week_11 - 98 Operational Amplifier Need for high gain...

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98 Operational Amplifier Need for high gain stage: in V . A out V DD V = A AV V in out problem is that with having even small DC component will be saturated to either or ground. in V out V DD V Assume ) 2 ( . 1 . 0 100 ) 2 ( . 1 1 000 , 100 3 ft Cos v v AV V ft Cos v mvDC V A v V in out in DD π µ + = = + = = = We don’t have 100v bias on the amplifier = v V V DD out 3 so the gain stage will not work if DC signal is present. How about noise? noise v ft Cos v V noise v ft Cos v V out in 1 ) 2 ( . 1 . 0 10 ) 2 ( . 1 + = + = Signal
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99 Therefore the noise masks the output performance because it is amplified with the same gain as signal being amplified. Solution Æ use high gain differential amplifier. If your input signal has DC component or common mode noise component it will not be amplified by the differential amplifier. in V . A out V DD + V ~ + ~ DC V 2 noise V Assume: ) ( . 1 100 ) 2 ( 1 01 . 0 000 , 100 mod mod mod mod noise DC e common diff in e diff out DC noise diff in e common e diff V V A V A V v V ac v V ft vCos V A A + + = = = = = = µ π amp diff the of noise added v v ft vCos noise + + + = 1 01 . 0 ) 2 ( 1 . 0
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100 High gain differential-stage are called OPAMPS . output DD V + ) ( input v e + . ) ( input v e Usually they are built as following: V DD is used to stabilize the system when if is need with feedback. cmp C Example of such OPAMP in V 1 A out V + 2 A 1 cmp C Diff-input stage High gain stage Buffer for driving high output current 6 . 1 300 2 Q cmp C in v b R . DD V . . 6 . 1 100 6 . 1 25 6 . 1 25 6 . 1 25 6 . 1 25 6 . 1 25 DD V . . DD V out + in v v .. 6 . 1 300 6 . 1 300 . . . DD V . 1 Q 4 Q 3 Q 6 Q 5 Q 8 Q 7 Q 9 Q 6 . 1 300 6 . 1 300 6 . 1 150 6 . 1 150 6 . 1 500 6 . 1 500 Biasing Diff-amplifier Gain stage buffer
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101 Let’s simplify it: Biasing : 2 PMOS current mirror 4 NMOS Æ cascode current source with source degeneration = K R b 8 . DD V . 15 Q 13 Q 13 I 15 I 10 I 11 I 2 13 2 15 13 15 2 15 15 15 15 2 13 13 13 13 15 11 10 ) ( ) ( ) ( 2 ) ( 2 13 15 15 15 13 13 n n n n t GS t GS t GS ox n b GS GS t GS ox n V V W V V W I I V V L W C I R I V V V V L W C I I I I I = = × = × + = × = = = = µ 2 15 15 15 2 15 15 15 15 15 13 15 ) ( 2 ) ( 2 2 2 ) ( ) ( 2 25 100 15 15 15 15 13 15 13 15 b ox n t GS ox n b t GS t b GS GS t GS GS t GS t GS R I L W C V V L W C I R I V V V R I V V V V V V V V V m W m W n n n n n n × = × = = = = = = = m W m L v A C K
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week_11 - 98 Operational Amplifier Need for high gain...

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