Sam PalermoAnalog & Mixed-Signal Center

Texas A&M University

Lecture 5: Transimpedance Amplifiers (TIAs)

ECEN689: Special Topics in Optical Interconnects Circuits and Systems

Spring 2020

Announcements• Reading

• Sackinger Chapter 5• Razavi Chapter 4

2

Agenda• Optical Receiver Overview• Transimpedance Amplifiers

• Common-Gate TIAs• Feedback TIAs• Common-Gate & Feedback TIA Combinations• Differential TIAs

• Integrating Optical Receivers• Equalization in Optical Front-Ends

3

Optical Receiver Technology• Photodetectors convert optical

power into current• p-i-n photodiodes• Waveguide Ge photodetectors

• Electrical amplifiers then convert the photocurrent into a voltage signal• Transimpedance amplifiers• Limiting amplifiers• Integrating optical receiver

4

TIA Output

LA Output

• Key design objectives• High transimpedance gain• Low input resistance for high bandwidth and efficient gain

• For large input currents, the TIA gain can compress and pulse-width distortion/jitter can result

Transimpedance Amplifier (TIA)

5

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log20by dB of unitsin expressed Also

anceTransimped

• Input Overload Current• The maximum peak-to-peak input current for which we can

achieve the desired BER• Assuming high extinction ratio

• Maximum Input Current for Linear Operation• Often quantified by the current level for a certain gain

compression (1dB)

Maximum Currents

6

ovlppovl PRi 2

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pplin ii

Resistive Front-End

• Direct trade-offs between transimpedance, bandwidth, and noise performance

7

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[Razavi]

Agenda• Optical Receiver Overview• Transimpedance Amplifiers

• Common-Gate TIAs• Feedback TIAs• Common-Gate & Feedback TIA Combinations• Differential TIAs

• Integrating Optical Receivers• Equalization in Optical Front-Ends

8

Common-Gate TIA

• Input resistance (input bandwidth) and transimpedance are decoupled

9

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[Razavi]

Common-Gate TIA Frequency Response

• Often the input pole may dominate due to large photodiode capacitance (100 – 500fF)

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[Razavi]

Common-Gate TIA Noise• Both the bias current source

and RD contribute to the input noise current

• RD can be increased to reduce noise, but voltage headroom can limit this

• Common-gate TIAs are generally not for low-noise applications

• However, they are relatively simple to design with high stability

11

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[Razavi]

Regulated Cascode (RGC) TIA

12

[Park ESSCIRC 2000]• Input transistor gm is boosted by common-source amplifier gain, resulting in reduced input resistance

• Requires additional voltage headroom

• Increased input-referred noise from the common-source stage

CMOS 20GHz TIA

13333222 RgARgA mm

• An additional common-gate stage in the feedback provides further gm-boosting and even lower input resistance

• Shunt-peaking inductors provide bandwidth extension at zero power cost, but very large area cost

[Kromer JSSC 2004]

Agenda• Optical Receiver Overview• Transimpedance Amplifiers

• Common-Gate TIAs• Feedback TIAs• Common-Gate & Feedback TIA Combinations• Differential TIAs

• Integrating Optical Receivers• Equalization in Optical Front-Ends

14

Feedback TIA w/ Ideal Amplifier

• Input bandwidth is extended by the factor A+1• Transimpedance is approximately RF• Can make RF large without worrying about voltage

headroom considerations 15

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Feedback TIA w/ Finite Bandwidth Amplifier

16

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• Finite bandwidth amplifier modifies the transimpedancetransfer function to a second-order low-pass function

Feedback TIA w/ Finite Bandwidth Amplifier

17

• Non-zero amplifier time constant can actually increase TIA bandwidth!!

• However, can result in peaking in frequency domain and overshoot/ringing in time domain

• Often either a Butterworth (Q=1/sqrt(2)) or Bessel response (Q=1/sqrt(3)) is used• Butterworth gives maximally flat

frequency response• Bessel gives maximally flat group-

delay Butterworth

Bessel

[Sackinger]

-A(s)

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vout

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Feedback TIA Transimpedance Limit

18

• Maximum RT proportional to amp gain-bandwidth product• If amp GBW is limited by technology fT, then in order to

increase bandwidth, RT must decrease quadratically!

3dB

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Feedback TIA

19

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• As power supply voltages drop, there is not much headroom left for RD and the amplifier gain degrades

• CMOS inverter-based TIAs allow for reduced voltage headroom operation

• Cascaded inverter-gm + TIA stage provide additional voltage gain

• Low-bandwidth feedback loop sets the amplifier output common-mode level

CMOS Inverter-Based Feedback TIA

20

[Li JSSC 2014]

• TIA noise is modeled with an input-referred noise current source that reproduces the output TIA output noise when passed through an ideal noiseless TIA

• This noise source will depend on the source impedance, which is determined mostly by the photodetector capacitance

Input-Referred Noise Current

21

Hz

pA2

• Input-referred noise current spectrum typically consists of uniform, high-frequency f2, & low-frequency 1/f components

• To compare TIAs, we need to see this noise graph out to ~2X the TIA bandwidth• Recall the noise bandwidth tables

Input-Referred Noise Current Spectrum

22

Hz

pA2

• The input-referred rms noise current can be calculated by dividing the rms output noise voltage by the TIA’s midbandtransimpedance value

Input-Referred RMS Noise Current

23

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• If we integrate the output noise, the upper bound isn’t too critical. Often this is infinity for derivations, or 2X the TIA bandwidth in simulation

• This rms current sets the TIA’s electrical sensitivityrms

TIAnppsens Qii ,2

• To determine the total optical receiver sensitivity, we need to consider the detector noise and responsivity

• TIA noise performance can also be quantified by the averaged input-referred noise current density

Averaged Input-Referred Noise Current Density

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• The feedback resistor and amplifier front-end noise components determine the input-referred noise current spectrum

FET Feedback TIA Input-Referred Noise Current Spectrum

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26

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designs CMOSfor small typicallyis This

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Total Input-Referred FET Feedback TIA Noise

28

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Feedback Resistor Gate Shot Noise FET Channel Noise

• Note that the TIA input-referred noise current spectrum begins to rise at a frequency lower than the TIA bandwidth

Agenda• Optical Receiver Overview• Transimpedance Amplifiers

• Common-Gate TIAs• Feedback TIAs• Common-Gate & Feedback TIA Combinations• Differential TIAs

• Integrating Optical Receivers• Equalization in Optical Front-Ends

29

Common-Gate & Feedback TIA

30

• Recall that the feedback TIA stability depends on the ratio of the input pole (set by CD) and the amplifier pole• Large variation in CD can degrade amplifier stability

• Common-gate input stage isolates CD from input amplifier capacitance, allowing for a stable response with a variety of different photodetectors

• Transimpedance is still approximately RFA/(1+A)

Common-Gate

Feedback TIA

[Mohan JSSC 2000]

RF RF

BJT Common-Base & Feedback TIA

31

• Transformer-based negative feedback boosts gm with low power and noise overhead

• Input series peaking inductor isolates the photodetectorcapacitance from the TIA input capacitance

• High frequency techniques allow for 26GHz bandwidth with group delay variation less than 19ps

[Li JSSC 2013]

Agenda• Optical Receiver Overview• Transimpedance Amplifiers

• Common-Gate TIAs• Feedback TIAs• Common-Gate & Feedback TIA Combinations• Differential TIAs

• Integrating Optical Receivers• Equalization in Optical Front-Ends

32

Differential TIAs

33

• Differential circuits have superior immunity to power supply/substrate noise

• A differential TIA output allows easy use of common differential main/limiting amplifiers• This comes at the cost of higher

noise and power• How to get a differential output

with a single-ended photocurrent input?• Two common approaches, based on

the amount of capacitance applied at the negative input

Balanced TIA

34

• A balanced TIA design attempts to match the capacitance of the two differential inputs

DX CC

• This provides the best power supply/substrate noise immunity, as the noise transfer functions are similar

• Due to double the circuitry, the input-referred rms noise current is increased by sqrt(2)

design ended-single theasfunction transfer Same

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Pseudo-Differential TIA

35

• A pseudo-differential TIA design uses a very large capacitor at the negative input, such that it can be approximated as an AC ground XC

• While not good to reject power supply/substrate noise, it does provide significant filtering of the RF’ noise

• The differential transimpedanceis approximately doubled relative to the single-ended case

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Offset Control

36

• Due to the single-ended photodetector signal, the differential output signal swings from 0 to Vppd, which can limit the dynamic range

• Adding offset control circuitry can allow for an output swing of ±Vppd/2

Differential Shunt Feedback TIA

37

Agenda• Optical Receiver Overview• Transimpedance Amplifiers

• Common-Gate TIAs• Feedback TIAs• Common-Gate & Feedback TIA Combinations• Differential TIAs

• Integrating Optical Receivers• Equalization in Optical Front-Ends

38

39

Optical RX Scaling Issues

Traditionally, TIA has high RT and low Rin

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Headrooom/Gain issues in 1V CMOS• A ≈ 2 – 3

Power/Area Costs 4

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40

Integrating Receiver Block Diagram

[Emami VLSI 2002]

41

Demultiplexing Receiver

• Demultiplexing with multiple clock phases allows higher data rate̶ Data Rate = #Clock Phases x Clock Frequency̶ Gives sense-amp time to resolve data̶ Allows continuous data resolution

42

1V Modified Integrating Receiver

Differential BufferFixes sense-amp common-mode input for improved

speed and offset performanceReduces kickback chargeCost of extra power and noise

Input Range = 0.6 – 1.1V

43

Receiver Sensitivity Analysis

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Residual SA Offset = 1.15mV

Max Vin(IAVG) = 0.6mV

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Vb for BER = 10-10 = 6.4tot + Offset = 11.9mV

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Gb/s Pavg (dBm)

10 -9.816 -7.8

44

Integrating Receiver Sensitivity

• Test Conditions̶ 8B/10B data patterns

(variance of 6 bits)̶ Long runlength data

(variance of 10 bits)

• BER < 10-10

[Palermo JSSC 2008]

45

Integrating RX with Dynamic Threshold

• Dynamic threshold adjustment allows for un-coded data

[Nazari ISSCC 2012]

46

Integrating RX with Dynamic Threshold

[Nazari ISSCC 2012]

Agenda• Optical Receiver Overview• Transimpedance Amplifiers

• Common-Gate TIAs• Feedback TIAs• Common-Gate & Feedback TIA Combinations• Differential TIAs

• Integrating Optical Receivers• Equalization in Optical Front-Ends

47

Low-BW TIA & CTLE Front-End

48

• Improved sensitivity is possible by increasing the first stage feedback resistor, resulting in a high-gain low-bandwidth TIA

• The resultant ISI is cancelled by a subsequent CTLE

[Li JSSC 2014]

Active CTLE Example

49

Din- Din+

Vo-Vo+

Low-BW TIA & CTLE Front-End

50

• Significant reduction in feedback resistor noise

• Low-frequency input and post amplifier noise is also reduced

[Li JSSC 2014]

Low-BW TIA & CTLE Front-End

51

[Li JSSC 2014]

25Gb/s Eye Diagram

Low-BW TIA & DFE RX

52

• In a similar manner, a high-gain low-bandwidth TIA is utilized

• The resultant ISI is cancelled by a subsequent 1-tap loop-unrolled DFE

[Ozkaya JSSC 2017]

DFE Example

53

• If only DFE equalization, DFE tap coefficients should equal the unequalized channel pulse response values [a1 a2 … an]

• With other equalization, DFE tap coefficients should equal the pre-DFE pulse response values

• DFE provides flexibility in the optimization of other equalizer circuits

• i.e., you can optimize a TX equalizer without caring about the ISI terms that the DFE will take care of

a1

a2

[w1 w2]=[a1 a2]

Low-BW TIA & DFE RX

54

• As RF is increased, the main cursor increases and the SNR improves is ISI is cancelled by a DFE

• Large performance benefit with a low-complexity 1-tap DFE

[Ozkaya JSSC 2017]

Low-BW TIA & DFE RX

55

• Self-referenced TIA is used for differential generation

• Actual 64Gb/s pulse response has a significant pre-cursor ISI tap, which requires a 2-tap TX FFE

[Ozkaya JSSC 2017]

64Gb/s Pulse Response & Timing Margin

Next Time• Main/Limiting Amplifiers

56