Lect. 28 MOS Differential amplifiers.ppt - Yonsei...

Lect. 28: MOS Differential Amplifiers (Razavi 10.3)

(vout1-vout2) vs. (vin1-vin2) ?

MOS Differential Pair

Prof. Woo-Young ChoiElectronic Circuits 1 (09/2)

Lect. 28: MOS Differential Amplifiers

MOS Differential Pair

What is Δvin,max?

2I1

2 SSG t

Iv v Wk

L

= +

2G tv v=

2 SSIΔ ,max

SSinv Wk

L

Δ =


Lect. 28: MOS Differential Amplifiers (Razavi 10.3)

MOS Differential PairVCM: Common Mode

vid: Differential Mode

1 2, 2 2id id

G CM G CMv v

v V v V= + = −

'R

1 2 ,2

G GCM

v vV

+= 1 2id G Gv v v= −

LR

Ideally, no CM response, large DM response



Common-Mode

1 2D Di i=

For identical Q1 and Q2

I=1 2D D

1 2D Dυ υ=

2

2DD DIV R= −

1 2 0D Dυ υ∴ − =

2

==> No CM output



Common-Mode

VCM, max?

DS GS tv v V≥ −DS GS t

( ) ( )2DD D CM GS GS tIV R V v v V− − − ≥ −

2DD D t CMIV R V V∴ − + ≥

,max 2CM DD D tIV V R V= − +

VCM, min?



Differential-Mode

If vid << VCM

Small-signal analysis can be usedSmall-signal analysis can be usedfor differential-mode!

V BiVCM: Bias

S ll i l t t

vid: Small-signal input

vod: Small-signal output



Differential-mode small-signal analysis

No voltage changesince Left and Right areanti-symmetric (odd)



Differential-mode small-signal half-circuitDifferential mode small signal half circuit



Differential-mode small-signal half-circuitg

1

/ 2o

id

vv

= ( || )m D og R r−

For the total circuit

1 2od o ov v v= −

2 1But o ov v= −

CS lifi f i t diff !

2 1o o

12od ov v∴ =

12od od

id id

v vA

v v= =

CS amplifier for input difference!( || )m D og R r= −


Lect. 28 MOS Differential amplifiers.ppt - Yonsei...

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