Experiment No.7 Active Low pass filter

AEEE239 Exp.7 Active low-pass filter

Experiment No.7 – Active low-pass filter Objectives • To design a first order low pass Butterworth filter. • To obtain the frequency response of the above filter and examine the effect of the variation of the circuit components. Procedure 1. Connect the circuit of the first order low pass Butterworth filter shown in fig.7.1 V+

R

+

Vi 1.2 KΩ Ω C 0.1 µF

Vo

V-

R1 10 KΩ Ω

R2 10 KΩ Ω

Fig.7.1 2. Calculate the cut-off frequency and the voltage gain of the above filter. fc=

Av=

3. For an input voltage Vi=1 V (peak) record the frequency and the output voltage of the above filter and complete Table 1.1. (Note. Obtain more readings close to the theoretical cut-off frequency)

f (KHz) V0 (V) Gain (dB) f (KHz) V0 (V) Gain (dB) Table 7.1

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AEEE239 Exp.7 Active low-pass filter

4. From the results obtained in Table 7.1 plot the frequency Response (Gain vs frequency) in Fig.7.2

Fig.7.2 5. Sketch the theoretical bode plot in Fig.7.2 and compare with the experimental curve. 6. Replace the series Resistor R with a 2.2KΩ. Hence, by keeping the same input voltage (1Vp) and varying the frequency of the signal generator locate and record the passband gain and the cut-off frequency (at -3dB). fc=

Av(pass)=

7. Keep the series resistor R at 2.2 KΩ and replace the feedback resistor R1 with a 2.2 KΩ. and repeat the procedure in step 6. fc=

Av(pass)=

2

AEEE239 Exp.7 Active low-pass filter

8. Connect the circuit of the second order low pass Butterworth filter shown in fig.7.3

CA 1 nF + 15 V

RA

RB

22 KΩ Ω

22 KΩ Ω

Vi

+ Vo

-

CB 1 nF

R1 - 15 V

5.9 KΩ Ω

R2 10 KΩ Ω Fig.7.3 9. Calculate the cut-off frequency and the voltage gain of the above filter. fc=

Av=

10. For an input voltage Vi=1 V (peak) record the frequency and the output voltage of the above filter and complete Table 7.2. (Note. Obtain more readings close to the theoretical cut-off frequency) f (KHz) V0 (V) Gain (dB) f (KHz) V0 (V) Gain (dB) Table 7.2

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AEEE239 Exp.7 Active low-pass filter

11. From the results obtained in Table 7.2 plot the frequency Response (Gain vs frequency) in Fig.7.4

Fig.7.4 12. From the frequency response in obtained in Fig.7.4, calculate the cut-off frequency and the voltage gain of the above filter. fc=

Av=

Theory: 1. Active Filters in: “Electronic Devices,” by Thomas L. Floyd, Prentice Hall; 6th edition (2001) Reviewed by: Dr C. Themistos (July 2004).

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