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Department of EECS University of California, Berkeley
Prof. Ali M. Niknejad
Differential Amplifiers
EE 105 Fall 2016 Prof. A. M. Niknejad
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Why Differential?
z Differential circuits are much less sensitive to noises and interferences
z Differential configuration enables us to bias amplifiers and connect multiple stages without using coupling or bypass capacitors
z Differential amplifiers are widely used in IC’s– Excellent matching of transistors, which is critical for
differential circuits– Differential circuits require more transistors Æ not an
issue for IC
EE 105 Fall 2016 Prof. A. M. Niknejad
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Goal: Differential Gm
z What’s wrong with this circuit?
EE 105 Fall 2016 Prof. A. M. Niknejad
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Add a buffer to source side…
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A symmetric pair is born…
EE 105 Fall 2016 Prof. A. M. Niknejad
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MOS Differential-Pair Basic Configuration
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Common Mode InputsDifferential Pair
Rejects Common-Mode Inputs
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Small Signal Operation
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Differential Symmetry
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Differential Half Circuit
Q1 biased at I2
Ad =vod2vid2
= vodvid
= gm RD || ro( )
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CM Symmetry
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CM AC Equivalent Circuit
Non-ideal current source consists of an ideal current source shunted by a large resistance, RSS
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Common Mode “Half Circuit”
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Ideal CM Output Voltage
Common-Source with degeneration
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Common Mode Gain with Mismatched RDHowever, any mismatch in the half circuitswill produce finite output voltage, e.g., RD2 = RD +DRD
vod = vo2 - vo1 =- DRD2RSS
vicm
Common mode gain:
Acm =vodvicm
= - DRD2RSS
= - RD2RSS
æ
èç
ö
ø÷DRDRD
æ
èç
ö
ø÷
Common Mode Rejection Ratio (CMRR):
CMRR ºAdAcm
, in dB: CMRR(dB) º 20 logAdAcm
CMRR should be as large as possible. For the above case,
CMRR = gmRDDRD2RSS
= 2gmRSSDRDRD
æ
èç
ö
ø÷
EE 105 Fall 2016 Prof. A. M. Niknejad
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Common Mode Gain with Mismatch of gm
Mismatch in gm :
gm1 = gm +12Dgm; gm2 = gm -
12Dgm
gm1 - gm2 =Dgm
(Derivation skipped)
Acm =vodvicm
= RD2RSS
Dgmgm
CMRR = gmRDRD
2RSSDgmgm
= 2gmRSSDgmgm
æ
èç
ö
ø÷
EE 105 Fall 2016 Prof. A. M. Niknejad
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DC Offset
Due to mismatch in RD, output voltage VO ¹ 0 even both inputsare grounded. To produce zero output, an input offset voltage
VOS =VOAd
, where Ad is differential gain, needs to be applied.
EE 105 Fall 2016 Prof. A. M. Niknejad
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DC Offset
For example, DC offset caused by mismatch in RD :
RD1 = RD +DRD / 2; RD2 = RD - DRD / 2; VO =VD2 - VD1 =I2DRD
VOS =VOAd= IDRD / 2gmRD
= IDRD / 2
2 I / 2VOV
æ
èç
ö
ø÷RD
= VOV2DRDRD
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Differential Input, Single-End Output
Differential-in, Differential-out Differential input, Single-ended Output
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Current Mirror Load (1)
AC equivalentcircuit for differential input
Current mirror forces small-signal currents through Q3 and Q4 to be the sameÆ output currents = 2x that of half circuit
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Current Mirror Load (2)
Short-circuit transconductance of differntial pair with current mirror load:
Gm =iovid
; io : output current with short-circuit load
io = gm2vid2
æèç
öø÷- gm4vgs4; vgs4 = vgs3 = - gm1
vid2
æèç
öø÷
1gm3
|| ro3 || ro1æèç
öø÷» - gm1
gm3
vid2
gm1 = gm2 = gm; gm3 = gm4 Þ io = gmvid Þ Gm = gm
EE 105 Fall 2016 Prof. A. M. Niknejad
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Current Mirror Load (3)
Output resistance,Rout = Rop || Ron = ro4 || ro2
Ad =GmRout = gm ro4 || ro2( )RopRon
Rout
EE 105 Fall 2016 Prof. A. M. Niknejad
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Common Mode Gain
Common mode gain (derivation skipped):
Acm » -1
2gm3RSS
CMRR = AdAcm
= gm ro4 || ro3( )×2gm3RSS
For ro4 = ro3 = ro and gm = gm3
CMRR = gmro( ) gmRSS( )
