22-02-2013, 11:52 AM
To design LPF using OPAMP.
LPF using OPAMP.docx (Size: 19 KB / Downloads: 15)
OBJECTIVE: - To design LPF using OPAMP.
(I) PRACTICAL SIGNIFICANCE: -LPF is used in AM receivers.
(II) COMPETENCY/SKILL: - Design and testing.
(III) EXPERIMENT OBJECTIVE: - To design LPF using OPAMP.
(IV) THEORETICAL BACKGROUND: -
A first order low pass butter worth filter that uses an RC network for filtering. The resistors R1 and RF determine the gain of the filter. The low pass filter has a constant gain AF from 0 HZ to the high cutoff frequency fH. At fH the gain is 0.707AF, and after fH it decreases at constant rate with an increase in frequency. That is when the frequency is increased tenfold (one decade), the voltage gain is divided by 10. In other words, the gain decreases 20db (=20 log 10) each time the frequency is increased by 10. Hence the rate at which the gain rolls of after fH is 20db/decade. The frequency f = fH is called the cutoff frequency because the gain of the filter at this frequency down by 3dB(= 20 log 0.707 ) from 0 HZ.
FREQUENCY SCALING:-
Once a filter is designed, there may sometimes be a need to change its cutoff frequency. The procedure used to convert an original cutoff frequency fH to a new cutoff frequency fH is called frequency scaling. To change a high cutoff frequency, multiply R or C but not both, by the ratio of original cutoff frequency to the new cutoff frequency. In filter design the needed values of R and C are often not standard. Besides a variable capacitor is not commonly used. Therefore, choose a standard value of capacitor, and then calculate the value of resistor for a desired cutoff frequency. This is because for a nonstandard value of resistor a potentiometer can be used.
PROCEDURE:
A low pass filter can be designed by implementing the following steps:
1. Choose the value of high cutoff frequency fH.
2. Select the value of C less than or equal to 1F.
3. Calculate the value of R using R = 1 / 2fHC.
4. Finally, select values of R1 and RF dependent on the desired pass band gain AF using AF = 1 + RF / R1.
5. Now connect the circuit as per diagram.
6. Apply sinusoidal input signal of constant amplitude to input terminal of the circuit.
7. Vary the input frequency from 10 Hz to maximum in steps and measure peak to peak amplitude of VO after each steps.
8. Take down the reading according to observation table.
9. From these reading draw the frequency response curve for the filter and calculate voltage gain, pass band & high cut off frequency.