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The Sallen–Key topology is an electronic filter topology used to implement second-order active filters that is particularly valued for its simplicity.〔("EE315A Course Notes - Chapter 2"-B. Murmann )〕 It is a degenerate form of a voltage-controlled voltage-source (VCVS) filter topology. A VCVS filter uses a super-unity-gain voltage amplifier with practically infinite input impedance and zero output impedance to implement a 2-pole (12 dB/octave) low-pass, high-pass, or bandpass response. The super-unity-gain amplifier allows for very high Q factor and passband gain without the use of inductors. A Sallen–Key filter is a variation on a VCVS filter that uses a unity-gain amplifier (i.e., a pure buffer amplifier with 0 dB gain). It was introduced by R. P. Sallen and E. L. Key of MIT Lincoln Laboratory in 1955. Because of its high input impedance and easily selectable gain, an operational amplifier in a conventional non-inverting configuration is often used in VCVS implementations. Implementations of Sallen–Key filters often use an operational amplifier configured as a voltage follower; however, emitter or source followers are other common choices for the buffer amplifier. VCVS filters are relatively resilient to component tolerance, but obtaining high Q factor may require extreme component value spread or high amplifier gain.〔 Higher-order filters can be obtained by cascading two or more stages. ==Generic Sallen–Key topology== The generic unity-gain Sallen–Key filter topology implemented with a unity-gain operational amplifier is shown in Figure 1. The following analysis is based on the assumption that the operational amplifier is ideal. Because the operational amplifier (OA) is in a negative-feedback configuration, its ''v''+ and ''v''- inputs must match (i.e., ''v''+ = ''v''-). However, the inverting input ''v''- is connected directly to the output ''v''out, and so : By combining Equations (1) and (2), : Applying Equation (1) and KCL at the OA's non-inverting input ''v''+ gives : which means that : Combining Equations (2) and (3) gives : Rearranging Equation (4) gives the transfer function : which typically describes a second-order LTI system. 抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)』 ■ウィキペディアで「Sallen–Key topology」の詳細全文を読む スポンサード リンク
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