Electronic Circuits for Optical Systems : Transimpedance Amplifier (TIA)

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

2011-1 Special Topics in Optical Communications Contents • 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

2011-1 Special Topics in Optical Communications Electrical Interconnects • 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

2011-1 Special Topics in Optical Communications Optical Interconnects • Photodetector • Current-voltage (I-V) conversion component / circuits • Direct modulation • - Driving circuits • - VCSEL • External modulation • - Laser • - Modulator • - Driving circuits

2011-1 Special Topics in Optical Communications Requirements for The First Block RF Receiver Front-End Optical Receiver Front-End • (Voltage / Power) Gain • (Channel) Bandwidth • Noise figure • (Transimpedance) Gain • (Broadband) Bandwidth • (Input) Noise current

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

2011-1 Special Topics in Optical Communications Integrated Total Noise • Output Noise Spectrum of Circuit Circuit bandwidth ↑  Integrated total noise ↑

2011-1 Special Topics in Optical Communications Bandwidth vs. Noise • BW = 1.4 fB • No ISI • High noise  • BW = 0.35 fB • High ISI  • Low noise • BW = 0.7 fB • No ISI • Medium Noise * ISI: Inter-Symbol Interference ** fB: data rate

2011-1 Special Topics in Optical Communications Bandwidth vs. Noise • Inter-Symbol Interference (ISI) vs. Noise Optimum bandwidth depends on data rates !!

2011-1 Special Topics in Optical Communications Gain vs. Noise • Signal-to-Noise Ratio (SNR) Noise Ringing Gain Bandwidth Noise

2011-1 Special Topics in Optical Communications Receiver Basics - Example ** p-i-n photodetector ** (N.Feng et al., Optics Express, 2010) Low impedance High impedance 1kΩ • High bandwidth • (fp = 15.9 GHz) •  Low sensitivity • (Vo,pp = 9 mVpp) • High noise •  Small bandwidth • (fp = 0.8 GHz) • High sensitivity • (Vo,pp = 1.8 Vpp) • Low noise • R = 0.9 A/W (TM polarization) •  IIN = 1.8 mApp @ 0 dBm • (ideal extinction ratio) • (2) Cpd = 200 fF •  RC time constant

2011-1 Special Topics in Optical Communications Transimpedance Amplifier (TIA) • (Shunt-Shunt) Feedback Amplifier • - Low input impedance & High transimpedance !! • (Transimpedance) Gain • (Broadband) Bandwidth • (Input) Noise current Rin A

2011-1 Special Topics in Optical Communications Comparison But, RF ↑  Rin ↑

2011-1 Special Topics in Optical Communications Advanced Techniques [1] • Shunt Peaking • Inductor implementation : • Spiral inductor • Active inductor • (a) Large power consumption & chip area • (b) PVT variation

2011-1 Special Topics in Optical Communications Advanced Techniques [2] • Input (current) Buffer Stage Common-gate TIA (CG-TIA) Regulated-cascode TIA (RGC-TIA) Rin Rin • Low input impedance • a) higher input noise current • b) Relatively low transimpedance gain  G = RD// (RF/(A+1))

2011-1 Special Topics in Optical Communications Advanced Techniques [3] • Current-mode Transimpedance Amplifier (CM-TIA) Rin • Low input impedance • a) Higher input noise current • b) Low transimpedance gain @ low supply voltage •  G = Rout,M6 // Rout,M5

2011-1 Special Topics in Optical Communications Receiver Front-End Integration • Photodetector and electronic circuits should be integrated on a single chip for optical interconnect applications. Photodetector • 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]

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

2011-1 Special Topics in Optical Communications Receiver-Less Approach: Example ** Avalanche photodetector ** (S.Assefa et al., Nature, 2010) (= 1kΩ) • R = 0.15 A/W •  IIN = 0.3 mApp @ 0 dBm • (ideal extinction ratio) • (2) Cpd = 10 fF •  RC time constant • High bandwidth (fp = 15.9 GHz) • High sensitivity (Vo,pp = 300 mVpp) • Low noise

2011-1 Special Topics in Optical Communications Conclusion • 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 !!

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

Electronic Circuits for Optical Systems : Transimpedance Amplifier (TIA)