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Unformatted text preview: ECE 209: Circuits and Electronics Laboratory Notes for Lab 4 (Frequency Response of FirstOrder 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 worstcase specification. 2. Firstorder active filters. Active filters allow for gain, simplicity, robustness, and can have hard knees without inductors. Use standard inverting/noninverting OA configuration, but use (equivalent) Laplacedomain impedances instead of simple resistances. To quickly determine characteristic of filter (i.e., lowpass, highpass, 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 lowpass 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). * Firstorder 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 lowpass 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 highpass 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). * Firstorder 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
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 Frequency

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