L09 lecture full

L09 lecture full - Feedback Benefits Reduced sensitivity to...

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EECS 240 Lecture 9: Feedback B. Boser 1 Feedback Benefits – Reduced sensitivity to Gain variations Nonlinearity – Increased bandwidth Caveat: potential instability Stability test – Bounded input, bounded output: no general test available – Linear system: Poles in LHP (“left half-plane”) •N y q u i s t c r i t e r i o n Bode criterion – Hand-analysis –S P I C E
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EECS 240 Lecture 9: Feedback B. Boser 2 Generic Feedback Circuit 1 for 1 1 1 1 1 : gain loop - closed : gain loop : factor feedback : gain loop - open 1 >> + = + = = = T f f T a V V A f a T f a T i o v v
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EECS 240 Lecture 9: Feedback B. Boser 3 Electronic Feedback Circuit f T R R R R T R R R R V V A f a T R R R f a i o v v 1 1 for 1 1 1 : gain loop - closed : gain loop isolate) to difficult (sometimes : factor feedback : gain loop - open 1 2 1 2 2 1 1 2 2 1 1 >> + = = = + =
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EECS 240 Lecture 9: Feedback B. Boser 4 Stability Analysis Depends on T(s) –NOT a(s) Finding T(s): – Hand analysis: Break loop at controlled source (e.g. g m ) •T = - s r / s t – SPICE: Controlled sources not accessible a) Break loop, model load (approximation), or b) Determine T from T v and T i (exact)
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EECS 240 Lecture 9: Feedback B. Boser 5 Simple Circuit Example freq, Hertz C 1 r o C GS Vi C 2 Vo C 1 C 2 g m v gs v i v o v gs Small Signal Equivalent Loop Gain = ? Feedback Amplifier (Biasing for V gs not shown)
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EECS 240 Lecture 9: Feedback B. Boser 6 Return Ratio Analysis [HLGM 01] freq, Hertz r o C GS C 2 g m v gs v gs C 1 i test i return 1. Set all independent sources to zero (v i =0) 2. Disconnect (ideal) controlled source from circuit 3. Replace with test source 4. Find ratio return signal/test signal = “Return Ratio“ = Loop Gain GS GS o GS m C C C C C C r p C C C C F p s r g F s T + + + = + + = + = 1 2 1 2 1 2 1 2 1 0 ) ( 1 1 1 ) ( Easy! Why not do the same thing in SPICE?
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EECS 240 Lecture 9: Feedback B. Boser 7 MOSFET AC Simulation Model freq, Hertz Small-signal model not accessible in SPICE!
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