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Unformatted text preview: ECE 209: Circuits and Electronics Laboratory Notes for Lab 4 (Frequency Response of First-Order Active Circuits) 1. Comments on returned lab report. Put units on tables and figures and show individual data points (i.e., not just interpolation). In meters lab, positive error on R v is a good thing. Manufacturer gives worst-case specification. 2. First-order active filters. Active filters allow for gain, simplicity, robustness, and can have hard knees without inductors. Use standard inverting/non-inverting OA configuration, but use (equivalent) Laplace-domain impedances instead of simple resistances. To quickly determine characteristic of filter (i.e., low-pass, high-pass, or bandpass) consider what happens to OA configurations gain at some sample frequencies. In todays lab, both filters have transfer function given by Z F ( s ) /Z I ( s ). In the low-pass filter, Z I ( s ) = R 1 and Z F ( s ) = R 2 bardbl ( sC ) 1 . * Z F (0) R 2 for low frequencies LF gain is R 2 /R 1 (i.e., R 2 /R 1 with 180 shift). * Z I ( j ) 0 for high frequencies HF gain is /R 1 0 (with 180 90 shift). * First-order filters time constant must depend on C , but because V is a virtual ground, the output does not feel the effect of R 1 . So time constant = R 2 C . * It is a low-pass filter with passband gain K = R 2 /R 1 and time constant = R 2 C : H LPF ( s ) defines K s + 1 = R 2 R 1 sR 2 C + 1 . In the high-pass filter, Z I ( s ) = R 1 + 1 / ( sC ) and Z F ( s ) = R 2 . * Z I (0) (i.e., open for VLF) LF gain is R 2 / 0 (with 180 + 90 shift). * Z I ( j ) R 1 for high frequencies HF gain is R 2 /R 1 (i.e., R 2 /R 1 with 180 shift). * First-order filters time constant must depend on C , but because V is a virtual ground, the input does not feel the effect of R 2 . So time constant = R 1 C ....
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- Fall '08