Lab-in-a-Box

Experiment 28: Active Filters

Name: ______Pledge: ______ID: ______Date: ______

Procedure +9V 7

3 + 5

+ V OS2 6 + R1 C 0 OUT

2 - 1

- V OS1 .1u LF356N RLoad

Vin 4 Vo 1Vac R3 -9V 2.2k 0Vdc

R2 0 - 0 0

C

.1u Figure 1: Circuit diagram for active bandpass filter. Analysis: 1. Design the bandpass filter in Figure 1 to have a center frequency of 1.0 kHz, a bandwidth of 200 Hz, and a voltage gain of -5 at the center frequency. (Note: The values of the capacitors and the load resistor have already been given to you and are determined by the components available in your parts kit.)

2. Using your answers from step 1, choose resistors from your kit to use for R1 and R2 . You may want to put two or more resistors in series to get close to your design values. Also, choose the

trim pot with the smallest range that contains the value you calculated for R3 . For greater sensitivity, you may want to put a smaller trim pot in series with a resistor such that the trim pot,

when set near its midpoint, plus the fixed resistor sum to R3. 3. Using your choice of scientific graphing program (e.g., Excel, MATLAB, Derive, etc.), plot the magnitude and phase Bode plots for the filter from 60 Hz to 3 kHz. Do not plot any data points; show only a line through them. Save these graphs for use in steps 13. A MATLAB program that serves as a template for preparing this plot is given in Appendix D.1. The program may be downloaded from the text WWW site at http://www.wiley.com/college/Hendricks. 4. Extra Credit: Derive Eqs (1) through (3) given in the text.

1 of 2 PSpice Analysis: 5. Construct the circuit of Figure 1 in PSpice.

6. Set R1 and R2 equal to the values determined in step 2.

7. Set R3 equal to the value calculated in step 1. 8. Use AC sweep analysis to plot the frequency response. 9. Using the plot of the frequency response, find the center frequency, gain, and bandwidth of the circuit. Calculate the percent deviation of each parameter from the design criteria given in step 1. 10. Print out a copy, or save an electronic copy, of the frequency response plot. Measurements:

11. Construct the circuit in Figure, setting the trim pot to the value of R3 measured in step 1. Set the frequency of the function generator to its lowest value and set its amplitude to 1 V. 12. Gradually increase the frequency of the circuit until the output reaches a maximum. Determine the amplitude, gain, frequency, and phase of the output at this point. 13. Calculate the percent deviation of the measured gain and center frequency from the design criteria given in step 1. Explain the relationship between the gain and the phase of the output. 14. Set the upper and lower voltage cursors at the -3 dB point from the signal maximum. 15. Decrease the input signal frequency from the center frequency until the maximum output signal just touches the upper and lower cursors. Record the frequency. 16. Repeat step 14 but increase the input signal frequency from the center frequency. Record the frequency at which the output signal again just touches the cursors. 17. Compute the bandwidth of the filter and compute the percent deviation of the bandwidth from the design criteria given in step 1.

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