Search

EP-3769417-B1 - DIFFERENTIAL TRANSIMPEDANCE AMPLIFIER

EP3769417B1EP 3769417 B1EP3769417 B1EP 3769417B1EP-3769417-B1

Inventors

  • LAMBRECHT, Joris
  • RAMON, Hannes
  • MOENECLAEY, Bart
  • YIN, XIN

Dates

Publication Date
20260506
Application Date
20190320

Claims (12)

  1. A transimpedance amplifier (100) for converting a current between its two input terminals (132, 134), wherein one of the input terminals is an inverting input terminal (134, VinN) and the other one is a non-inverting input terminal (132, VinP), to a voltage over its two output terminals (112, 114), the transimpedance amplifier comprising a voltage amplifier (110) wherein the output terminals (112, 114) of the transimpedance amplifier (100) are corresponding with output terminals of the voltage amplifier (110), a differential DC-coupled feedback network (120), an input biasing network (130), and a level shifter (140) which is transparent for alternating voltages, wherein the level shifter (140) is configured for creating a tunable difference in DC voltage between the input terminals (132, 134) for reverse biasing a photodiode or multiple photodiodes connected to at least one of the input terminals (132, 134), wherein the input biasing network comprises two current sources per input terminal, one configured for sourcing current to the photodiode or multiple photodiodes and one configured for sinking current from the photodiode or multiple photodiodes, wherein a current needed for creating the tunable difference in DC voltage between the input terminals is provided by the input biasing network, and wherein the input biasing network is transparent for a feedback signal from the feedback network (120), wherein the feedback network (120) is differentially and DC-coupled with the output terminals (112, 114) of the voltage amplifier (110) and outputs of the feedback network are differentially and DC-coupled with the input biasing network (130), wherein the feedback network comprises a first feedback network which is configured to set a common mode voltage on the output terminals (112, 114) of the voltage amplifier (110) to a reference value and to set a differential output voltage between the output terminals (112, 114) of the voltage amplifier (110) to zero in DC, by tuning one of the current sources per input terminal, and wherein the feedback network comprises a second feedback network which is configured for defining a transfer function of the transimpedance amplifier, and wherein outputs of the input biasing network (130) are coupled with inputs of the level shifter (140) and the level shifter is differentially and DC-coupled with input terminals of the voltage amplifier (110).
  2. A transimpedance amplifier (100) according to claim 1 wherein the level shifter (140) comprises a current source and a parallel RC chain which are configured for generating a DC voltage between the input terminals of the voltage amplifier.
  3. A transimpedance amplifier (100) according to any of the previous claims wherein the feedback network (120) is an active feedback network.
  4. An optical receiver comprising a transimpedance amplifier (100) according to any of the claims 1 to 3, and at least one photodiode wherein the at least one photodiode is connected to at least one of the input terminals of the transimpedance amplifier (100).
  5. An optical receiver according to claim 4 wherein the photodiode is connected to both input terminals of the transimpedance amplifier (100).
  6. An optical receiver according to claim 4 wherein the photodiode is connected to the inverting or non-inverting input terminal of the transimpedance amplifier (100).
  7. An optical receiver according to claim 4 wherein one photodiode is connected to the inverting input terminal of the transimpedance amplifier and another photodiode is connected to the non-inverting input terminal of the transimpedance amplifier.
  8. An optical receiver according to claim 4 wherein one photodiode is connected with its anode to the inverting input terminal of the transimpedance amplifier and another photodiode is connected with its cathode to the same inverting input terminal of the transimpedance amplifier.
  9. A datacenter optical link (300) comprising an optical receiver (100) in accordance with any of the claims 4 to 8.
  10. A passive optical network comprising an optical receiver in accordance with any of the claims 4 to 8.
  11. A coherent optical receiver (200) comprising an optical receiver in accordance with any of the claims 4 to 8.
  12. An equalizer comprising a transimpedance amplifier (100) according to any of the claims 1 to 4, and a plurality of photodiodes arranged in a first array (310) connected to the non-inverting input terminal of the transimpedance amplifier (100) and in a second array (320) connected with the inverting input terminal of the transimpedance amplifier (100).

Description

Field of the invention The present invention relates to the field of transimpedance amplifiers. More specifically it relates to transimpedance amplifiers which can be deployed in datacenter applications as well as in coherent links. Background of the invention In high-speed transimpedance amplifier (TIA) design for broadband optical communications, the ever-increasing demand for more bandwidth and less power consumption forces design trade-offs, typically reducing gain and sacrificing noise performance. The best noise performance has traditionally been obtained with shunt-feedback (SFB) transimpedance amplifiers (TIA). Other topologies, such as the common gate TIA, the regulated cascade (RGC) TIA or invertor TIAs in CMOS, promise advantages - such as a higher bandwidth, a larger obtainable transimpedance or strongly reduced complexity and power consumption - but at the cost of a large noise penalty, poor power supply rejection (PSRR), and less tunability. For higher-end applications, shunt-feedback (SFB) TIAs are most often the topology of choice (see the schematic drawing in FIG. 1). Even with the SFB topology, there is a limit on the combination of transimpedance (noise) and bandwidth that can be obtained, as given by the transimpedance limit (Broadband circuits of optical fiber communication, E.Sackinger, ISBN: 9780471712336, p. 117). RT≤A.fA2πCTBW3dB2 In this equation RT is the effective transimpedance, A is the open loop gain of the error voltage amplifier, CT denotes the total input capacitance and BW3dB is the total closed loop bandwidth of the TIA stage. According to this limit, doubling the bandwidth forces a reduction of RT by a factor of four, severely degrading noise performance and reducing gain. Finding ways to go past this limit proves very challenging. Each new generation of high-speed TIAs needs a larger bandwidth to comply with increasing data rate specifications. Therefore, the input-referred noise current continues to grow. To ensure sufficient SNR, the average input signal amplitude needs to be increased. Since photodiodes are optical power detectors and provide a non-balanced output, high-speed TIAs must be able to sink or source a larger average photocurrent to have a reasonable dynamic range. High-speed optical transmitters for short-reach optical links (datacenter, e.g. EAMs) typically have a relatively low extinction ratio (e.g. 6 dB or less), further increasing the average photodiode (PD) current produced to obtain the same modulated (signal) current. The average PD current cannot be neglected when sufficient dynamic range is required, and relatively strong signals need to be processed. Transimpedance amplifiers employing feedback, such as the SFB TIA, are exposed to stability risks due to non-zero parasitic impedances to ground (e.g. emitter inductance) or supply (excessive inductive peaking or resistance, lowering the phase margin), or due to coupling between ground or supply bondwires from the same or different channels (Design of Integrated Circuits for Optical Communications, B.Razavi, p. 102). Most often, PSRR or substrate noise rejection is not good, unless e.g. an on-chip LDO (low-dropout) voltage regulator and/or dummy structures are used (extra design effort, power consumption, area). In a multichannel receiver, numerous layout measures need to be taken to limit channel-to-channel crosstalk from occurring through either substrate, power supply or ground. For example: using starconnections to provide supply and ground to channels, use isolation barriers (e.g. in BiCMOS: deep trench isolation), extensive use of guard rings and triple wells, etc. US20170338782 and US20140097331 relate to feedback transimpedance amplifiers. In US20140097331 a light reception circuit includes a direct current (DC) level shift circuit that shifts a DC voltage level of a first signal or a second signal and outputs a third signal or a fourth signal, so that a DC voltage level of the first signal output from a cathode of a photodiode that generates a signal by photo conversion and a DC voltage level of the second signal output from an anode of the photodiode agree. In US20140097331 the level shifter is in front of a TIA. This TIA comprises the feedback path which is transparent for alternating voltages and consists of resistors which close the loop between the output and the input of the amplifier. Thus, in US20140097331 the impedance of the level shifting circuit of D1 must be much lower that the impedance of the TIA. US20120281991, US20140145789, and JP2011171812 relate to differential transimpedance amplifiers and illustrate ways of connect photodiodes to the differential transimpedance amplifiers. US8872585B2 discloses an optical receiver which includes two transimpedance amplifiers. The optical receiver, furthermore, comprises a level detector to detect an average level between outputs of the transimpedance amplifiers, a controller to detect the difference between the outputs of the transimp