter circuit as in the Figure 1.
2. Apply an input of 1 Vrms and vary the signal frequency from 1kHz to 1MHz while measuring
and recording the output voltage.
3. Plot the output gain-frequency response curve.
Part 2: First-order high pass active filter
1. Construct the first-order high pass filter circuit as in the figure 2.
2. Apply an input of 1 Vrms and vary the signal frequency from 1kH to 10Hz while measuring
and recording the output voltage.
3. Plot the output gain-frequency response curve.
Part 3: Second-order low pass filter
1. Use the circuit in part 1 and 2 to construct a second-order low pass filter.
2. Determine the critical frequency fo, pass-band gain APB, damping factor ξ, and the roll-off rate
for a second-order low pass filter.
3. Plot the output gain-frequency response curve.
5.0
Discussion
Compare the critical frequency from calculation result in part 1, 2 and 3 with that obtained from
graph. If there is a difference, explain why?