week_13 - 129 Advanced Current Mirrors and OPAMPs Short...

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129 Advanced Current Mirrors and OPAMPs Short channel length L Æ degraded output resistance Æ current mirrors such as cascode with high output resistance are needed Æ signal swing is limited in cascode as 2 transistors have to stay in saturation. Wide-swing Cascode Current Mirror Function of 4 Q Circuit can work without : 4 Q Forces same for and if and have the same size and and have the same size. DS V 3 Q 2 Q 1 Q 4 Q 2 Q 3 Q 3 Q in I 2 Q 3 Q in I 2 Q . . bias I . out I 1 Q Fig.1 . . 4 Q bias I . out I 1 Q 5 Q Fig.2 The difference that makes is to reduce drain-source voltage across Æ and drain-source voltage will be very close in the presence of . 4 Q 3 Q 3 Q 2 Q 4 Q
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130 Æ Since output impedance (resistance) of the transistors and are finite, keeping their drain-source voltage the same helps making 3 Q 4 Q in out I I = independent of their . ds r So all - does is to act like a diode connected transistor as shown in Fig2. 3 Q 4 Q Transistor sizing is important for propos function of the current source. For now let’s assume all the drain current are equal. L W 3 Q 4 Q in I 2 Q . . bias I . out I 1 Q 5 Q Smaller . . 2 n L W 2 ) 1 ( + n L W L W 2 n L W Smaller Smaller 3 Q and are current mirrors Æ 4 Q ) ( 2 2 3 2 L W C I V V V V ox n D tn gs tn gs µ = = our assumption (needs to be verified) Æ 5 2 D D I I = Æ 2 2 2 ) ( ) 1 ( 2 ) )( 2 ( 5 2 tn GS ox n tn GS ox n V V n L W C V V L W C + = Æ 1 5 2 + = n V V V V tn GS tn GS Æ ) )( 1 ( 2 5 tn GS tn GS V V n V V + = For the same reason:
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131 ) ( 2 4 1 tn GS tn GS tn GS V V n V V V V = = tn tn GS GS V V V n V + = ) ( 2 1 Gate of , and are connected together Æ 4 Q 5 Q 1 Q
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week_13 - 129 Advanced Current Mirrors and OPAMPs Short...

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