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Sam PalermoAnalog & Mixed-Signal Center
Texas A&M University
ECEN620: Network TheoryBroadband Circuit Design
Fall 2020
Lecture 14: Limiting Amplifiers (LAs)
Announcements• Project descriptions posted on website
• Preliminary report due Nov 3 (HW4)• Final report due Nov 24• Presentation on Dec 4 (2PM-4:30PM)
2
Agenda• Multi-stage limiting amplifiers
• Bandwidth extension techniques
• Offset compensation
3
Limiting Amplifiers• Limiting amplifier amplifies the
TIA output to a reliable level to achieve a given BER with a certain decision element (comparator)
• Typically designed with a bandwidth of 1-1.2X data rate
• Want group delay variation
How to Achieve an Ampilfier GBW > fT?
5
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Optimum Number of Gain Stages
12
• Note, while this is the optimum number of stages from a maximum GBW perspective, the bandwidth doesn’t falloff too dramatically with lower n
• Thus, from a power and noise perspective, it may make sense to use a lower number of LA stages
• Typically high-gain LAs use between 3-7 stages
[Galal JSSC 2003]
Agenda• Multi-stage limiting amplifiers
• Bandwidth extension techniques
• Offset compensation
13
Bandwidth Extension Techniques• In order to increase the bandwidth of our multi-
stage amplifiers, we need to increase the bandwidth of the individual stages
• Passive bandwidth extension techniques• Shunt Peaking• Series Peaking• T-coil Peaking
• An excellent reference
14
Shunt Peaking
15
• Adding an inductor in series with the load resistor introduces a zero in the impedance transfer function
• This zero increases the impedance with frequency, compensating the decrease caused by the capacitor, and extending the bandwidth
Shunt Peaking
16
• While the inductor can increase the bandwidth significantly, frequency peaking can occur if the inductor is too big
• For a flat frequency response, ~70% bandwidth increase can be achieved
• A maximum 85% bandwidth increase is possible with 1.5dB of peaking
constants timeRC of Ratio
RL
Bridged-Shunt Peaking
17
• Adding a bridge capacitor in parallel with the inductor allows for compensation of the frequency peaking with the possible maximum shunt peaking bandwidth increase
• A real inductor will always have some parasitic CB, and thus kB will be >0 in practice even without an extra cap
No L, CB
Max BW L, No CB
L CB
Series Peaking
• Introducing a series peaking inductor is useful to “split” the load capacitance between the amplifier drain capacitance and the next stage gate capacitance
• Without L, the transistor has to charge the total capacitance at the same time
• With L, initially only C1 is charged, reducing the risetimeat the drain and increasing bandwidth
18
Series Peaking
• As the capacitance is more distributed with a higher kCvalue, a higher BWER is achieved
• Up to 2.5x bandwidth increase is achieved with no peaking
• Higher BWER is possible with some frequency peaking
19
No L
C1=0
Bridged-Shunt-Series Peaking
20
• Combining both shunt and series peaking can yield even higher bandwidth extension
Bridged-Shunt-Series Peaking
21
• Proper choice of component values can yield close to 4x increase in bandwidth with no peaking
• However, this requires tight control of these components, which can be difficult with PVT variations
T-Coil Peaking
22
• If the input transistor drain capacitance (C1) is relatively small, then the bandwidth extension through shunt-series peaking is limited
• T-coil peaking, which utilizes the magnetic coupling of a transformer, provides better bandwidth extension in this case• L2 performs capacitive splitting, such that the
initial current charges only C1• As current begins to flow through L2,
magnetically coupled current also flows through L1, providing increased current to charge C2which improves bandwidth and transition times
T-Coil Peaking
23
T-Coil Peaking
• A bandwidth extension of 4x is possible without any frequency peaking
• If peaking is acceptable, then a BWER near 5 can be achieved, depending on the size of C1
24
Active Bandwidth Extension Techniques• While passive techniques offer excellent bandwidth
extension at near zero power cost, there are some disadvantages• Generally large area• Process support/characterization of inductors/transformers
• Active circuit techniques can also be employed to extend amplifier bandwidth
• Some active bandwidth extension techniques• Negative Miller Capacitance• Active Negative Feedback
• There are numerous other techniques, but that is all we have time for this semester
25
Negative Miller Capacitance• In modern technologies,
Cgd is a significant (50% to near 100%) fraction of Cgs
• Amplifier effective input capacitance can increase significantly due to the Miller multiplication of Cgd
• Without additional Cn:
26
ecapacitancinput effective in the increaset significan
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Negative Miller Capacitance• In order to mitigate this Cgd
multiplication, additional cross-coupled capacitors can be added from the amplifier inputs to the outputs
• Effectively, the charge on this additional capacitor charges a (large) portion of the Cgdcapacitor
27
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28
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Cherry Hooper Amplifier
29
Active Negative Feedback• Instead of using simple first-order amplifier cells, a
second-order cell with active negative feedback can provide bandwidth enhancement
30
Inverting
Active Negative Feedback• This second-order amplifier cell can be optimized for different
objectives, but Gmf can be set to yield a Butterworth response with a maximally-flat frequency response
31
Active Negative Feedback
• The second-order cell gain-bandwidth can potentially achieve a value greater than the technology fT
32
dB
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Limiting Amplifier Example 1
33
[Galal JSSC 2003]
Resistive Load Only Active Negative Feedback Shunt Inductive Peaking Negative Miller Capacitance
Limiting Amplifier Example 2
• T-coils in LA stages allow for a combination of series and shunt peaking and close to 3x bandwidth extension
34
[Proesel ISSCC 2012]
Agenda• Multi-stage limiting amplifiers
• Bandwidth extension techniques
• Offset compensation
35
Offset Compensation• The receiver sensitivity is degraded if the limiting
amplifier has an input-referred offset• This is often quantified in terms of a Power Penalty, PP
36
• It is important to minimize the offset of these multi-stage limiting amplifiers!
Offset Compensation
• The DC offset, Vos, of the limiting amplifier is compensated by a low-frequency negative feedback loop
37tagesoffset vol eduncorrelatwith
becomesoffset the, offset,an hasamplifier error theif However,
1
offset to thereduces thisIdeally,
21
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AV
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V
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osos
os
osos
Offset Compensation
• The low-pass filtering in the feedback loop causes a low-frequency cutoff
38
11
1 12/21
CRAAfLF
Note, the AA1/2 factor assume a 50 driver source• Thus, the feedback loop bandwidth should be made much lower than
the lowest frequency content of the input data• This may lead to large-area passive in the offset correction feedback• Some designs leverage Miller capacitive multiplication with the error
amplifier to reduce this filter area
Next Time• High-Speed Transmitters
39