07_diff_amps

07_diff_amps - EECE 416/501 Analog Circuit Design...

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Unformatted text preview: EECE 416/501 Analog Circuit Design Differential Amplifiers Christopher M. Twigg Assistant Professor BINGHAMTON U N I V E R S I T Y STATE UNIVERSITY OF NEW YORK Department of Electrical and Computer Engineering October 31, 2009 Diff Amps: Differential Signals Differential Inputs, Differential Outputs Differential Inputs, Single-Ended Output Vi+ Vi- + - + V oV o+ Vi+ Vi- + - Vo Absolute Voltages Differential-Mode and Common-Mode Voltages Vid 2 Vid 2 + - + + Vo + Vi+ + + - - Vi- + - Vic EECE 416/501, C. M. Twigg October 31, 2009 2/16 Diff Amps: Differential Pair ("Diff Pair") Ideal Differential Pair I+ IIb Vi+ ViVi+ Ib I+ IVi- Practical Differential Pair I+ I- Ib ViVi+ ViI+ I- Vi+ Ib EECE 416/501, C. M. Twigg October 31, 2009 3/16 Diff Amps: Diff Pair, Sub-Threshold I + I - I+ = (W/L)i I0 e(Vi Vi- +-V) /UT I- = (W/L)i I0 e(Vi --V) /UT Vi+ () Vb W L i () V W L i () Ib W L b Iout = I+ - I- = IB tanh IB Vid 2UT Common-Mode Input Range IB = I+ + I- IB = (W/L)i I0 e(Vi UT ln L W i +-V) /UT 0 IB I0 = Vi+ - V L W i V = Vi+ - UT ln Vi+ UT IB I0 I+ I- I+ - I - -6*UT -4*UT -2*UT 0 Vid 2*UT 4*UT 6*UT 8*UT 4 + ln L W i IB I0 -IB -8*UT EECE 416/501, C. M. Twigg October 31, 2009 4/16 Diff Amps: Diff Pair, Above-Threshold I+ = I = + IB 2 i 2 [Vi+ - VT0 - V]2 2 i Vid 4IB I- = i 2 Vi- - VT0 - V 2 1 + Vid i IB 1- I = - IB 2 1 - Vid 1- 2 i Vid 4IB i IB 1- 2 i Vid 4IB Iout = I - I = IB Vid Model Predicts IB IB + - i IB Actual Response 0 0 -IB 0 Vid I I- + - I -I + -IB 0 Vid I+ I- I+ - I - EECE 416/501, C. M. Twigg October 31, 2009 5/16 Diff Amps: Diff Pair, Above-Threshold (cont) Common-Mode Input Range Assume =1 IB = I+ + I- IB = 2IB i i 2 [Vi+ - VT0 - V]2 = Vi+ - VT0 - V 2IB i V = Vi+ - VT0 - Vi+ Vb + 2IB i EECE 416/501, C. M. Twigg October 31, 2009 6/16 Diff Amps: OpAmps vs. OTAs Operational Transconductance Amplifier + - Operational Amplifier + - EECE 416/501, C. M. Twigg October 31, 2009 7/16 Diff Amps: OTA, Resistive Load (Differential-Mode Gain) VDD VDD R Vo Vi+ V Vb Ib I+ I- R Vo Vi- EECE 416/501, C. M. Twigg October 31, 2009 8/16 Diff Amps: OTA, Resistive Load (Common-Mode Gain) VDD VDD R Vo Vi+ V Vb Ib I+ I- R Vo Vi- EECE 416/501, C. M. Twigg October 31, 2009 9/16 Diff Amps: OTA, Resistive Load (Power Supply Gain) VDD VDD R Vo Vi+ V Vb Ib I+ I- R Vo Vi- EECE 416/501, C. M. Twigg October 31, 2009 10/16 Diff Amps: OTA, Diode and Current Source Loads VDD VDD VDD Vbp Vo Vi+ V Vbn Ib Vbn I+ IVo ViVo Vi+ V Ib I+ IVo ViVDD EECE 416/501, C. M. Twigg October 31, 2009 11/16 Diff Amps: OTA, Current Mirror Load VDD VDD I+ Vo I Vi+ V Vb Ib - Vi- EECE 416/501, C. M. Twigg October 31, 2009 12/16 Diff Amps: OTA, Problems with the 5T OTA VDD VDD I+ Vo I Vi+ V Vb Ib 4 3.5 3 Vo (V) 2.5 2 1.5 1 0.5 0 0 0.5 1 1.5 2 2.5 Vi+ (V) EECE 416/501, C. M. Twigg October 31, 2009 13/16 - 5 Vi4.5 Vi- = 1 V Vi- = 2 V Vi- = 3 V Vi- = 4 V Vi- = 5 V 3 3.5 4 4.5 5 Diff Amps: Wide-Range OTA VDD Vi- V + i I+ IVo Vb Ib EECE 416/501, C. M. Twigg October 31, 2009 14/16 Diff Amps: Why Gain Matters Open Loop Vi+ Vi- + - Vo Vo = Closed Loop (Feedback) Vi + - Vo Vo = How do we increase gain? EECE 416/501, C. M. Twigg October 31, 2009 15/16 Diff Amps: Cascoded, Wide-Range OTA VDD Vcp ViIb Vi+ Vo Vb Vcn EECE 416/501, C. M. Twigg October 31, 2009 16/16 ...
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