Download A Low Noise Transimpedance Preamplifier for Fiber Optics

A Low Noise Transimpedance Preamplifier for
Fiber Optics Applications.
Alejandro Díaz Sánchez, Jaime Martínez-Castillo, Guillermo Espinosa Flores-Verdad
National Institute for Astrophysics, Optics y Electronics.
Luis Enrique Erro # 1. Tonantzintla, Puebla, México.
E-mail: [email protected] and [email protected].
ABSTRACT.
A fully CMOS transimpedance preamplifier for
fiber optics receiver is presented. It is based on the
common-gate topology, with negative feedback to
increase the bandwidth of the circuit. With a
bandwidth of 2.8 GHz an important noise reduction
was achieved. Intermodulation products of 42 dB
below the carrier were obtained. Simulations were
performed using HSPICE CMOS level 53 models
for a 0.35 µm AMS process.
INTRODUCTION
Communication systems based on fiber optics data
transmission have an important role nowadays. Due
to that applications that systems have increased
their bandwidth in the range from 1 to 5 GHz.
The input stage of those receivers requires fulfilling
though requirements, such as high gain, low noise
figure and wide bandwidth. Therefore its design is
one of the most important issues in fiber optic
communication systems.
Since any direct-detection fiber optic receiver uses a
photodiode as input device, transimpedance
amplifiers are commonly used due its better
insensitivity
to
the
photodiode-associated
capacitance. However, new data transmission speed
requirements have lead to a use optimization in the
tradeoff low-noise/wide-bandwidth of the input
stage.
Several input stages for fiber optic receivers based
on bipolar transistors have been reported in recent
literature. Some of these implementations have
used the
shunt-series feedback over several
common emitter stages to increase the
transimpedance amplifier bandwidth [2, 3].
However, that structure presents peaks in its
frequency response, which affect the reception
process [8, 9]. Darlington topologies with shuntshunt feedback have been also used [4], with some
improvement
in
the
frequency
response
characteristics. Some other structures reported use
an AGC to increase the dynamic range of the
receiver [5, 6].
Despite MOS transistors amplifiers have not a
performance good in high frequencies, compared
with bipolar transistor based amplifiers, but their
low cost, low power consumption and small silicon
area required have motivated some research
nowadays [1]. A fully MOS implementation was
reported by Park [7], using common gate and
regulated cascode implementations to increase the
bandwidth and decrease the input impedance.
The work present describes a fully CMOS
transimpedance preamplifier for fiber optic
applications. A negative feedback is used at the
input transistor to increase the bandwidth and
reduce the noise levels. The section II presents the
theoretical considerations and the effect of that
feedback in the preamplifier characteristics. Section
III describes the simulated results, while section IV
will present the conclusions.
TRANSIMPEDANCE PREAMPLIFIER
The basic diagram of a transimpedance amplifier
with negative feedback, which is commonly used in
fiber optic receivers, is shown in figure 1. The
transimpedance, ZT, is given by:
ZT =
ZF

Z 
1 + F 
Zi 
1+ 
A
(1)
Where Zi is the input impedance, ZF is the feedback
impedance and A is the open-loop gain of the
amplifier. When the open loop gain A is large, the
transimpedance is approximately equal to:
vo
≈ ZF
is
(2)
The amplifier bandwidth f-3
equation:
f −3dB =
dB
is defined by the
1
2 * Z in * (c s + cin )
(3)
Some bandwidth increasing can be achieved using a
local feedback net as shown in figure 3. The biasing
of the transimpedance preamplifier is implemented
using a gain stage. Now, the structure shows from
equation (6), what there is an increase in the
dominant polo due a the negative feedback
Where cs is the photodiode capacitance and cin is the
input capacitance of the amplifier. The equivalent
input noise source In is given by:

 4 kT ∆ f
1
1 
I n =  w (c s + c in ) +
+
 * E in + 
Z
Z
F
in
 RF





(4)
The proposed preamplifier has a common-gate
topology, with capacitive coupling, as shown in
figure 2 [8]. Its transfer function is given by the
expression:
vout
gm + gds
=
iS S2c2c1 + S c2(gds + gm) +c1 gf + gds + gf (gds + gm)
{
(
)}
gm
c1
and
S p2 =
gf
c2
Z in
(6)
gm
2 π * (C S + C gs )
If S → 0 the gain of the transimpedance amplifier
is given by:
[
]
g m1 g m 2 g m 3 + g f 2 g L1
vout
=
is
g f 1 g m1 g f 2 g L1 + g m 2 g m 3 + g L1 g f 2 g m 2
{ [
]
}
v out
1
1
*
=
is
g f1 

g L1 g f 2 g m 2
1 +

 g m1 g m 2 g m 3 + g f 2 g L1 
[
if
[
]
g L1 g f 2 g m 2
g m1 g m 2 g m 3 + g f 2 g L1
amplifier is given by
(12)
]
< < 1 the gain of the
vout
≈ R f 1 from equation (2).
is
The negative feedback also reduces the total noise
of the circuit, which is now given by the equation:
2
iint
=
[(
)]
 S c + c (A +1) + A c 2 i2 +
S
gs
gd
c

*
2 
gm (A +1) − AScgd  + S c + c (A +1) + g ( A +1 ) 2 i2
S
gs
m
Rf
î
[
1
]
[(
)
]





(13)
SIMULATED RESULTS
(8)
and from equation (2) the gain of the amplifier is
given by:
v out
≅ RF
iS
If k = 0, the obtained results are similar to the
obtained for the simple common gate amplifier
case.
(7)
From equation (3) we obtain
f −3dB =
(10)
(11)
Because rds is large, we assume gds << gm. On the
other side, C in = C1 is input capacitance of M1, and
the input impedance is given by:
1
=
gm
(k + 1)g m
C S + (k + 1)C gs
(5)
Where C1=Cs+Cgs is the preamplifier input
capacitance, and Cgs is the transistor M1 source
capacitance. The output capacitance can be defined
as C2=Cgd+Cdb+Cp1, where Cgd and Cdb are the
gate-drain and drain-bulk capacitances of transistor
M1, and Cp1 is the coupling capacitance to the next
stage. The transfer function in (5) has two real poles
at:
S p1 =
S p1 = −
(9)
The circuit was simulated using CMOS level 53
models in HSPICE, and a transimpedance gain of
40 dB was obtained with an RF of 100 Ω, the output
capacitance was 0.1 pf and the estimated
photodiode capacitance value was 0.5 pf.
Figures 4 and 5 shown the transfer function and the
input equivalent noise of the modified common gate
preamplifier, respectively.
In table 1 a comparison of the high frequency
characteristics of both amplifiers is presented, while
figure 6 shows the results of the signal
intermodulation distortion in the modified common
gate structure. The tests signals were 1.1 and 1.2
GHz. In table 2 the intermodulation products and
harmonic distortion are presented. A single power
supply of 3 volts was used.
[8]
J. Martínez-Castillo and J. Silva-Martínez,
“Transimpedance Amplifiers for Optical Fiber
Systems Based on Common-Base Transistors,”
IEEE ISCAS, July 1999.
[9]
M. Vadipour, “Capacitive Feedback Technique
for Wide-Band Amplifiers,” IEEE J. Solid-State
Circuits, vol. 28, no. 1, January 1993.
IV. Conclusions
A design of a new topology of transimpedance
amplifier for fiber optic receivers was shown. Two
transimpedance amplifiers have been compared.
The best transimpedance amplifier was the
modified common-gate structure. This structure
improves bandwidth, noise level reduction, and
performance in dynamic range.
Structure f(-3dB) Peak Current
GHz dB
mA
CG
1.29
0
3.5
CG-MOD 2.80 1.5
5.5
Table 1. High-frequency characteristics.
ZF
V. References
[1]
M. Ohara, Y. Akazawa, N. Ishihara and S.
Konaka, “Bipolar Monolithic Amplifier for a
Gigabit Optical Repeater”, IEEE J. Solid-State
Circuits, vol. sc-19, no. 4, August 1984.
[3]
R. G. Meyer and R. A. Blauschild, “A WideBand Low-Noise Monolithic Transimpedance
Amplifier,” IEEE J. Solid-State Circuits, vol.
sc-21, no. 4, August 1986.
[4]
Chris T. Armijo and R. G. Meyer, “A New
Wide-Band Darlington Amplifier”, IEEE J.
Solid-State Circuits, vol. 24, no. 4, August
1989.
[5]
[6]
[7]
Amplificador
Y. Suzuki, T. Suzaki, Y. Ogawa, S. Fujita, W.
Liu and A. Okamoto, “Pseudomorphic 2DEG
FET IC´s for 10-Gb/s Optical Communication
Systems with External Optical Modulation”,
IEEE J. Solid-State Circuits, vol. 27, no. 10,
October 1992.
[2]
is
vo
-A
Zi
Figure 1. Classical transimpedance preamplifier for fiber
optic receivers applications.
VDD
RF
K. Yamashita, T. Kinoshita, Y. Takasaki, M.
Maeda, T. Kaji and N. Maeda, “A Variable
Transimpedance Preamplifier for use in Wide
Dynamic Range Optical Receivers”, IEEE J.
Solid-State Circuits, sc-16, no. 2, April 1986.
R. G. Meyer, and W. D. Mack, “A Wideband
Low-Noise
Variable-Gain
BiCMOS
Transimpedance Amplifier”, IEEE J. SolidState Circuits, vol. 29, no. 6, June 1994.
S.M. Park and C. Toumazou, “Low Noise
Current-Mode
CMOS
Transimpedance
Amplifier
for
Giga-Bit
Optical
Communication,” IEEE ISCAS 1998.
Level Noise ( pA/rt Hz)
1 GHz
1.6 GHz 3.2 GHz
28.1
32.5
53.9
18.7
52.0
20.3
M2
v 01
M1
O
Vbias
ii
CS
IB
IB
v
0
CL
VSS
Figure 2. Common-gate transimpedance amplifier for
capacitive sources.
VDD
Rf1
VSS
IBias2
M1
vout
VDD
VSS
VSS
VSS
VSS
Rf2
M3
M2
iS
cS
VSS
IBias1
VBias
VSS
Figure 3. Modified
common-gate
transimpedance amplifiers.
structure
for
Figure 6. Harmonic distortion and intermodulation
products for the modified common-gate
structure.
Figure 4. Magnitude response for the transimpedance
amplifiers (40 dB dc gain).
IM2
w2-w1
1.00E+08
IM3
F
F
IM3
2w1-w2
w1
w2
2w2-w1
1.00E+09
1.10E+09
1.20E+09
1.30E+09
HD2
IM2
HD2
2w1
w2+w1
2w2
2.20E+09
2.30E+09
2.40E+09
HD3
IM3
IM3
HD3
3w1
2w1+w2
2w2+w1
3w2
3.30E+09
3.40E+09
3.50E+09
3.60E+09
Table 2. Results for the intermodulation products and
harmonics distortions.
Figure 5. Equivalent input noise for the transimpedance
amplifier.