Lect_15 - ECE442 SolidStateDevices&Circuits...

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ECE 342–Jose Schutt Aine 1 ECE 442 Solid State Devices & Circuits 15. Differential Amplifiers Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois jschutt@emlab.uiuc.edu
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ECE 342–Jose Schutt Aine 2 Background Differential Amplifiers – The input stage of every op amp is a differential amplifier – Immunity to temperature effects – 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
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ECE 342–Jose Schutt Aine 3 Differential Amplifiers Practical Considerations – 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 – Power supply noise is a common-mode signal and has little effect on the output signal
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4 ECE 342–Jose Schutt Aine MOS Differential Pair ¾ Assume current source is ideal ¾ Transistors should not enter triode region
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5 ECE 342–Jose Schutt Aine Common-Mode Operation ¾ Input voltage v cm to both gates ¾ Difference in voltage between the two drains is zero
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6 ECE 342–Jose Schutt Aine Differential Input Voltage ¾ Differential pair responds to differntial input signals by providing corresponding differential output signal between the two drains.
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ECE 342–Jose Schutt Aine 7 MOS Differential Pair Assume current source is ideal v ID =v gs1 -v gs2 Output is collected as v D2 -v D1
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ECE 342–Jose Schutt Aine 8 - If v ID is positive, v D2 -v D1 is positive v ID >0 Î v gs1 >v gs2 I D1 > I D2 v D1 lower voltage point than v D2 MOS Differential Pair For proper operation, MOSFETS should not enter triode region
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ECE 342–Jose Schutt Aine 9 DC Analysis 1 2 D DD D IR VV =− 2 2 D D IR () 2 2 ox DG S T CW IV V L μ 2 D I I = GS T ox LI =+ SQ T ox LI +
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ECE 342–Jose Schutt Aine 10 Incremental Analysis ' 1 2 in om D v vg R =− Neglecting the body effect ' 2 2 in D v R = ' D Do u t R Rr = & ' 21 oo D mD in vv Ag R v == 1 1 2 =+ g cm id v 2 1 2 g cm id v
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ECE 342–Jose Schutt Aine 11 ' 1 2 high out D f CR π = Frequency Response When driven by a low-impedance signal source, the upper corner frequency is determined by the output circuit
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ECE 342–Jose Schutt Aine 12 12 1 2 oo D icm icm SS m vv R R g == + Assume R SS >> 1/g m Common-Mode Rejection Ratio
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ECE 342–Jose Schutt Aine 13 12 2 oo D icm icm SS vv R R = ± 1 , 22 D cm d m D SS R AA g R R == (a) For single-ended output: d mS S cm A CMRR g R A Common-Mode Rejection Ratio
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ECE 342–Jose Schutt Aine 14 Common-Mode Rejection Ratio (b) For differential output: 21 0 oo cm icm vv A v == dm D id Ag R v CMRR = ∞
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ECE 342–Jose Schutt Aine 15 BJT Differential Pair Assume perfect match between the devices and symmetry in the circuit
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ECE 342–Jose Schutt
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Lect_15 - ECE442 SolidStateDevices&Circuits...

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