Berkeley ELENG 105 - Lecture 24 (25 pages)

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Lecture 24



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Lecture 24

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Lecture Notes


Pages:
25
School:
University of California, Berkeley
Course:
Eleng 105 - Microelectronic Devices and Circuits
Microelectronic Devices and Circuits Documents

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Lecture 24 ANNOUNCEMENTS The last HW assignment HW 12 will be due 12 6 Prof Liu will be away on Tuesday 12 4 no office hour that day OUTLINE MOSFET Differential Amplifiers Reading Chapter 10 3 10 6 EE105 Fall 2007 Lecture 24 Slide 1 Prof Liu UC Berkeley Common Mode CM Response Similarly to its BJT counterpart a MOSFET differential pair produces zero differential output as VCM changes V X VY VDD EE105 Fall 2007 Lecture 24 Slide 2 I SS RD 2 Prof Liu UC Berkeley Equilibrium Overdrive Voltage The equilibrium overdrive voltage is defined as VGS VTH when M1 and M2 each carry a current of ISS 2 VGS VTH equil EE105 Fall 2007 Lecture 24 Slide 3 I SS W n Cox L Prof Liu UC Berkeley Minimum CM Output Voltage In order to maintain M1 and M2 in saturation the common mode output voltage cannot fall below VCM VTH This value usually limits voltage gain VDD EE105 Fall 2007 Lecture 24 Slide 4 I SS RD VCM VTH 2 Prof Liu UC Berkeley Differential Response EE105 Fall 2007 Lecture 24 Slide 5 Prof Liu UC Berkeley Small Signal Response For small input voltages V and V the gm values are equal so the increase in ID1 and decrease in ID2 are equal in magnitude Thus the voltage at node P is constant and can be considered as AC ground I EE I D1 I 2 I D2 I EE I 2 VP 0 I D1 g m V I D 2 g m V EE105 Fall 2007 Lecture 24 Slide 6 Prof Liu UC Berkeley Small Signal Differential Gain Since the output signal changes by 2gm VRD when the input signal changes by 2 V the small signal voltage gain is gmRD Note that the voltage gain is the same as for a CS stage but that the power dissipation is doubled EE105 Fall 2007 Lecture 24 Slide 7 Prof Liu UC Berkeley Large Signal Analysis I D1 I D 2 EE105 Fall 2007 4 I SS 1 W 2 n Cox Vin1 V in 2 Vin1 Vin 2 W 2 L n Cox L Lecture 24 Slide 8 Prof Liu UC Berkeley Maximum Differential Input Voltage There exists a finite differential input voltage that completely steers the tail current from one transistor to the other This value is known as the maximum differential input



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