Lect2UP270_(100328)

Lect2UP270_(100328) - Lecture 270 High Speed Op Amps...

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Lecture 270 – High Speed Op Amps (3/28/10) Page 270-1 CMOS Analog Circuit Design © P.E. Allen - 2010 LECTURE 270 – HIGH SPEED OP AMPS LECTURE ORGANIZATION Outline Extending the GB of conventional op amps Cascade Amplifiers - Voltage amplifiers - Voltage amplifiers using current feedback • Summary CMOS Analog Circuit Design, 2 nd Edition Reference Pages 368-384 Lecture 270 – High Speed Op Amps (3/28/10) Page 270-2 CMOS Analog Circuit Design © P.E. Allen - 2010 INCREASING THE GB OF OP AMPS What is the Influence of GB on the Frequency Response? The unity-gainbandwidth represents a limit in the trade-off between closed loop voltage gain and the closed-loop -3dB frequency. Example of a gain of -10 voltage amplifier: 0dB 20dB | A vd (0)| dB Magnitude log 10 ( ω ) GB ω A ω -3dB Op amp frequency response Amplifier with a gain of -10 Fig. 7.2-1 What defines the GB ? We know that GB = g m C where g m is the transconductance that converts the input voltage to current and C is the capacitor that causes the dominant pole. This relationship assumes that all higher-order poles are greater than GB .
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Lecture 270 – High Speed Op Amps (3/28/10) Page 270-3 CMOS Analog Circuit Design © P.E. Allen - 2010 What is the Limit of GB ? The following illustrates what happens when the next higher pole is not greater than GB : For a two-stage op amp, the poles and zeros are: 1.) Dominant pole p 1 = - g m 1 A v (0) C c 2.) Output pole p 2 = - g m 6 C L 3.) Mirror pole p 3 = - g m 3 C gs 3 + C gs 4 and z 3 = 2 p 3 4.) Nulling pole p 4 = -1 R z C I 5.) Nulling zero z 1 = -1 R z C c -( C c / g m 6 ) 0dB 20dB | A vd (0)| dB Magnitude log 10 ( ω ) GB ω A ω -3dB Op amp frequency response Amplifier with a gain of -10 Fig. 7.2-2 Next higher pole -40dB/dec Lecture 270 – High Speed Op Amps (3/28/10) Page 270-4 CMOS Analog Circuit Design © P.E. Allen - 2010 Higher-Order Poles For reasonable phase margin, the smallest higher-order pole should be 2-3 times larger than GB if all other higher-order poles are larger than 10 GB . 060709-01 -GB - 10 GB Dominant pole Smallest non- dominant pole Larger non- dominant poles GB 10 GB 0dB A v (0) dB GB A v (0) log 10 ω If the higher-order poles are not greater than 10 GB , then the distance from GB to the smallest non-dominant pole should be increased for reasonable phase margin.
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Lecture 270 – High Speed Op Amps (3/28/10) Page 270-5 CMOS Analog Circuit Design © P.E. Allen - 2010 Increasing the GB of a Two-Stage Op Amp 1.) Use the nulling zero to cancel the closest pole beyond the dominant pole. 2.) The maximum GB would be equal to the magnitude of the second closest pole beyond the dominant pole. 3.) Adjust the dominant pole so that 2.2 GB ± (second closest pole beyond the dominant pole) Illustration which assumes that p 2 is the next closest pole beyond the dominant pole: 0dB | A vd (0)| dB Magnitude log 10 ( ω ) Fig. 7.2-3 -40dB/dec - p 1 - p 2 = z 1 - p 4 - p 3 | p 1 | | p 2 | | p 4 | | p 3 | -60dB/dec -80dB/dec Before cancelling p 2 by z 1 and increasing p 1 j ω σ | p 1 | GB - p 1 New Old GB Increase Old GB New Old New Lecture 270 – High Speed Op Amps (3/28/10) Page 270-6 CMOS Analog Circuit Design © P.E. Allen - 2010 Example 270-1 - Increasing the GB of the Op Amp Designed in Ex. 230-1
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Lect2UP270_(100328) - Lecture 270 High Speed Op Amps...

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