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Lecture 28 – Differential-In, Differential-Out Op Amps (6/25/14) Page 28-1
CMOS Analog Circuit Design © P.E. Allen - 2016
LECTURE 28 – DIFFERENTIAL-IN, DIFFERENTIAL-OUT OP
AMPS
LECTURE ORGANIZATION
Outline
• Introduction
• Examples of differential output op amps
• Common mode output voltage stabilization
• Summary
CMOS Analog Circuit Design, 3rd Edition Reference
Pages 386-397
Lecture 28 – Differential-In, Differential-Out Op Amps (6/25/14) Page 28-2
CMOS Analog Circuit Design © P.E. Allen - 2016
INTRODUCTION
Why Differential Output Op Amps?
• Cancellation of common mode signals including clock feedthrough
• Increased signal swing
• Cancellation of even-order harmonics
Symbol:
v1
v2
v1-v2t
t
t
A
-AA
-A
2A
-2A Fig. 7.3-1
Lecture 28 – Differential-In, Differential-Out Op Amps (6/25/14) Page 28-3
CMOS Analog Circuit Design © P.E. Allen - 2016
Common Mode Output Voltage Stabilization
If the common mode gain not small, it may cause the common mode output voltage to
be poorly defined.
Illustration:
Remember that:
vOUT = Avd(vID) ± Acm(vCM)
VDD
vod
t
070506-01
VSS
0
VDD
vod
t
VSS
0
VDD
vod
t
VSS
0
CM output voltage properly defined,
Vcm = 0
CM output voltage too large,
Vcm= 0.5VDD
CM output voltage too small,
Vcm= 0.5VSS
Lecture 28 – Differential-In, Differential-Out Op Amps (6/25/14) Page 28-4
CMOS Analog Circuit Design © P.E. Allen - 2016
EXAMPLES OF DIFFERENTIAL OUTPUT OP AMPS (OTA’S)
Two-Stage, Miller, Differential-In, Differential-Out Op Amp
Note that the
upper ICMR is
VDD - VSGP + VTN
(OCMR) = VDD+ |VSS| - VSDP(sat) - VDSN(sat)
The maximum peak-to-peak output voltage 2·OCMR
Conversion between differential outputs and single-ended outputs:
vi1 M1 M2
M3 M4
M5
M6M8
VDD
VSS
VBN+
-
Cc
M9
Cc
VBP+
-
vi2
vo1vo2RzRz
M7
Fig. 7.3-3
Lecture 28 – Differential-In, Differential-Out Op Amps (6/25/14) Page 28-5
CMOS Analog Circuit Design © P.E. Allen - 2016
Two-Stage, Miller, Differential-In, Differential-Out Op Amp with Push-Pull Output
Comments:
• Able to actively source and sink output current
• Output quiescent current poorly defined
vi1M1 M2
M3 M4
M5
M6
M7
VDD
VSS
VBN+
-
Cc
M9
Cc
VBP+
-
vi2
vo1 vo2RzRz
M8
Fig. 7.3-6
M10 M12
M13
M14
Lecture 28 – Differential-In, Differential-Out Op Amps (6/25/14) Page 28-6
CMOS Analog Circuit Design © P.E. Allen - 2016
Folded-Cascode, Differential Output Op Amp
• No longer has the low-frequency asymmetry in signal path gains.
• Class A
060717-01
VPB1
M4 M5
I6 VPB2
I4 I5
VDD
I7M6 M7
VNB2
M8 M9
M10M11
+
-vIN
vOUT
VNB1
I1 I2
M1 M2
M3I3
CL
VNB1
+-CL
Lecture 28 – Differential-In, Differential-Out Op Amps (6/25/14) Page 28-7
CMOS Analog Circuit Design © P.E. Allen - 2016
Enhanced-Gain, Folded-Cascode, Differential Output Op Amp
What about the A amplifier?
Below is the upper A amplifier:
Note that VBias controls the dc voltage at the
input of the A amplifier through the negative
feedback loop.
• Balanced inputs
• Class A
060718-02
VDD
VPB1
VPB2 VPB2
VNB1 VBias
vin+ vin
-
vout+vout
- M1M2 M3
M4
M5 M6
M7 M8
M9 M10
M11M12
060718-01
vOUT
M4M5
M3
M7
M8 M9
M10 M11
M6
VDD
VPB1
+
-
vIN
M1 M2
VNB1
A+- +
-
+- +
-A
+-
VPB1
Lecture 28 – Differential-In, Differential-Out Op Amps (6/25/14) Page 28-8
CMOS Analog Circuit Design © P.E. Allen - 2016
Push-Pull Cascode Op Amp with Differential-Outputs
• Output quiescent currents are well defined
• Self-biased circuits can be replaced with VNB2 and VPB2
VDD
VSS
VBias
R2 M22
M23
R1
M19
M20
M1 M2
M3 M4M5 M6M7 M8
M9 M10
M11
M12M13
M14
M15 M16M17
M18
M21
vo1
vi1 vi2vo2
Fig. 7.3-8
Lecture 28 – Differential-In, Differential-Out Op Amps (6/25/14) Page 28-9
CMOS Analog Circuit Design © P.E. Allen - 2016
Folded-Cascode, Push-Pull, Differential Output Op Amp
I6 = I7 = I14 = I15 > 0.5I5
I5 = I1 + I2 + I3 + I4
Av = gmRout(diff)
060717-02
VPB1
M6 M7
I6
VPB2I4
I7
I9M8 M9
VNB2
M10 M11
M12 M13
+
-vIN
vOUTVNB1
I3
I2
M2 M3
M5I5
CL+-CL
I1
M1
VNB2
M18M19
M20M21
VPB1
M14 M15
I16 VPB2
I14 I15
VDD
I17
M16 M17
M4
I8
0.5gm1vin
0.5gm3vin
0.5gm2vin
0.5gm4vin
0.5gm3vin
0.5gm4vin0.5gm2vin
0.5gm1vin
Lecture 28 – Differential-In, Differential-Out Op Amps (6/25/14) Page 28-10
CMOS Analog Circuit Design © P.E. Allen - 2016
Enhanced-Gain, Folded-Cascode with Push-Pull Outputs
• Gain approaches gm3rds
3
060718-06
VPB1
M6 M7
M8 M9
M20
M19
M18
M21
+
-vIN
VNB1
M2 M3
M5CL+-CL
M1
VNB2M14
M17M16
M15
VPB1
M10 M11
VPB2
VDD
M12 M13
M4
vOUT
A+- +
-
+- +
-AVNB2
Lecture 28 – Differential-In, Differential-Out Op Amps (6/25/14) Page 28-11
CMOS Analog Circuit Design © P.E. Allen - 2016
Cross-Coupled Differential Amplifier Stage
The cross-coupled input stage allows the push-pull output quiescent current to be well
defined.
Operation:
Voltage loop vi1 - vi2 = -VGS1+ vGS1 + vSG4 - VSG4 = VSG3 - vSG3 - vGS2 + VGS2
Using the notation for ac, dc, and total variables gives,
vi2 - vi1 = vid = (vsg1 + vgs4) = -(vsg3 + vgs2)
If gm1 = gm2 = gm3 = gm4, then half of the differential input is applied across each
transistor with the correct polarity.
i1 = gm1vid
2 =
gm4vid
2 and i2 = -
gm2vid
2 = -
gm3vid
2
M1 M2
M3 M4
VGS2
VSG4
VGS1
VSG3
i1
i1i2
i2
vi1 vi2
Fig. 7.3-9
vGS1+
-
vSG4
+
-vSG3
+
-
vGS2+
-
Lecture 28 – Differential-In, Differential-Out Op Amps (6/25/14) Page 28-12
CMOS Analog Circuit Design © P.E. Allen - 2016
Class AB, Differential Output Op Amp using a Cross-Coupled Differential Input
Stage
Quiescent output currents are defined by the current in the input cross-coupled
differential amplifier.
M1 M2
M3 M4
M5 M6
M7
VDD
VSS
VBias+
-
R1
M24
M25
R2
M27
M28
M8M9 M10
M11 M12
M13
M14
M15
M16
M17 M18
M19 M20
M21 M22
M26
M23
vo2
vi2vi1
vo1
Fig. 7.3-10
Lecture 28 – Differential-In, Differential-Out Op Amps (6/25/14) Page 28-13
CMOS Analog Circuit Design © P.E. Allen - 2016
COMMON MODE OUTPUT VOLTAGE STABILIZATION
Common Mode Feedback Circuits
Because the common mode gain is undefined, any common mode signal at the input
can cause the output common mode voltage to be improperly defined. The common
mode output voltage is stabilized by sensing the common mode output voltage and using
negative feedback to adjust the common mode voltage to the desired value.
Model for the Output of Differential Output Op Amps:
Roi represents the self-resistance of the output sink/sources.
1.) If the common mode output voltage increases the sourcing current is too large.
2.) If the common mode output voltage decreases the sinking current is too large.
VDD
io1(source) Ro1
vo1
io1(sink) Ro3
io2(source)
io2(sink)
Ro2
Ro4
vo2
VDD
io1(source) Ro1
vo1
Io1(sink) Ro3
io2(source)
Io2(sink)
Ro2
Ro4
vo2
Class A Output Push-Pull Output 060718-08
Lecture 28 – Differential-In, Differential-Out Op Amps (6/25/14) Page 28-14
CMOS Analog Circuit Design © P.E. Allen - 2016
Conceptual View of Common-Mode Feedback
Function of the common-mode feedback circuit:
1.) If the common-mode output voltage increases, decrease the upper currents sources or
increase the lower current sink until the common-mode voltage is equal to VCMREF.
2.) If the common-mode output voltage decreases, increase the upper currents sources or
decrease the lower current sink until the common-mode voltage is equal to VCMREF.
Lecture 28 – Differential-In, Differential-Out Op Amps (6/25/14) Page 28-15
CMOS Analog Circuit Design © P.E. Allen - 2016
Two-Stage, Miller, Differential-In, Differential-Out Op Amp with Common-Mode
Feedback
Comments:
• Simple
• Unreferenced – value of common mode output voltage determined by the circuit
characteristics
vi1 M1 M2
M3 M4
M5
M6M7
VDD
VSS
VBN+
-
Cc
M9
Cc
VBP
+
-
vi2
vo1vo2RzRz
M8
Fig. 7.3-12
M10 M11
Lecture 28 – Differential-In, Differential-Out Op Amps (6/25/14) Page 28-16
CMOS Analog Circuit Design © P.E. Allen - 2016
Common Mode Feedback Circuits
Implementation of common mode feedback circuit:
This scheme can be applied to any differential output amplifier.
CM Loop Gain = -gmC1Ro1 which can be large if the output of the differential output
amplifier is cascaded or a gain-enhanced cascode.
The common-mode loop gain may need to be compensated for proper dynamic
performance.
060718-10
vi1M1 M2
M3 M4
M5
VDD
IBias
VCM
vo1
MC2A
MC2B
MC1
MC3
MC4
MC5MB
I3 I4IC4
IC3Common-
mode feed-
back circuit
Ro1 Ro2
vi2
vo2
Lecture 28 – Differential-In, Differential-Out Op Amps (6/25/14) Page 28-17
CMOS Analog Circuit Design © P.E. Allen - 2016
Common Mode Feedback Circuits – Continued
The previous circuit suffers when the input common mode voltage is low because the
transistors MC2A and MC2B have a poor negative input common mode voltage.
The following circuit alleviates this disadvantage:
060718-11
vi1M1 M2
M3 M4
M5
VDD
IBias
VCM
vo2vo1
vi2MC2MC1
MC3
MC4
MC5MB
I3 I4
IC4IC3
Common-
mode feed-
back circuitRCM1
RCM2
Lecture 28 – Differential-In, Differential-Out Op Amps (6/25/14) Page 28-18
CMOS Analog Circuit Design © P.E. Allen - 2016
An Improved Common-Mode Feedback Circuit
The resistance loading of the previous circuit can be avoided in the following CM
feedback implementation:
This circuit is capable of sustaining a large differential voltage without loading the
output of the differential output op amp.
VDD
060718-12
VCMREF
RCM RCM
vo1 vo2
CM Correction Circuitry
M1 M2 M3 M4
M5 M6
Lecture 28 – Differential-In, Differential-Out Op Amps (6/25/14) Page 28-19
CMOS Analog Circuit Design © P.E. Allen - 2016
Frequency Response of the CM Feedback Circuit
Consider the following CM feedback circuit implementation:
The CM feedback path has two poles – one at the gates of M10 and M11 and the
dominant output pole of the differential output op amp.
Can compensate with Miller capacitors as shown.
070506-02
VPB1
M4 M5
VPB2
VDD
M6 M7
VNB2M8
M10
M11
+
-vIN
vOUT
VNB1
M1 M2
M3 VCMREF
+-
M9
M12
M13 M14
M15 M16
CcCc
Lecture 28 – Differential-In, Differential-Out Op Amps (6/25/14) Page 28-20
CMOS Analog Circuit Design © P.E. Allen - 2016
Improved CM Feedback Frequency Response
The circuit on the previous page can be modified to eliminate the pole at the gates of
M10 and M11 as follows:
• The need for compensation of the common mode loop no longer exists since there is
only one dominant pole
• The dominant pole of the differential amplifier becomes the dominant pole of the
common mode feedback
060718-14
VPB1
M4 M5
VPB2
VDD
M6 M7
VNB2M8
M10 M11
+
-vin
vo1
VNB1
M1 M2
M3
VCMREF
M9
M12
M13 M14
M16M15
VNB1
VNB2
M17
M18M19
vo2
Lecture 28 – Differential-In, Differential-Out Op Amps (6/25/14) Page 28-21
CMOS Analog Circuit Design © P.E. Allen - 2016
Implications of the Common Mode Feedback Correction Bandwidth
We have seen from the previous slides that the bandwidth of the common mode feedback
correction circuit can be equal, less, or larger than the bandwidth of the differential gain
of the op amp.
Common mode feedback BW < Differential mode BW:
- Common mode feedback correction circuit responds slower than the differential
output voltage changes
- It is possible that the CM feedback correction circuit will not correct for the
highest frequency CM disturbances and the differential output signal may be
saturated or clipped
Common mode feedback BW ≈ Differential mode BW:
- The CM feedback correction circuit should be able to correct for all CM
disturbances
- The compensation of the differential mode can be used for the CM feedback
correction circuit
Common mode feedback BW > Differential mode BW:
- The CM feedback correction circuit will be able to correct for all CM disturbances
- Difficult to keep stable because of higher BW and can amplify noise
Lecture 28 – Differential-In, Differential-Out Op Amps (6/25/14) Page 28-22
CMOS Analog Circuit Design © P.E. Allen - 2016
A Common Mode Feedback Correction Scheme for Discrete Time Applications
Correction Scheme:
Operation:
1.) During the 1 phase, both Ccm are charged to the desired value of Vocm and CMbias
= Vocm.
2.) During the 2 phase, the Ccm capacitors are connected between the differential
outputs and the CMbias node. The average value applied to the CMbias node will be
Vocm.
Lecture 28 – Differential-In, Differential-Out Op Amps (6/25/14) Page 28-23
CMOS Analog Circuit Design © P.E. Allen - 2016
Example of a Common-Mode Output Voltage Stabilization Scheme for Discrete-
Time Applications
Common mode
adjustment phase:
Switches S1, S2 and S3
are closed. C1 and C2
are charged to the value
necessary for I12 and I13
to keep the common
mode output voltage at
VCM.
Amplification phase:
Switches S4 and S5 are
closed. If the common
mode output voltage is
not at VCM, the currents I12 and I13 will change to force the value of the common mode
output voltage back to VCM.
070506-03
VPB1
M4 M5
VPB2
VDD
M6 M7
VNB2M8
M10 M11
+
-vIN
vOUT
VNB1
M1 M2
M3
VCM
+-
M9
M12 M13 M14
M15
C1
C2
VNB1
S4
S5
S1
S2
S3
I12
I13
Discrete time common
mode correction circuit
Lecture 28 – Differential-In, Differential-Out Op Amps (6/25/14) Page 28-24
CMOS Analog Circuit Design © P.E. Allen - 2016
Correction of Channel Charge and Clock Feedthrough
In the discrete-time common mode correction schemes, the switches can introduce
error due to channel charge and clock feedthrough.
Through simulation, these errors can be predicted and corrected by applying a
correction signal superimposed upon the error signal to achieve the desired (target)
common mode voltage.
General principle:
Lecture 28 – Differential-In, Differential-Out Op Amps (6/25/14) Page 28-25
CMOS Analog Circuit Design © P.E. Allen - 2016
SUMMARY
• Advantages of differential output op amps:
- 6 dB increase in signal amplitude
- Cancellation of even harmonics
- Cancellation of common mode signals including clock feedthrough
• Disadvantages of differential output op amps:
- Need for common mode output voltage stabilization
- Compensation of common mode feedback loop
- Difficult to interface with single-ended circuits
• Most differential output op amps are truly balanced
• For push-pull outputs, the quiescent current should be well defined
• Common mode feedback schemes include,
- Continuous time
- Discrete time