Section Index | Owatonna Steele County Amateur Radio |
20-Jul-2007 |

Active FiltersActive filters can be used to limit the range of frequencies that may be passed to an amplifier. Filters can be designed to pass high frequencies, low frequencies, or a range of middle frequencies. Filters can be designed utilizing op-amps in an inverting amplifier configuration.
**Cutoff Frequency**: The frequency fc where the filter output voltage falls to 70.7% of its peak output. This is equivalent to 50% (-3dB) of the peak power output.**Low Pass Filter**: Attenuates signals with frequencies*above*Fc.**High Pass Filter**: Attenuates signals with frequencies*below*Fc.**Band-Pass Filter**: Attenuates signals with frequencies*outside*of a range of frequencies (passband).**Q**: The ratio of a Band-Pass Filter's center frequency to the bandwith of its passband. A higher "Q" indicates a narrow passband for a given center frequency.**Roll-Off**: The gradual reduction in signal amplitude beyond a Fc.**Vi**: The inverting input of the op-amp.
High Pass FilterA high pass filter is as simple as adding a capacitor (Ci) in series with the input signal. The reactance of a capacitor is frequency dependent: - Xc = 1 / ( 2 * π * fc ).
roll-off. Ci is calculated as:
- Ci = 1 / ( 2 * π * fc * Ri )
- The capacitor reactance decreases.
- The impedance of the feedback signal decreases, resulting in a lower voltage drop across Rf and Cf.
- Because RF, Cf, and R form a voltage divider, Vi increases.
- This means that less output voltage is required to balance the input current at higher frequencies.
Ci is calculated as: - Cf = 1 / ( 2 * π * fc * Rf )
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