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Transimpedance Amplifier (TIA) with Coupled Inductor in the Feedback Path for Bandwidth Enhancement

IP.com Disclosure Number: IPCOM000016006D
Original Publication Date: 2002-Aug-27
Included in the Prior Art Database: 2003-Jun-21
Document File: 5 page(s) / 97K

Publishing Venue

IBM

Abstract

Abstract This proposal describes a new method for broadening the bandwidth of a shunt-shunt feedback-type transimpedance amplifier by applying a coupled inductor in the feedback path. The inductive coupling occurs between the inductor in the feedback path and an inductor used for shunt or series peaking [1] (in the following series peaking is assumed). Introduction The block diagram of a typical optical receiver front-end is shown in Fig. 1. It consists of a reverse biased photodiode (PD), a transimpedance amplifier (TIA), a limiting amplifier (LA) and a clock and data recovery (CDR) circuit with decision (DEC) unit. The PD delivers an output current that is proportional to the optic power. The TIA acts as current-to-voltage converter and the LA receives the small voltage signal from the TIA and amplifies it to a level sufficient for a reliable operation of the CDR circuit.

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  Transimpedance Amplifier (TIA) with Coupled Inductor in the Feedback Path for Bandwidth Enhancement

Abstract

This proposal describes a new method for broadening the bandwidth of a shunt-shunt feedback-type transimpedance amplifier by applying a coupled inductor in the feedback path. The inductive coupling occurs between the inductor in the feedback path and an inductor used for shunt or series peaking [1] (in the following series peaking is assumed).

Introduction

The block diagram of a typical optical receiver front-end is shown in Fig. 1. It consists of a reverse biased photodiode (PD), a transimpedance amplifier (TIA), a limiting amplifier (LA) and a clock and data recovery (CDR) circuit with decision (DEC) unit. The PD delivers an output current that is proportional to the optic power. The TIA acts as current-to-voltage converter and the LA receives the small voltage signal from the TIA and amplifies it to a level sufficient for a reliable operation of the CDR circuit.

The main requirements for a TIA include: - low input impedance - maximum transimpedance gain and sensitivity for a given bandwidth (or vice versa: maximize the bandwidth for a given transimpedance gain) - wide dynamic range - provide the PD with sufficient reverse bias if TIA and PD are not AC-coupled

The bandwidth of a TIA is typically chosen to be equal to 0.7 times the bit rate. This is a reasonable compromise between the total integrated noise and intersymbol interference (ISI) resulting from limited bandwidth.

Basic TIA Topology

The topology of a shunt-shunt feedback TIA is depicted in Fig. 2. The closed loop transimpedance gain Gtrans and the input impedance Zin are given by [2]

                trans
==20log()20logGZutrans i , (1)

()inL

-

R

Z

f

trans

C

R

  ⋅


s

+⋅+11inf( ) outout

 

sC

R


A

, (2)

1

Page 2 of 5


R

C

R

1

. (3)

The bandwidth of the TIA is specified by the first pole of Gtrans and is given by

in


s


f

out

out

out

+

R

R

++≈()





f


Z

R

R


in

f

out

C

R

  ⋅

+⋅+11inf( ) outout

 

sC

R


A

BW

A

π

2

R

f C

k

M

=

, (5)

where M denotes the mutual inductance. The indication of direction of the magnetic flux through Lf (indicated by the point next to the inductor symbol) is chosen such that the induced voltage caused by the mutual inductance is directed contrary to the voltage drop across the inductor due to the self-inductance. The total voltage across Lf is thus given by

L) dt

di

di

   t fU s f
=Lf -

(

M


dt

.

(6)

In an MMIC design the described inductive coupling can be obtained by two coupled spiral inductors that show opposite sense of direction of their windings (see below).

2

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The series inductor Ls separates the load capacitance Cout by the output capacitance of the CS stage. It performs series peaking, which is a well-known technique for bandwidth extension [1]. The basic idea of the peaking technique is to defer the current flow in...