Sam PalermoAnalog & Mixed-Signal Center

Texas A&M University

Lecture 6: Limiting Amplifiers (LAs)

ECEN689: Special Topics in Optical Interconnects Circuits and Systems

Spring 2016

Announcements & Agenda

• Multi-stage limiting amplifiers

• Bandwidth extension techniques

• Offset compensation

2

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

3

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

11

Shunt Peaking

12

• 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

13

• 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

14

• 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

15

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

16

No L

C1=0

Bridged-Shunt-Series Peaking

17

• 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

20

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

21

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• TIA Load• Active Negative Feedback

22

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:

23

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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

24

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Cherry Hooper Amplifier

26

Active Negative Feedback

• Instead of using simple first-order amplifier cells, a second-order cell with active negative feedback can provide bandwidth enhancement

27

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

28

Active Negative Feedback

• The second-order cell gain-bandwidth can potentially achieve a value greater than the technology fT

29

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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

31

[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

32

• 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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• The low-pass filtering in the feedback loop causes a low-frequency cutoff

34

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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

35