lecture_08_diff_amplifiers

lecture_08_diff_amplifiers - Handout #8 EE 214 Winter 2009...

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B. Murmann, B. Wooley EE214 Winter 2008-09 1 Differential Amplifiers Handout #8 EE 214 Winter 2009 B. Murmann and B. A. Wooley Stanford University B. Murmann, B. Wooley EE214 Winter 2008-09 2 Differential Circuits Enables straightforward biasing w/o large capacitors Information is carried in “differential” signals that are insensitive to “common-mode” perturbations, such as power supply noise Fully differential circuits are increasingly used for I/O and clock-and- data recovery (CDR) circuits to allow the use of small signals at high speeds, which generate less noise V I1 V I2 V O2 V O1 + +
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B. Murmann, B. Wooley EE214 Winter 2008-09 3 Diffential (Emitter-Coupled) Pair Voltages V i1 , V i2 , V o1 and V o2 are with reference to ground Q 2 Q 1 R C1 R C2 V i1 V i2 I C1 V o2 V o1 V CC V EE I C2 I EE R EE I EE I ref V EE V i1 ! V BE1 + V BE2 ! V i2 = 0 I C1 + I C2 ! I EE B. Murmann, B. Wooley EE214 Winter 2008-09 4 Large-Signal Analysis Q 1 and Q 2 are identical R C1 = R C2 = R C Q 1 and Q 2 don’t saturate Neglect r b , r o (most of the time) and r μ (as well as r c and r e ) Neglect R EE (R EE ! 0) except for common-mode analysis Assumptions Differential Signals Differential Input Voltage Differential Collector Current Differential Output Voltage V id ! V ! V i2 V od ! V o1 ! V o2 I Cd ! I C1 ! I C2
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B. Murmann, B. Wooley EE214 Winter 2008-09 5 If Q 1 and Q 2 don’t saturate I C1 ! I S1 e V BE1 /V T and I C2 ! I S2 e V BE2 T where V T ! kT / q If Q 1 and Q 2 are identical, I S1 = I S2 = I S and I C1 I C2 = e (V BE1 ! V BE2 )/V T = e (V i1 ! V i2 T = e V id T Summing currents at the tail node with ! F1 = ! F2 ! ! F I EE = ! (I E1 + I E2 ) = 1 " F C1 + I C2 ) ! " F I EE = I C1 1 + e # V id T ( ) Thus I C1 = ! F I EE 1 + e " V id T and I C2 = ! F I EE 1 + e V id T B. Murmann, B. Wooley EE214 Winter 2008-09 6 Transition is narrow compared to MOS (higher gain) Linear only for |V id | < V T I C1 ,I C2 -2 -4 4 2 V id /V T ! F I EE ! F I EE /2 I C1 I C2 I Cd = I C1 ! I C2 = " F I EE 1 1 + e ! V id T !
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lecture_08_diff_amplifiers - Handout #8 EE 214 Winter 2009...

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