HO12_214W09_Noise_part2_2pp

HO12_214W09_Noise_part2_2pp - Handout #12 EE 214 Winter...

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Handout #12 EE 214 Winter 2009 Electronic Noise Part II B. Murmann and B. A. Wooley Stanford University Noise in Circuits (1) ± Most circuits have more than one relevant noise source ± In order to quantify the net effect of all noise sources, we must refer the noise sources to a single "interesting" port of the circuit – Usually the output or input B. A. Wooley, B. Murmann EE214 Winter 2008-09 2
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Noise in Circuits (2) Output referred noise Input referred noise Refer noise to output via individual noise transfer functions Physical concept, exactly Represent total noise via a fictitious input source that captures all circuit-internal noise sources Physical concept, exactly what one would measure in the lab – Useful primarily for “general purpose” components, in which the output noise depends on how the component is used B. A. Wooley, B. Murmann EE214 Winter 2008-09 3 Circuit Example ± For simplicity, let’s neglect – Source impedance All capacitances – Burst and flicker noise –r o , r b , and r μ B. A. Wooley, B. Murmann EE214 Winter 2008-09 4
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Output Referred Noise PSD 22 1 42 vk T f q I f R ⎛⎞ + Δ ⎜⎟ () 2 ou tC m R kT f R g R ⎝⎠ + 2 2 21 m m kTg f R gR + ± Shot noise due to base current is absorbed by the input source and does not contribute to noise at the output – We will revisit the case of finite R s later ± For large gain (g m R), the collector shot noise dominates B. A. Wooley, B. Murmann EE214 Winter 2008-09 5 Input Referred Noise PSD ± From the previous calculation, we know that 2 out m m T g f R = Δ⋅ + ± Since out v in vA v = 2 out v out v m v w h e r e A g R == ± We can write 2 2 2 2 12 m m in mm m kTg f R T f gg R + = + ( ) ± Larger g m translates in lower input referred voltage noise B. A. Wooley, B. Murmann EE214 Winter 2008-09 6
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Spice Simulation *** EE214 BJT noise example *** biasing ib vcc vb 100u g m =3.67mS, R=10k, A v =36.7 q1 vb vb 0 npn214 c1 vb 0 1 *** main circuit v1 vcc 0 2.5 vi vi vb ac 1 rl vcc vo 10k q2 vo vi 0 npn214 .op .ac dec 100 100 10e9 .noise v(vo) vi .options post brief .inc 'ee214_hspice.sp' d B. A. Wooley, B. Murmann EE214 Winter 2008-09 7 .end Output Referred Noise PSD with Load Capacitance 2 2 11 42 out C vk T f q I f R | | Rj C ⎛⎞ + Δ ⎜⎟ 2 2 1 C kT f qI f R R C ω ⎝⎠ + Δ 2 2 21 1 m m kTg f R gR jRC + ± Same calculation as before except that now the noise current drops into Same calculation as before, except that now the noise current drops into parallel combination of R and C ± Output PSD is shaped by squared magnitude of first order response B. A. Wooley, B. Murmann EE214 Winter 2008-09 8
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Input Referred Noise PSD with Load Capacitance ± Same calculation as before, except that the voltage gain is now frequency dependent 2 2 2 1 v ω ( ) ( ) () 2 2 2 0 1 out in v v v vw h e r e A j A j RC Aj ω= ω = ω ⎛⎞ ( ) 2 2 21 1 1 0 m m v kTg f R gR jRC A Δ⋅ + ⋅ ⎜⎟ ⎝⎠ = 1 j RC + ω ± Input referred noise is frequency independent, because the output noise and gain have the same frequency rol of 2 12 in mm vk T f gg R Δ + and gain have the same frequency roll-off B. A. Wooley, B. Murmann EE214 Winter 2008-09 9
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HO12_214W09_Noise_part2_2pp - Handout #12 EE 214 Winter...

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