TRANSIMPEDANCE AMPLIFIER CIRCUIT FOR OPTICAL RECEIVER IN OPTICAL COMMUNICATION SYSTEM
A transimpedance amplifier circuit for an optical receiver in an optical communication system in which a range of an increase/decrease in bandwidth according to gain change is reduced by a bandwidth adjustor. The circuit includes: a photodiode (PD) for generating a current signal by photoelectric conversion of an input optical signal; a Transimpedance Amplifier (TIA) for converting the current signal input from the photodiode into a voltage signal; an auto gain adjustor for adjusting feedback resistances of the transimpedance amplifier; a photodiode parallel capacitor reducer for reducing a parallel capacitor current of the photodiode; and a bandwidth adjustor for reducing a range of an increase/decrease in bandwidth according to an increase/decrease in gain of the transimpedance amplifier.
1 . A transimpedance amplifier circuit for an optical receiver in an optical communication system, comprising:
a photodiode (PD) for receiving an input optical signal, and for generating a current signal by photoelectric conversion of the input optical signal;
a transimpedance amplifier (TIA) for converting the current signal input from the PD into a voltage signal;
an auto gain adjustor for adjusting a feedback resistance of the TIA;
a photodiode parallel capacitor reducer for reducing a parallel capacitor current of the PD; and
a bandwidth adjustor for reducing a range of an increase/decrease in bandwidth according to an increase/decrease in gain of the TIA.
2 . The transimpedance amplifier circuit as claimed in claim 1 , wherein the auto gain adjustor comprises:
a bottom-hold circuit connected to an output stage of the transimpedance amplifier and detecting a low level of an output signal of the transimpedance amplifier;
a comparator having two input stages, an inverted (−) input stage which receives an output signal of the bottom-hold circuit, and a non-inverted (+) input stage which receives a reference voltage Vref, and for comparing input signals of the two input stages;
a resistance R 4 and a resistance R 5 connected in parallel to an input and an output of the transimpedance amplifier, respectively; and
a transistor TR 6 connected in series to resistance R 4 and for performing a switching operation of the resistance R 4 according to an output signal of the comparator.
3 . The transimpedance amplifier circuit as claimed in claim 2 , wherein the reference voltage comprises a predetermined comparing voltage for determining by the comparator whether an input optical signal is overloaded.
4 . The transimpedance amplifier circuit as claimed in claim 2 , wherein:
when an output signal of the comparator comprises a ‘Logic High’, the sixth transistor is connected, and
when the output signal of the comparator comprises a ‘Logic Low’, the sixth transistor is not connected.
5 . The transimpedance amplifier circuit as claimed in claim 1 , wherein the photodiode parallel capacitor reducer comprises:
a capacitor (C 1 ) connected in series with an anode terminal of the PD;
a resistance (R 3 ) connected in series with the capacitor C 1 ; and
a transistor (TR 4 ) whose collector terminal is connected to a power source, whose base terminal is connected to between the capacitor C 1 and the resistance R 3 , and whose emitter terminal is connected to a cathode terminal of the PD.
6 . The transimpedance amplifier circuit as claimed in claim 5 , wherein:
said PD having two ends and comprising a current source portion ipd and a capacitive portion C 2 being parallel to the current source portion, wherein a current flowing through the parallel capacitor C 2 of the PD is proportional to a voltage difference between both ends of the PD; and
means for reducing a voltage difference between both ends of the PD as a frequency increases, and for reducing the current flowing through the parallel capacitive portion C 2 of the PD.
7 . The transimpedance amplifier circuit as claimed in claim 1 , wherein the bandwidth adjustor comprises:
a transistor Tr 1 for switching according to the output signal of the comparator; and
a current mirror circuit comprising transistors TR 2 , TR 3 and TR 5 .
8 . The transimpedance amplifier circuit as claimed in claim 7 , wherein an input/output transfer function of the transimpedance amplifier circuit is defined by the following equation,
Gain
=
V
out
i
pd
=
A
·
R
f
(
1
+
A
)
+
sC
2
·
[
R
f
+
(
1
+
A
)
·
R
o
]
,
wherein A is −(Vout/Vin), s is a complex frequency (i.e. s=jw), w=2πf, Rf is a total feedback resistance value of the transimpedance amplifier, and Ro is a resistance value of a resistance Ro of an equivalent circuit.
9 . The transimpedance amplifier circuit as claimed in claim 8 , wherein a pole frequency below 3 dB, is defined by the following equation,
f
3
dB
=
1
+
A
2
·
π
·
C
2
·
[
R
f
+
(
1
+
A
)
·
R
o
]
,
wherein A is −(Vout/Vin), s is a complex frequency (i.e. s=jw), w=2πf, Rf is a total feedback resistance value of the transimpedance amplifier, and Ro is a resistance value of a resistance Ro of an equivalent circuit shown in FIG. 4 .
10 . The transimpedance amplifier circuit as claimed in claim 9 , wherein Ro is defined by the following equation,
(
R
o
=
1
g
m
+
g
mb
≈
1
g
m
g
m
=
2
·
k
′
·
(
W
L
)
4
·
I
4
)
,
wherein k′ is u*Cox, u is a mobility, Cox is a capacitance of a gate oxide film, W is a channel width of a transistor TR 4 , and L is a channel length of TR 4 .
11 . The transimpedance amplifier circuit as claimed in claim 10 , wherein the transimpedance amplifier circuit for changing Ro for reducing an increase/decrease in bandwidth according to gain change, and for changing a bias current I 4 of transistor TR 4 in order to change Ro.
12 . The transimpedance amplifier circuit as claimed in claim 11 , wherein:
when the output of the comparator is ‘Logic Low’, the bandwidth adjustor adjusts the I 4 to a value twice an Is value of the current mirror circuit; and
when an Output of the comparator is a ‘Logic High’, the bandwidth adjustor adjusts I 4 to have a same value as the Is value.
13 . The transimpedance amplifier circuit as claimed in claim 10 , wherein the transimpedance amplifier circuit changes Ro in order to reduce an increase/decrease in bandwidth according to gain change, a channel width W and a channel length L of the transistor TR 4 changed in order to change Ro.
14 . The transimpedance amplifier circuit as claimed in claim 1 , wherein the bandwidth adjustor comprises:
a current mirror circuit comprising a transistor TR 2 and a transistor TR 3 ;
a resistance R 1 and a resistance R 2 connected in series with each other on a path where a current (Is) of the current mirror circuit flows;
an inverter connected in series with an output stage of the comparator; and
a transistor TR 1 connected in parallel to the resistance R 2 and for performing a switching operation according to an output signal of the comparator through the inverter.
15 . The transimpedance amplifier circuit as claimed in claim 14 , wherein:
when the comparator outputs a ‘Logic High’ signal, the bandwidth adjustor turns off the transistor TR 1 , and the resistance R 1 is connected to R 2 , so that the current Is being reduced to a value lower than when a ‘Logic Low’ signal is output;
when the comparator outputs a ‘Logic Low’ signal, the bandwidth adjustor turns on the transistor TR 1 and current flows through the resistance R 1 and the transistor TR 1 , so that mirror current Is being increased more than an output of a ‘Logic High’ signal; and
a determined Is becomes I 4 by the current mirror circuit.