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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. - f" o6 y- \3 x2 d# d& ]The signal bandwidth is determined by RF and not by the circuit gain. / z' x5 Z# G p1 ~9 @( W7 xThe circuit gain is independently set with RS. $ a0 _0 Y; w+ D- kThe signal bandwidth remains stable for all gain settings .5 z0 l$ J. W5 o$ g& _9 t, V6 ]
Even for unity-gain operation, an RF resistor is required. ! N$ w' i0 j- @( B7 G& |- WThe 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.5 q n, Z5 U0 i4 D- C% Y9 V ?% Z; ?
: ~9 J# i. j( z4 b% ?. n6 B n | m(1) The closed-loop gain can be modified by changing RS, leaving the closed-loop bandwidth unchanged., H* r* p& z# k. e4 M* O/ F
(2) For a given RF, frequency compensation can be optimized. _/ ~2 O. A# V! Q
(3)Suitable for high frequency applications. 2 e$ Q' |# {/ H% u1 \+ b, K
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1. First order Sigma-Delta patterns in the output spectrum+ Y5 J, k& Z. {0 m+ E0 t/ Z$ \- g
=> Idle tones(pattern noise, limit cycles)) @, e' K! u) k0 B
2. Second order Sigma-delta 輸入太大時還是會不穩定