1. High-Speed Circuits & Systems Laboratory Electronic Circuits for Optical Systems : Transimpedance Amplifier (TIA) Jin-Sung Youn (jsyoun@tera.yonsei.ac.kr)jsyoun@tera.yonsei.ac.kr High-Speed Circuits & Systems Laboratory 2011-1 Special Topics in Optical Communications

2. High-Speed Circuits & Systems Laboratory Contents 2 2011-1 Special Topics in Optical Communications  Electrical interconnects vs. optical interconnects  Electronic circuits for optical interconnects - Receiver basics - Transimpedance amplifier (TIA)  TIA design considerations  Advanced (wideband) techniques  Recent research topics  Conclusion

3. High-Speed Circuits & Systems Laboratory 3 Electrical Interconnects 2011-1 Special Topics in Optical Communications  Transmitter - Serializer: slow parallel data  fast serial data - Phase-Locked Loop (PLL): generate reference clock - Pre-emphasis: compensate high-frequency loss  Receiver - Equalizer: compensate high-frequency loss - Limiting amplifier: amplify signal up to digital level - Clock and Data Recovery (CDR): recover synchronous clock and data - De-serializer: fast serial data  slow parallel data

4. High-Speed Circuits & Systems Laboratory 4 Optical Interconnects 2011-1 Special Topics in Optical Communications  Direct modulation - Driving circuits - VCSEL  External modulation - Laser - Modulator - Driving circuits  Photodetector  Current-voltage (I-V) conversion component / circuits

5. High-Speed Circuits & Systems Laboratory 5 Requirements for The First Block 2011-1 Special Topics in Optical Communications  (Voltage / Power) Gain  (Channel) Bandwidth  Noise figure RF Receiver Front-EndOptical Receiver Front-End  (Transimpedance) Gain  (Broadband) Bandwidth  (Input) Noise current

6. High-Speed Circuits & Systems Laboratory 6 Receiver Basics 2011-1 Special Topics in Optical Communications  (Transimpedance) Gain  (Broadband) Bandwidth  (Input) Noise current ► Resistor performs a current-to-voltage conversion. Trade-off between gain, speed and noise !!

7. High-Speed Circuits & Systems Laboratory 7 Integrated Total Noise 2011-1 Special Topics in Optical Communications  Output Noise Spectrum of Circuit Circuit bandwidth ↑  Integrated total noise ↑

8. High-Speed Circuits & Systems Laboratory 8 Bandwidth vs. Noise 2011-1 Special Topics in Optical Communications  BW = 1.4 f B  No ISI  High noise   BW = 0.35 f B  High ISI   Low noise  BW = 0.7 f B  No ISI  Medium Noise * ISI: Inter-Symbol Interference ** f B : data rate

9. High-Speed Circuits & Systems Laboratory 9 Bandwidth vs. Noise 2011-1 Special Topics in Optical Communications  Inter-Symbol Interference (ISI) vs. Noise Optimum bandwidth depends on data rates !!

10. High-Speed Circuits & Systems Laboratory 10 Gain vs. Noise 2011-1 Special Topics in Optical Communications  Signal-to-Noise Ratio (SNR) NoiseRinging Gain Bandwidth Noise

11. High-Speed Circuits & Systems Laboratory 11 Receiver Basics - Example 2011-1 Special Topics in Optical Communications High bandwidth (f p = 15.9 GHz)  Low sensitivity (V o,pp = 9 mV pp )  High noise  Small bandwidth (f p = 0.8 GHz) High sensitivity (V o,pp = 1.8 V pp ) Low noise Low impedanceHigh impedance ** p-i-n photodetector ** (N.Feng et al., Optics Express, 2010) 1kΩ (1)R = 0.9 A/W (TM polarization)  I IN = 1.8 mA pp @ 0 dBm (ideal extinction ratio) (2) C pd = 200 fF  RC time constant

12. High-Speed Circuits & Systems Laboratory 12 Transimpedance Amplifier (TIA) 2011-1 Special Topics in Optical Communications  (Shunt-Shunt) Feedback Amplifier - Low input impedance & High transimpedance !!  (Transimpedance) Gain  (Broadband) Bandwidth  (Input) Noise current A R in

13. High-Speed Circuits & Systems Laboratory 13 Comparison 2011-1 Special Topics in Optical Communications But, R F ↑  R in ↑

14. High-Speed Circuits & Systems Laboratory 14 Advanced Techniques [1] 2011-1 Special Topics in Optical Communications  Shunt Peaking Inductor implementation : (a) Spiral inductor (b) Active inductor  (a) Large power consumption & chip area (b) PVT variation

15. High-Speed Circuits & Systems Laboratory 15 Advanced Techniques [2] 2011-1 Special Topics in Optical Communications  Input (current) Buffer Stage Common-gate TIA (CG-TIA)Regulated-cascode TIA (RGC-TIA) R in Low input impedance  a) higher input noise current b) Relatively low transimpedance gain  G = R D // (R F /(A+1))

16. High-Speed Circuits & Systems Laboratory 16 Advanced Techniques [3] 2011-1 Special Topics in Optical Communications  Current-mode Transimpedance Amplifier (CM-TIA) R in Low input impedance  a) Higher input noise current b) Low transimpedance gain @ low supply voltage  G = R out,M6 // R out,M5

17. High-Speed Circuits & Systems Laboratory 17 Receiver Front-End Integration 2011-1 Special Topics in Optical Communications  Photodetector and electronic circuits should be integrated on a single chip for optical interconnect applications.  Responsivity  Transimpedance gain [to meet limiting amplifier input sensitivity]  Pole frequency - Transit effect - RC time constant  TIA Input impedance [to avoid RC time effect]  EQ gain [to compensate limited bandwidth]  Noise  Low input-referred circuit noise [to achieve high SNR at TIA input node] Photodetector

18. High-Speed Circuits & Systems Laboratory 18 Recent Research Topics 2011-1 Special Topics in Optical Communications  “Receiver-less approach” Electronic circuit - Low input MOSFET capacitance - Low input sensitivity Photodetector - High responsivity - Low junction capacitance (~ fF)  Avalanche photodetector (> p-i-n photodetector)  Power consumption of optical system should be minimized to take over a substantial fraction of interconnect applications. Requirements Buffer (inverter) chain

19. High-Speed Circuits & Systems Laboratory 19 Receiver-Less Approach: Example 2011-1 Special Topics in Optical Communications ** Avalanche photodetector ** (S.Assefa et al., Nature, 2010) (1)R = 0.15 A/W  I IN = 0.3 mA pp @ 0 dBm (ideal extinction ratio) (2) C pd = 10 fF  RC time constant High bandwidth (f p = 15.9 GHz) High sensitivity (V o,pp = 300 mV pp ) Low noise (= 1kΩ)

20. High-Speed Circuits & Systems Laboratory 20 Conclusion 2011-1 Special Topics in Optical Communications  Receiver System for Optical Interconnects  Current-Voltage Conversion  Resistor termination  Transimpedance amplifier (TIA)  TIA design considerations & methods  Advanced (Wideband) techniques  Shunt-peaking  Common-gate & regulated cascode  Recent research topics  Receiver-less approach !!

21. High-Speed Circuits & Systems Laboratory 21 2011-1 Special Topics in Optical Communications Thank you for listening !! Jin-Sung Youn (jsyoun@tera.yonsei.ac.kr)jsyoun@tera.yonsei.ac.kr High-Speed Circuits & Systems Laboratory