EECS140 ANALOG CIRCUIT DESIGN MORE ON OP AMPS TELESCOPIC AND FOLDED CASCODE University of California Berkeley College of Engineering Department of Electrical Engineering and Computer Science Robert W Brodersen Fall 2002 EECS140 Analog Circuit Design More on Op Amps TELESCOPIC and FOLDED CASCODE ROBERT W BRODERSEN LECTURE 22 EECS140 ANALOG CIRCUIT DESIGN MORE ON OP AMPS TELESCOPIC AND FOLDED CASCODE MOA 1 V dd I1 M3 2 1 M4 2 1 V G6 M5 2 1 M7 2 1 V G8 Telescopic OP AMP Circuit 1 Poor Bias Strategy M11 2 1 I REF RC M2 10 1 M13 4 1 M12 2 1 V dd ROBERT W BRODERSEN OUT M8 2 1 2 ic M10 2 1 V id 2 M1 10 1 V I2 V id 2 1 M6 2 1 M9 2 1 V dd LECTURE 22 EECS140 ANALOG CIRCUIT DESIGN MORE ON OP AMPS TELESCOPIC AND FOLDED CASCODE Circuit 1 Telecopic OP AMP with BAD Bias of cascode The currents are balanced so that all transistors have I DS I REF except for M13 which has I DS13 2 I REF Since I 1 I2 I REf then V GS3 V GS4 V GS9 all W L s are equal also This also implies that V DS4 V DS3 similarily V DS5 V DS6 So if the input has V id 0 ROBERT W BRODERSEN LECTURE 22 MOA 2 EECS140 ANALOG CIRCUIT DESIGN MORE ON OP AMPS TELESCOPIC AND FOLDED CASCODE MOA 3 then V OUT V DD 2 V To 2 V DSAT V G6 if 0 The gate of M8 is also at V DD 2 V To 2 V DSAT due to its connection to M11 so V OUT V G6 V G8 The swing in the positive direction will be V OUT MAX V G6 V T But since V OUT V G6 this means the swing is only 1 V T in the positive direction In the negative direction V OUT MIN V G8 V T but since V OUT V G8 the swing is only VT again ROBERT W BRODERSEN LECTURE 22 EECS140 ANALOG CIRCUIT DESIGN MORE ON OP AMPS TELESCOPIC AND FOLDED CASCODE MOA 4 Thus the total swing is only 2V T not too good In the positive direction we could use a high swing configuration as was described for the cascoded current source On the low side we can use a better circuit Circuit 2 Telescopic with a cascode bias that gives a better swing in the negative direction Maximum voltage in the positive direction is given by M6 going linear when V OUT MAX V DD V GS3 V DSAT6 Negative swing is limited by M8 going linear V OUT MIN V A V T8 Set VA so that M1 M2 are at the edge of saturation ROBERT W BRODERSEN LECTURE 22 EECS140 ANALOG CIRCUIT DESIGN LECTURES ON MOS DEVICE MODELS MOA 5 V dd 5V M3 2 1 M4 2 1 d4 d3 M5 2 1 I REF 2 M7 2 1 M6 2 1 Ib d5 VA d1 1 M1 10 1 M9 2 30 VS d12 OUT IREF 2 M8 2 1 d9 I REF d2 2 M2 10 1 M12 2 1 RC M13 2 1 V D D V dd 5V ROBERT W BRODERSEN Telescopic OP AMP Circuit 2 Better Bias LECTURE 2 FALL 1998 EECS140 ANALOG CIRCUIT DESIGN LECTURES ON MOS DEVICE MODELS V DD Telescopic Op Amp MOA 6 Max swing positive M3 M4 V DD 2 V V T V DD V T V M5 M6 g r 2 m o V DD 2 V T V V DD 2 V V T M7 M8 g m r o2 RC M9 M1 M2 M12 M13 V D D V D D ROBERT W BRODERSEN LECTURE 2 FALL 1998 EECS140 ANALOG CIRCUIT DESIGN LECTURES ON MOS DEVICE MODELS Telescopic Op Amp Cont MOA 7 V A V S V DSAT 9 V T 9 V DS2 V DSAT 2 for setting M2 at EOS V DS2 V A V T8 V DSAT8 V S V S V DSAT9 V T9 V T8 V DSAT 8 V S V DS2 V DSAT9 V DSAT 8 V DSAT2 for EOS V DSAT 9 V DSAT2 V DSAT8 W W L L 8 7 ROBERT W BRODERSEN W W L L 1 2 LECTURE 2 FALL 1998 EECS140 ANALOG CIRCUIT DESIGN MORE ON OP AMPS TELESCOPIC AND FOLDED CASCODE MOA 8 This means that V A V S V DSAT 2 V T 8 V DSAT 8 Since this will set V DS2 V DSAT 2 To calculate the W L 9 and IB to do this we set V DS 9 V A V S V DSAT2 V T8 V DSAT8 since V DS 9 V T9 V DSAT9 V DSAT 9 V T 8 V T9 V DSAT2 V DSAT8 If we say V T 8 V T9 then 1 2 2 1 2 2 IB 1 1 2 I DS2 1 2 1 2 k n k W W W n L L L 8 2 9 ROBERT W BRODERSEN LECTURE 22 EECS140 ANALOG CIRCUIT DESIGN MORE ON OP AMPS TELESCOPIC AND FOLDED CASCODE MOA 9 so 1 2 W I B W L IS L 8 9 1 2 W L 2 2 if then W 10 L 2 W 2 L I B 0 79 A 8 1 W L 9 15 Gain and Rout GM g m1 cascoding has no effect on g m R OUT g m6 r o4 r o6 g m8 r o2 r o8 m6 g m 8 r o4 r o6 r o2 r o8 A d g m 1 g g m 6 r o4 r o6 g m 8 r o2 r o8 ROBERT W BRODERSEN LECTURE 22 I S 10 5 A EECS140 ANALOG CIRCUIT DESIGN MORE ON OP AMPS TELESCOPIC AND FOLDED CASCODE I DS 1 1 g m 6 r o4 r o6 2 V DSAT 6 p I DS p I DS I DS 2 1 1 A d 2 2 2 V DSAT1 I DS V DSAT6 p V DSAT8 n 4 1 V DSAT1 V DSAT6 p2 V DSAT8 n2 To see the current dependence let g m g m1 g m 6 and r o r o4 r o6 r o2 r o8 1 DS A d g m2 r o2 I 2 I DS I DS Gain keeps increasing as we decrease the current ROBERT W BRODERSEN LECTURE 22 MOA 10 EECS140 ANALOG CIRCUIT DESIGN DIFCAS2 SW0 V VOUT LECTURES ON MOS DEVICE MODELS CASCODED DIFF PAIR GATE VOLTAGE SWEEP 96 10 05 14 40 14 MOA 11 4 50 4 250 4 0 Gain 3 5k 3 750 3 50 3 250 3 0 2 750 V O L T S 2 50 2 250 2 0 L I N 1 750 1 50 1 250 1 0 Gain 240k 750 0M 500 0M 250 0M 0 250 0M 500 0M 20 0M ROBERT W BRODERSEN 10 0M VOLTS LIN 0 LECTURE 2 10 0M 20 0M FALL 1998 EECS140 …
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