note5 - Handout 5 MOS Differential Pairs and Amplifiers...

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Handout 5 ECE 315, Cornell University 1 MOS Differential Pairs and Amplifiers What you will learn: • Basic large-signal operational range of MOS diff pair in common mode and differential mode • Small signal operations with differential gain and CMRR • Nonideal effects of r o , load mismatch and g m mismatch • Inclusion of active loads in differential amplifiers Textbook reading: 7.1, 7.2 and 7.5 Handout 5
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Handout 5 ECE 315, Cornell University 2 5.1 Diff Pair Large-/Small-Signal Operations • Common-mode and differential mode of MOS diff pair. • Choice of the biasing current and voltage swing that can keep the diff pair in saturation • Range and magnitude of differential gain Textbook reading: 7.1
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Handout 5 ECE 315, Cornell University 3 The Advantage of Differential Pairs v S • Errors mostly from match of the two differential paths: well suited for IC • Common-mode rejection (CMR): – Better noise immunity – No need to use bypass or coupling C in small signal cascade –Im p l em e n t t h e amplifier two times Fig. 7.1
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Handout 5 ECE 315, Cornell University 4 Common and Differential Modes v S Fig. 7.1 2 2 2 2 1 2 1 2 1 id CM G id CM G G G id G G CM v v v v v v v v v v v v = + = = + = Notice that we think here the common mode v CM is large signal and the differential mode v id is small signal (i.e., the slope information), although from definition, they are both large signal.
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Handout 5 ECE 315, Cornell University 5 Large-Signal Common-Mode Operations GS CM S V v v = () 2 ' 2 1 2 th GS n V V L W k I = ) / ( ' L W k I V V V n th GS OVcm = = V CS + - Fig. 7.2 v S Once we set I, then V OVcm , V GS , v D1 , and v D2 are known, only v S will be determined by v CM SS CS S CM GS S V V v v V v = = +
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Handout 5 ECE 315, Cornell University 6 Range of Common-Mode Input Voltage Q 1 in saturation: v D1 –v S > v Dsat = V GS –V th 2 / max D DD th CM IR V V V + = Q 1 above threshold: V GS > V th OVcm th CS SS CM V V V V V + + + = min •L a r g e I or R D will decrease the range for Q 1 and Q 2 to be in saturation. •L a r g e V OVcm will decrease the range for Q 1 and Q 2 to be above- threshold.
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Handout 5 ECE 315, Cornell University 7 Large-Signal Common-Mode Example Two requirements on v S : (1) V GS = V CM –v S (2) v D1 –v S > V dsat and v D2 –v S > V dsat Assume Q 1 and Q 2 are saturated and I/2 > I th , Voltage drop across the current source = 0.68V: if V CM < 0, this may not be sufficient. Check: V GS = 0.82V V DS = 1.82V V G1 = V G2 = V CM =0 Fig. 7.3 v S
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Handout 5 ECE 315, Cornell University 8 Large-Signal Differential Mode If v id > 0, then V GS1 > V GS2 , i D1 > i D2 , v D1 < v D2 , v D2 –v D1 > 0, V DS1 < V DS2 I 1 I 2 ( I 1 and I 2 are still between 0 and I ) If v id < 0, then V GS1 < V GS2 , i D1 < i D2 , v D1 > v D2 , v D2 –v D1 < 0, V DS1 > V DS2 Differential input: v id ; Differential output: v D2 –v D1 positive input negative input positive output negative output Fig. 7.4
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This note was uploaded on 02/02/2008 for the course ECE 3150 taught by Professor Spencer during the Fall '07 term at Cornell University (Engineering School).

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note5 - Handout 5 MOS Differential Pairs and Amplifiers...

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