Coherent light generators – Particular component circuitry – For driving or controlling laser
Reexamination Certificate
2001-07-18
2003-05-13
Ip, Paul (Department: 2828)
Coherent light generators
Particular component circuitry
For driving or controlling laser
C372S038100, C372S038070, C315S20000A, C315S24100S, C315S24100S
Reexamination Certificate
active
06563849
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a drive circuit for a light emitting device.
2. Related Background Art
A conventional drive circuit for driving a laser diode as a light emitting device was constructed in the structure as shown in FIG.
4
. Specifically, the drain terminal of PMOS-FET
41
is connected to the anode side of laser diode
40
with the cathode grounded, to drive the laser diode
40
directly. In the case of CD-R/W, DVD, etc., however, the laser diode
40
and driving IC (PMOS-FET
41
in this case) are spaced several cm or more apart from each other. In this configuration, a line
42
connects the laser diode
40
to the PMOS-FET
41
. Since this line
42
definitely produces an inductance component, peaking and ringing occur because of resonance, which was a serious problem in use of products.
FIG. 5
is a drawing to show the simulation result of the conventional drive circuit for driving the light emitting device.
FIG. 5
shows occurrence of heavy peaking and ringing due to resonance as described above. Efforts have been made heretofore to use wiring materials resisting the resonance, and research has been conducted on a method of interposing a resistor R and a capacitor C in series between bonding pad
43
and the ground shown in FIG.
4
.
SUMMARY OF THE INVENTION
However, extra cost for the wiring materials makes it difficult to decrease the product cost. The method of interposing the resistor R and capacitor C was not a desirable method in view of yield and dispersion, either.
FIG. 6
is a diagram showing an example of a drive circuit for a laser diode by means of a PMOS-FET, and
FIG. 7
a diagram showing an equivalent circuit of the drive circuit shown in FIG.
6
. The result of theoretical computation of resonance constant Q in this circuit will be presented below. In the discussion hereinafter, g
m1
and g
m2
represent mutual conductances, g
d1
a drain conductance, L an inductance, and C capacitances.
g
m1
V
gs1
+
V
1
(
sC
1
+
gd
1
)
+
(
V
1
-
V
out
)
sL
=
0
(
1
)
(
V
out
-
V
1
)
sL
+
V
out
(
gm
2
+
sC
out
)
=
0
(
2
)
From Eq (2), we can derive V
1
as follows.
(
V
out
-
V
1
)
sL
=
-
V
out
(
g
m2
+
sC
out
)
(
3
)
V
out
=−V
out
sL
(
g
m2
+sC
out
)+
V
1
(4)
V
1
=V
out
{1+
sL
(
g
m2
+sC
out
)} (2)′
By substituting (2)′ into (1), we can modify Eq (1) as follows.
g
m1
V
in
+
V
1
(
sC
1
+
gd
1
+
1
sL
)
-
V
out
sL
=
0
(
5
)
g
m1
V
in
+
V
out
{
1
+
sL
(
g
m2
+
sC
out
)
}
(
sC
1
+
gd
1
+
1
sL
)
-
V
out
sL
=
0
(
6
)
V
out
[
{
1
+
sL
(
g
m2
+
sC
out
)
}
(
sC
1
+
gd
1
+
1
sL
)
-
1
sL
]
=
-
g
m1
V
in
(
7
)
Then we obtain V
out
/V
in
as follows.
V
out
V
in
=
-
g
m1
{
1
+
sL
(
g
m2
+
sC
out
)
}
(
sC
1
+
gd
1
+
1
sL
)
=
-
g
m1
sC
1
+
gd
1
+
(
g
m2
+
sC
out
)
(
s
2
LC
1
+
sLgd
1
+
1
)
(
8
)
Assuming g
m2
(=200 mS)>>sC
out
(=5 mS), we obtain the following.
V
out
V
in
=
-
g
m1
sC
1
+
gd
1
+
s
2
LC
1
g
m2
+
sLgd
1
g
m2
+
g
m2
=
-
g
m1
/
LC
1
g
m2
s
2
+
s
(
1
Lg
m2
+
gd
1
C
1
)
+
gd
1
LC
1
g
m2
+
1
LC
1
(
9
)
From this, s, &ohgr;
0
, and Q are derived as follows.
s
=
-
(
1
Lg
m2
+
gd
1
C
1
)
±
1
Lg
m2
+
gd
1
C
1
)
2
-
4
(
gd
1
LC
1
g
m2
+
1
LC
1
)
2
(
10
)
w
0
=
gd
1
LC
1
g
m2
+
1
LC
1
=
gd
1
+
g
m2
LC
1
g
m2
≈
1
LC
1
(
11
)
Q
=
w
0
1
Lg
m2
+
gd
1
C
1
≥
10
(
12
)
When specific parameters are substituted into the result of inequality (12), the Q factor becomes approximately 10. With the Q factor larger than 1 as in this case, there will occur peaking and ringing as shown in FIG.
5
. It is seen from the above result that it is important to set the resonance constant Q at a possible minimum value in order to restrain the peaking and ringing. For suppressing the influence of the inductance L as much as possible to control the value of resonance constant Q to near 1, it is common practice to interpose a resistor in series with L. For example, since a source follower circuit permits an is equivalent resistance to be freely controlled by electric current values, the source follower circuit is also interposed instead of the resistor in certain cases. Let us investigate a configuration incorporating the source follower circuit instead of the resistor.
FIG. 8
is a diagram showing an example of the drive circuit for the laser diode by means of a simple source follower circuit, and
FIG. 9
a diagram showing an equivalent circuit of FIG.
8
. The result of theoretical computation of resonance constant Q in this circuit will be provided below. In the discussion hereinafter, g
m1
and g
m2
represent the mutual conductances, gd
1
the drain conductance, L the inductance, and C the capacitances.
-
V
gs1
g
m1
+
V
1
(
gd
1
+
sC
1
)
+
(
V
1
-
V
out
)
sL
=
0
(
13
)
(
V
out
-
V
1
)
sL
+
V
out
(
g
m2
+
sC
out
)
=
0
(
14
)
Using the relation of V
gs1
=V
in
−V
out
, Eq (13) can be rewritten as follows.
-
(
V
i
n
-
V
out
)
g
m1
+
V
1
(
gd
1
+
sC
1
)
+
(
V
1
-
V
out
)
sL
=
0
,
(
13
)
′
For Eq (14), since s
COUt
≈30 mS at g
m2
≈200 mS and f=1 GHz, we can assume g
m2
>>sC
out
.
(
V
out
-
V
1
)
sL
+
V
out
g
m2
=
0
(
15
)
(
1
sL
+
g
m2
)
V
out
=
V
1
sL
(
16
)
V
1
=
sL
(
1
sL
+
g
m2
)
V
out
=
(
1
+
sLg
m2
)
V
out
,
(
14
)
′
By substituting Eq (14)′ into Eq (13)′, we obtain the following relation.
-
(
V
in
-
V
out
)
g
m1
+
V
out
(
1
+
sLg
m2
)
(
gd
1
+
sC
1
)
+
(
1
+
sLg
m2
)
V
out
-
V
out
sL
=
0
(
17
)
-
V
in
g
m1
+
g
m1
V
out
+
V
out
(
1
+
sLg
m2
)
(
gd
1
+
sC
1
)
+
g
m2
V
out
=
0
(
18
)
Accordingly, V
out
/V
in
can be derived as follows.
V
out
V
in
=
g
m1
g
m1
+
(
1
+
sLg
m2
)
(
gd
1
+
sC
1
)
+
g
m2
=
g
m1
s
2
Lg
m2
C
1
+
s
(
Lg
m2
gd
1
+
C
1
)
+
(
gd
1
+
g
m1
+
g
m2
)
≈
g
m1
/
Lg
m2
C
1
s
2
+
s
(
gd
1
C
1
+
1
Lg
m2
)
+
1
LC
1
+
g
m1
Lg
m2
C
1
(
19
)
From this, s, &ohgr;
0
, and Q are obtained as follows.
s
=
-
(
gd
1
C
1
+
1
Lg
m2
)
±
(
gd
1
C
1
+
1
Lg
m2
)
2
-
4
(
1
LC
1
+
g
m1
Lg
m2
C
1
+
)
2
(
20
)
w
0
=
1
LC
1
+
g
m1
Lg
m2
C
1
(
21
)
Q
=
w
0
gd
1
C
1
+
1
Lg
m2
(
22
)
From the above computation result, the resonance frequency &ohgr;
0
increased a little, but Q itself was not affected at all. Namely, it was found that the resonance constant Q itself did not vary depending upon whether the current source was the common source of PMOS-FET or the common drain circuit of NMOS-FET, and that there was little effect thereby. Since the value of Q itself was unable to be suppressed even by the attempt to control the influence of L by the method of simply interposing the resistor R as described above, it was difficult to restrain the ringing and peaking. Since the number of portions requiring supply of electric current increased in order to solve these issues, it was also difficult to drive the circuit by the low supply voltage of 3.3 V or the like.
U.S. Pat. No. 5,898,334 discloses a method of lowering the parasitic capacitance by means of a single drive source, but this method involves such requirements that the size of MQ
1
has to be small and the gate voltage has to be large. For this reason, it is necessary to use the voltage of 5 V or more, which makes driving at a low supply voltage difficult and poses the problem of heat generation.
The present invention has been accomplished under such circumstances and an object of the invention is to provide a drive circuit for a light emitt
Hamamatsu Photonics K.K.
Ip Paul
Morgan & Lewis & Bockius, LLP
Rodriguez Armando
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