Page 240 - Op Amps Design, Applications, and Troubleshooting
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222    ACTIVE FILTERS
















           FIGURE 5.7 A 40-dB-per-decade
           high-pass filter circuit.




                  gain circuit, but it has no input signal. At low frequencies, we expect little or no
                  output signal.
                       At high frequencies, the capacitors will have a low reactance and wiM begin
                  to appear as short circuits. The high-frequency equivalent circuit is shown in Fig-
                  ure 5.9. Here we see that the capacitors have been replaced with direct connec-
                  tions. Also, resistor R 2 has been removed, because it is connected between two
                  points that have the same signal amplitude and phase (i.e., input and output of a
                  voltage follower). Because it has the same potential on both ends, it will have no
                  current flow and is essentially open. The resulting equivalent circuit indicates that
                  for high frequencies, our high-pass filter will act as a simple voltage follower.














           FIGURE 5.8 A low-frequency
           equivalent circuit for the high-pass
           filter shown in Figure 5.7.













           FIGURE 5.9 A high-frequency
           equivalent circuit for the high-pass
           filter shown in Figure 5.7.
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