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For a CFB amplifier the open-loop gain is ZT and its feedback factor is 1/RF, making the loop-gain TCFB = ZT/RF. 1 U6 K/ o% k- Y8 c9 q+ _2 lThe signal bandwidth is determined by RF and not by the circuit gain.+ o+ `# C9 j: `/ L& |6 v
The circuit gain is independently set with RS. * r4 a4 j6 h2 G3 H8 P& SThe signal bandwidth remains stable for all gain settings .3 M1 v6 e: B3 k ~& L" w: g4 s
Even for unity-gain operation, an RF resistor is required. * n+ a; U+ V& W# XThe best practice when designing with CFB op-amps is to use the RF values given in the datasheet and to adjust the desired gain level through RS. . O, x. x( \: l5 S5 Q" i
slide 1020 2 x2 k2 ^$ ]9 t: m% e k. |2 A & ~/ J" \! p" `4 K& b(1) The closed-loop gain can be modified by changing RS, leaving the closed-loop bandwidth unchanged.' I w' P* j7 ]4 ^+ O6 y
(2) For a given RF, frequency compensation can be optimized. 4 ]; V$ }2 T4 m(3)Suitable for high frequency applications. 4 U: ?& \* T% _
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1. First order Sigma-Delta patterns in the output spectrum & v1 _$ |. ~+ z) P5 d1 ?5 t, \=> Idle tones(pattern noise, limit cycles) 3 x D0 I9 U( W" `2. Second order Sigma-delta 輸入太大時還是會不穩定