l320-diffoutputopamps-2up
DESCRIPTION
differential opTRANSCRIPT
Lecture 320 – Differential Output Op Amps (3/27/02) Page 320-1
ECE 6412 - Analog Integrated Circuit Design - II © P.E. Allen - 2002
LECTURE 320 – DIFFERENTIAL OUTPUT OP AMPS(READING: AH – 384-393, GHLM – 808-857)
ObjectiveThe objective of this presentation is:1.) Design and analysis of differential output op amps2.) Examine the problem of common mode stabilizationOutline• Advantages and disadvantages of fully differential operation• Six different differential output op amps• Techniques of stabilizing the common mode output voltage• Summary
Lecture 320 – Differential Output Op Amps (3/27/02) Page 320-2
ECE 6412 - Analog Integrated Circuit Design - II © P.E. Allen - 2002
Why Differential Output Op Amps?
• Cancellation of common mode signals including clock feedthrough
• Increased signal swing
v1
v2
v1-v2t
t
t
A
-AA
-A
2A
-2A Fig. 7.3-1
• Cancellation of even-order harmonics
Symbol:
-
+vin vout+
-
-
+
-
+
Fig. 7.3-1A
Lecture 320 – Differential Output Op Amps (3/27/02) Page 320-3
ECE 6412 - Analog Integrated Circuit Design - II © P.E. Allen - 2002
Common Mode Output Voltage StabilizationIf the common mode gain not small, it may cause the common mode output voltage to
be poorly defined.Illustration:
VDD
vod
t
Fig. 7.3-2
VSS
0
VDD
vod
t
VSS
0
VDD
vod
t
VSS
0
CM output voltage = 0
CM output voltage =0.5VDD
CM output voltage =0.5VSS
Lecture 320 – Differential Output Op Amps (3/27/02) Page 320-4
ECE 6412 - Analog Integrated Circuit Design - II © P.E. Allen - 2002
Two-Stage, Miller, Differential-In, Differential-Out Op Amp
vi1 M1 M2
M3 M4
M5
M6M8
VDD
VSS
VBN+-
Cc
M9
Cc
VBP+-
vi2
vo1vo2RzRz
M7
Fig. 7.3-3
Output common mode range (OCMR) = VDD+ |VSS| - VSDP(sat) - VDSN(sat)
The maximum peak-to-peak output voltage ≤ 2·OCMRConversion between differential outputs and single-ended outputs:
vod CL
+
-+-+
-vid
+
-vo1 2CL
+
-+-+
-vid
+
-
vo2+
-2CL
Fig. 7.3-4
Lecture 320 – Differential Output Op Amps (3/27/02) Page 320-5
ECE 6412 - Analog Integrated Circuit Design - II © P.E. Allen - 2002
Differential-Output, Folded-Cascode, Class-A Op Amp
VDD
VSS
R1
M14
R2
M8
M10VBias
vi1 vi2 vo1vo2M1 M2
M3
M4 M5
M7M6
M9
M11
M12
M13
M15
M16
M17Fig. 7.3-5
OCMR = VDD + |VSS| - 2VSDP(sat) -2VDSN(sat)
Lecture 320 – Differential Output Op Amps (3/27/02) Page 320-6
ECE 6412 - Analog Integrated Circuit Design - II © P.E. Allen - 2002
Two-Stage, Miller, Differential-In, Differential-Out Op Amp with Push-Pull Output
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
Comments:• Able to actively source and sink output current• Output quiescent current poorly defined
Lecture 320 – Differential Output Op Amps (3/27/02) Page 320-7
ECE 6412 - Analog Integrated Circuit Design - II © P.E. Allen - 2002
Two-Stage, Differential Output, Folded-Cascode Op Amp
VDD
VSS
R1
M16 M8
M10
VBias
vi1 vi2vo1vo2 M1 M2
M3
M4 M5
M7M6
M9
M11
M14 M15
M12 M13
CcRz CcRz
M17
M18
M19
M20
Fig. 7.3-7A
Note that the followers M11-M13 and M10-M12 are necessary for level translation to theoutput stage.
Lecture 320 – Differential Output Op Amps (3/27/02) Page 320-8
ECE 6412 - Analog Integrated Circuit Design - II © P.E. Allen - 2002
Unfolded Cascode Op Amp with Differential-Outputs
VDD
VSS
VBias
R2 M22
M23
R1
M19
M20
M1 M2
M3 M4M5 M6M7 M8
M9 M10
M11
M12M13
M14
M15 M16M17
M18
M21
vo1vi1 vi2
vo2
Fig. 7.3-8
Lecture 320 – Differential Output Op Amps (3/27/02) Page 320-9
ECE 6412 - Analog Integrated Circuit Design - II © P.E. Allen - 2002
Cross-Coupled Differential Amplifier StageOne of the problems with some of the previous stages, is that the quiescent output currentwas not well defined.The following input stage solves this problem.
M1 M2
M3 M4
VGS2
VSG4
VGS1
VSG3
i1
i1i2
i2
vi1 vi2
Fig. 7.3-9
vGS1+
-
vSG4+
-vSG3
+
-
vGS2+
-
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 M1 = M2 = M3 = M4, then half of the differential input is applied across each transistorwith the correct polarity.
∴ i1 = gm1vid
2 = gm4vid
2 and i2 = -gm2vid
2 = -gm3vid
2
Lecture 320 – Differential Output Op Amps (3/27/02) Page 320-10
ECE 6412 - Analog Integrated Circuit Design - II © P.E. Allen - 2002
Class AB, Differential Output Op Amp using a Cross-Coupled Differential InputStage
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
Quiescent output currents are defined by the current in the input cross-coupleddifferential amplifier.
Lecture 320 – Differential Output Op Amps (3/27/02) Page 320-11
ECE 6412 - Analog Integrated Circuit Design - II © P.E. Allen - 2002
Common-Mode Output Voltage Stabilization
VDD
VSS
io2(source)
io2(sink)
io1(source)
io1(sink)
Ro1
Ro3
Ro2
Ro4
M1 M2Common-modefeedback circuit
vo1 vo2
Fig. 7.3-11
Model of output of differentialoutput op amp
Operation:M1 and M2 sense the common-mode output voltage.If this voltage rises, the currents in M1 and M2 decrease.This decreased current flowing through Ro3 and Ro4 cause the common-mode outputvoltage to decrease with respect to VSS.
Lecture 320 – Differential Output Op Amps (3/27/02) Page 320-12
ECE 6412 - Analog Integrated Circuit Design - II © P.E. Allen - 2002
Two-Stage, Miller, Differential-In, Differential-Out Op Amp with Common-ModeStabilization
vi1 M1 M2
M3 M4
M5
M6M7
VDD
VSS
VBN+-
Cc
M9
Cc
VBP
+
-
vi2
vo1vo2RzRz
M8
Fig. 7.3-12
M10 M11
Comments:• Simple• Unreferenced
Lecture 320 – Differential Output Op Amps (3/27/02) Page 320-13
ECE 6412 - Analog Integrated Circuit Design - II © P.E. Allen - 2002
A Referenced Common-Mode Output Voltage Stabilization Scheme
VDD
VSS
M1 M2M3 M4
M5 M6
Vocm To correctioncircuitry
vo1 vo2
I5I6
Iocm
Fig. 7.3-13
Operation:1.) The desired common-mode output voltage, Vocm, creates Iocm.
2.) The actual common-mode output voltage creates current I5 which is mirrored to I6 .
3.) If M1 through M4 are matched and the current mirror is ideal, then when Iocm = I6the actual common-mode output voltage should be equal to the desired common-mode output voltage.
4.) The above steps assume that a correction circuitry exists that changes the common-mode output voltage in the correct manner.
Lecture 320 – Differential Output Op Amps (3/27/02) Page 320-14
ECE 6412 - Analog Integrated Circuit Design - II © P.E. Allen - 2002
Common Mode Feedback CircuitsImplementation of common mode feedback circuit:
v1M1 M2
M3 M4
M5
VDD
VSS
IBias
VCM
v4v3
v2
MC2A
MC2B
MC1
MC3
MC4
MC5MB
I3 I4
IC4IC3
Fig. 7.3-13A
Common-mode feed-back circuit
Self-resistancesof M1-M4
This scheme can be applied to any differential output amplifier.Caution:
Be sure to check the stability of common-mode feedback loops, particularly those thatare connected to op amps that have a cascode output. The gain of the common-modefeedback loop can easily reach that of a two-stage amplifier.
Lecture 320 – Differential Output Op Amps (3/27/02) Page 320-15
ECE 6412 - Analog Integrated Circuit Design - II © P.E. Allen - 2002
Common Mode Feedback Circuits – ContinuedThe previous circuit suffers in performance when the differential output voltage becomestoo large and one of the MC2A-MC2B transistors shuts off.The following circuit alleviates this disadvantage:
v1M1 M2
M3 M4
M5
VDD
VSS
IBias
VCM
v4v3
v2MC2MC1
MC3
MC4
MC5MB
I3 I4
IC4IC3Common-mode feed-back circuit
RCM1 RCM2
Fig. 7.3-15New
Lecture 320 – Differential Output Op Amps (3/27/02) Page 320-16
ECE 6412 - Analog Integrated Circuit Design - II © P.E. Allen - 2002
External Common-Mode Output Voltage Stabilization Scheme for Discrete-TimeApplications
Ccm+
-+-+
-vid
vo1
vo2CMbias
φ1 φ2
Ccm Vocm
φ1 φ2
φ1
φ1
φ1 Fig. 7.3-14
Operation:1.) During the φ1 phase, both Ccm are charged to the desired value of Vocm and CMbias
= Vocm.
2.) During the φ2 phase, the Ccm capacitors are connected between the differentialoutputs and the CMbias node. The average value applied to the CMbias node will beVocm.
Lecture 320 – Differential Output Op Amps (3/27/02) Page 320-17
ECE 6412 - Analog Integrated Circuit Design - II © P.E. Allen - 2002
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,
- Unreferenced
- Referenced