Sam PalermoAnalog & Mixed-Signal Center
Texas A&M University
ECEN620: Network TheoryBroadband Circuit Design
Fall 2019
Lecture 14: Limiting Amplifiers (LAs)
Announcements
• Exam 2 Tuesday Nov 26• One double-sided 8.5x11 notes page allowed• Bring your calculator• Covers through Lecture 14
• Project report due Dec 3
• Project presentations Dec 6 3PM-5PM
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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 <10% over bandwidth of interest to limit DDJ
4
TIA Output
LA Output
How to Achieve an Ampilfier GBW > fT?
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• 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
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
12
Shunt Peaking
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• 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
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• 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
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Bridged-Shunt Peaking
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• 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
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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
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No L
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Bridged-Shunt-Series Peaking
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• Combining both shunt and series peaking can yield even higher bandwidth extension
Bridged-Shunt-Series Peaking
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• 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
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• 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
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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
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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
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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:
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Negative Miller Capacitance
• In order to mitigate this Cgdmultiplication, 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
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Active Negative Feedback
• Instead of using simple first-order amplifier cells, a second-order cell with active negative feedback can provide bandwidth enhancement
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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
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Active Negative Feedback
• The second-order cell gain-bandwidth can potentially achieve a value greater than the technology fT
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Limiting Amplifier Example 1
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[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
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[Proesel ISSCC 2012]
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
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• 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
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Offset Compensation
• The low-pass filtering in the feedback loop causes a low-frequency cutoff
33
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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
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