Transcript opamp_2 - Lane Department of Computer Science and Electrical

Opamps
Part 2
Dr. David W. Graham
West Virginia University
Lane Department of Computer Science and Electrical Engineering
© 2009 David W. Graham
1
High Gain
• Goal of opamp design – High gain
• Previous opamps do not have very high gain
• Example – 5T Opamp
– Gain = -gm1ro2||r04
– Subthreshold operation – |Gain| ≈ 650
– Above threshold operation – |Gain| ≈ 50
• Need much higher gain
– Cascode structures provide high gain
– Cascade of multiple amplifiers
2
Telescopic Opamps
Vb3
M7
M8
Av  g m1 ro 2 g x 4 ro 4 || ro8 g x 6 ro 6 
Vb2
M5
M6
Vo1
Vo2
Vb1
M3
Vi1
M4
M1
Vb
Approximately the square of the original gain
M2
Mb
Vi2
This is a high-speed opamp design
Major Drawback
• Very limited allowable signal swing
• Must ensure all transistors stay in saturation
• Limited signal swing at both the input and
the output
3
Telescopic Opamps – Single-Ended Output
M7
M7
M8
M8
Vb2
M5
M5
M6
M6
Vout
Vout
Vb1
Vb1
M3
Vi1
M1
Vb
M3
M4
M2
Mb
Vi2
Vi1
M4
M1
Vb
M2
Vi2
Mb
• Increased output signal swing
• Requires an additional bias
4
Unity-Gain Feedback Connection
• Another major drawback to the telescopic opamp is the very limited
range for unity-feedback connections
• Therefore, this opamp is rarely used as a unity-gain buffer
• Often used in switched-capacitor circuits, where the output is fed
back to the input only for short durations of time
For M2 and M4 to stay in saturation
M7
M8
M5
M6
Vout  Vx  VT 2  Vb1  Vgs 4  VT 2 for M 2
Vout  Vb1  VT 4 for M 4
Vout
Vb1
M3
M4
Vx
Vi1
M1
M2
Vb1  VT 4  Vout  Vb1  Vgs 4  VT 2
Voltage range for Vout
Vmax  Vmin  VT 4  Vgs 4  VT 2
 VT 2  Vov 4
Vb
Mb
Always less than a threshold voltage
5
Folded Cascode Structure
Used in opamps to increase input/output voltage ranges
Iref1
Iref1
Iref2
Vb
M3
Vb
Vi1
Vin
M1
M2
Vout
Iref2
Vi2
Vo1
Vo2
Iref3
Vb
M4
Iref4
Mb
I ref 3 
Ib
 I ref 1
2
• Iref1 is typically greater than Ib to improve
response after slewing
• Burns more power than the telescopic version
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Folded Cascode Opamp
Vout
Vb4
M9
M10
M5
M6
M3
M4
Vb3
M7
Vi1
M1
M2
Vi2
M8
Vo1
Vo2
Vb2
M5
Vb
Mb
M6
Vb1
M3
Vout
Vb1
M4
M5
M6
M3
M4
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Differential Gain of the Folded Cascode Opamp
• Resistance looking into the source of
M7 is much less than ro1||r09
• Virtually all current flowing out of M1
will flow into the source of M7
Vb4
M9
M10
Av  g m1 ro8 g x8 ro10 || ro 2  || ro 6 g x 6 ro 4 
Vb3
M7
Vi1
M1
M2
M8
Vout
Vi2
Vb1
M5
Vb
M6
Mb
M3
[Slightly] reduced gain from
telescopic amplifier
M4
ICMR
Vgs1  Vsat,b to Vdd  Vsat,9  Vsat,1  Vgs1
 Vdd  Vov,9  VT 1
Can use pFET inputs for
operation to ground
Output range
2Vsat to Vdd  2Vsat
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Folded Cascode Summary
Comparison to Telescopic Opamp
• Larger input/output swings
• Can be used in unity-gain configuration
• One less voltage is required to be set
• Do not need to worry about the CM voltage
• Decreased voltage gain
• Increased power consumption (plus, I9 should
be ~1.2-1.5 times Ib)
• Lower frequency of operation
• More noise
Overall, the folded cascode opamp is a good,
widely used opamp
9
Two-Stage Opamp
• Cascade of two amplifier stages
– First stage – Differential amplifier
– Second stage – High-gain amplifier
Vb2
M3
M4
M5
Vo1
M6
V1
M1
Vb1
M2
V2
Vo2
Mb
Vb3
M7
M8
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Two-Stage Opamp (Single-Ended Output)
• Cascade of two amplifier stages
Av1   g m1ro 2 || ro 4
– First stage – Differential amplifier
– Second stage – High-gain amplifier (CS Amp) Av 2   g m5ro5 || ro6
Av  g m1ro 2 || ro 4 g m5ro5 || ro 6 
M3
M4
M5
V1
M1
Vb
M2
Mb
V2
Vout
M6
• Large output swing (Vsat,6 to Vdd – Vsat,5)
• ICMR same as 5T opamp
• Unity-gain configuration sets a
minimum voltage to Vgs1-Vsat,b
• Can include cascodes, as well
• Adding an amplifier stage adds a pole
• Typically requires compensation to
remain stable
11
Feedback Systems
Vin(s)
H(s)
+
Vout(s)
Vin
Vout
F(s)
If F(s)=1, then unity gain feedback
12
Opamp Poles
•
•
Several poles in an opamp
Typically, one pole dominates
– Dominant pole is closest to the origin (Re-Im Plot)
– Dominant pole has the largest time constant
•
Dominant pole is often associated with the output node in an unbuffered
opamp
– Large Rout and load capacitance
|H(j)|dB
Gain Bandwidth, GB
Av,dc
GB  Av ,dc3dB
 1 

  Gm Rout 
R
C
 out out 
G
 m
Cout
-3dB
GB
log()
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Multiple Poles
• For multi-pole systems, other poles may be
close enough to the dominant pole to affect
stability
• Typically two poles are of primary concern
• Typically, for a two-stage, unbuffered opamp
– Pole at output of stage 1
– Pole at output of stage 2
– Dominant pole is usually associated with a large load
capacitance (i.e. output node)
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Multiple Poles
Vx
Vin
Gm1Vin
Rout1
Vout
Cout1
p1 
1
R1C1
p2 
1
R2C2
Gm2Vx
Rout2
Cout2
p2 typically dominates because of the load capacitance
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Multiple Poles
|H(j)|dB
Av,dc
Unity Gain
Phase of -180°
2
1
log()
log()
-90o
-180o
arg(H(j))
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Negative Feedback
In negative feedback configuration, if
H  j   1 and H  j   180
Then, combined with subtraction (-180°) at the input
• Results in -360° phase shift
• This is addition (positive feedback)
• Since the gain is > 1 at this frequency, the output will grow without bound
• Therefore, this system is unstable at this frequency
• For stability, must ensure that
H  j   1 for  where H  j   180
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Phase Margin
• Typically, we like to design to provide a margin
of error
– These conditions (magnitude and phase) can deviate
from their designed values due to processes like
noise and temperature drift
• Phase margin
– A measure of how far away from a complete 360°
phase shift
– Phase margin = 180° - arg(H(jω))
– Measure at ω where |H(jω)| = 1
• Typical designs call for Phase margins of greater
than 45°
– Often higher, e.g. 60° - 90°
18
Miller Compensation
• Need to spread the poles apart
• Add a capacitor from input to output of
stage 2
M3
M4
M5
Cc
V1
M1
Vb
M2
Mb
V2
Vout
M6
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Miller Compensation
Vx
Vin
Gm1Vin
Rout1
Cc
Vout
Cout1
Gm2Vx
Rout2
Cout2
Vout s 
Gm1Gm 2 R1 R2 1  sC 2 Gm 2 
 2
Vin s  s R1 R2 C1C2  Cc C1  Cc C2   sR1 C1  Cc   R2 C2  Cc   Gm 2 R1 R2Cc   1
p1 
1
Gm 2 R1 R2Cc
p2 
 Gm 2Cc
 Gm 2

C1C2  C2Cc  C1Cc
C2
If C2 >> C1 and Cc > C1
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Miller Compensation
|H(j)|dB
Av,dc
GB 
Gm1
Cc
ω2 should be ≥ GB
1
2
log()
log()
-90o
Phase Margin
-180o
arg(H(j))
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Opamp Comparison
Gain
Output
Swing
Speed
Power
Dissipation
Noise
Telescopic
Medium
Medium
Highest
Low
Low
FoldedCascode
Medium
Medium
High
Medium
Medium
Two-Stage
High
Highest
Low
Medium
Low
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