A Linear Differential Transimpedance Amplifier for 100-Gb/s Integrated Coherent Optical Fiber Receivers

This paper presents the design and measurements of a 32-Gb/s differential-input differential-output transimpedance amplifier (TIA) employed in dual polarization integrated coherent receivers for 100-Gb Ethernet. A circuit technique is shown that uses a replica TIA to stabilize the operating point of the two shunt-feedback input stages as well as to cancel the dc part of the two complementary input currents and balances their offset. The TIA can be operated in two modes, an automatic gain control mode to retain a good total harmonic distortion (THD) over a wide dynamic range and a manual gain control mode. Electrical as well as optical-electrical characterization of the TIA are presented. It achieves a maximum differential transimpedance of 74 dB
$\Omega $
, 33 GHz of 3-dB bandwidth, 12.2 pA/
$\sqrt {\text {Hz}}$
of average input-referred noise current density with the photodiode, 900 mV
_pp
of maximum differential output swing, less than 1% of THD for 600 mV
_pp
differential output swing, and 500
$\mu \text{A}_{\text {pp}}$
differential input current. The linearity of the TIA is furthermore demonstrated with PAM4 measurements at 25 Gbaud. The dual TIA chip is fabricated in a 0.13-
$\mu \text{m}$
SiGe:C BiCMOS technology, dissipates 436 mW of power and occupies 2 mm
²
of area.