Lect_15 - CE 442 ECE 442 Solid State Devices & Circuits...

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Unformatted text preview: CE 442 ECE 442 Solid State Devices & Circuits 15. Differential Amplifiers Jose E. Schutt-Aine lectrical & Computer Engineering Electrical & Computer Engineering University of Illinois jschutt@emlab.uiuc.edu ECE 442 Jose Schutt Aine 1 Background Differential Amplifiers The input stage of every op amp is a differential mplifier amplifier Immunity to temperature effects bility to amplify dc signals Ability to amplify dc signals Well-suited for IC fabrication because (a) they depend on matching of elements (b) they use more components Less sensitive to noise and interference Enable to bias amplifier and connect to other stage without the use of coupling capacitors ECE 442 Jose Schutt Aine 2 Differential Amplifiers Practical Considerations oth inputs to a differential amplifier may have Both inputs to a differential amplifier may have different voltages applied to them In the ideal situation with perfectly symmetric stages, the common-mode input would lead to zero output Temperature drifts in each stage are often common-mode signals ower supply noise is a common ode signal Power supply noise is a common-mode signal and has little effect on the output signal ECE 442 Jose Schutt Aine 3 MOS Differential Pair ssume current Assume current source is ideal Transistors should not enter triode region 4 ECE 442 Jose Schutt Aine Common-Mode Operation Input voltage v cm to both gates ifference in Difference in voltage between the two drains is zero 5 ECE 442 Jose Schutt Aine Differential Input Voltage Differential pair responds to ifferntial put differntial input signals by providing corresponding iffe e tial out ut differential output signal between the two drains. 6 ECE 442 Jose Schutt Aine MOS Differential Pair Assume current source is ideal v ID =v gs1-v gs2 Output is collected as v D2-v D1 ECE 442 Jose Schutt Aine 7 MOS Differential Pair- If v ID is positive, v D2-v D1 is ositive positive v ID >0 v gs1 >v gs2 I D1 > I D2 1 wer voltage point than 2 v D1 lower voltage point than v D2 or proper operation For proper operation, MOSFETS should not enter triode region ECE 442 Jose Schutt Aine 8 DC Analysis 1 D D DD IR V V 2 D D D D IR V V 2 2 2 x C W V V I I 2 ox D G S T I V V L 2 D S T LI V V Q T LI V V GS T ox C W SQ T ox C W ECE 442 Jose Schutt Aine 9 Incremental Analysis eglecting the body effect 1 1 2 g cm id v v v 2 1 2 g cm id v v v ' 1 2 in o m D v v g R Neglecting the body effect ' 2 2 in o m D v v g R ' D D o u t R R r ' 2 1 o o D m D v v A g R ECE 442 Jose Schutt Aine 10 in v Frequency Response When driven by a low-impedance signal source, the upper corner frequency is determined by the output circuit ' 1 2 high ut D f C R out D ECE 442 Jose Schutt Aine 11 Common-Mode Rejection Ratio 1 2 o o D v v R 1 2 icm icm SS m v v R g Assume R SS >> 1/g m ECE 442 Jose Schutt Aine 12 Common-Mode Rejection Ratio...
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This note was uploaded on 05/05/2010 for the course ECE 329 taught by Professor Franke during the Spring '08 term at University of Illinois at Urbana–Champaign.

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Lect_15 - CE 442 ECE 442 Solid State Devices & Circuits...

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