Coherent light generators – Particular component circuitry – For driving or controlling laser
Reexamination Certificate
2002-08-26
2004-05-11
Ip, Paul (Department: 2828)
Coherent light generators
Particular component circuitry
For driving or controlling laser
C372S038040, C372S038070
Reexamination Certificate
active
06735228
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to a driving control circuit for a light-emitting device. In particular, the present invention relates to a circuit to control the range of a driving current of a light-emitting device.
2. Description of the Related Art
In the present driving circuit of the light-emitting device, the bias current and the modulated current are controlled separately. The modulated current is provided by a differential output stage. The bias current is provided by a circuit source.
FIG. 1
is a circuit diagram of a conventional driving control circuit for a light-emitting device. In
FIG. 1
, the conventional driving control circuit drives a laser diode LD
100
. This circuit includes a pair of input terminals
100
and
101
, and a differential output circuit
102
to be applied with a digital data signal to drive the laser diode LD
100
, a current source circuit
105
for supplying a constant current I
101
as a driving current to the laser diode LD
100
through the differential output circuit
102
, and a current source circuit
106
for supplying a constant current I
102
as a dc bias current to the laser diode LD
100
.
The differential output circuit
102
is formed by n-channel MOS transistors Q
101
and Q
102
whose sources are coupled together, such as by a source-coupled pair of MOS transistors Q
101
and Q
102
. The digital data signal is applied across the non-inverted input terminal
100
and the inverted input terminal
101
. The terminal
100
is connected to a gate of the MOS transistor Q
102
. The terminal
101
is connected to a gate of the MOS transistor Q
101
. A drain of the transistor Q
101
is connected to one end of a load resistor R
101
. The other end of the resistor R
101
is applied with a power supply voltage V
DD
. A drain of the transistor Q
102
, which serves as an output terminal of the conventional driving control circuit, is connected to the cathode of the laser diode LD
100
. The anode of the laser diode LD
100
is applied with the power supply voltage V
DD
.
The current source circuit
105
is formed by n-channel MOS transistors Q
103
and Q
104
serving as a current mirror, and a reference current source
103
for supplying a constant reference current I
ref1
to the transistor Q
104
. A drain of the transistor Q
103
is connected to the coupled sources of the transistors Q
101
and Q
102
. A source of the transistor Q
103
is connected to the ground. A drain and a gate of the transistor Q
104
are connected in common to a gate of the transistor Q
103
. A source of the transistor Q
104
are connected to the ground. The commonly connected drain and gate of the transistor Q
104
are connected to one end of the reference current source
103
. The other end of the reference current source
103
is applied with the power supply voltage V
DD
.
The current source circuit
106
is formed by n-channel MOS transistors Q
105
and Q
106
serving as a current mirror, and a reference current source
104
for supplying a constant reference current I
ref2
to the transistor Q
106
. A drain of the transistor Q
105
is connected to the drain of the cathode of the laser diode LD
100
, i.e., the output terminal of the conventional driving control circuit of
FIG. 1. A
source of the transistor Q
105
is connected to the ground. A drain and a gate of the transistor Q
106
is connected in common to a gate of the transistor Q
105
. A source of the transistor Q
106
is connected to the ground. The commonly connected drain and gate of the transistor Q
106
are connected to one end of the reference current source
104
. The other end of the reference current source
104
is applied with the power supply voltage V
DD
.
The above-described conventional driving control circuit of
FIG. 1
has the following problems:
The first problem is that the current consumption of the circuit of
FIG. 1
is large, because the dc bias current I
102
is always consumed during operation irrespective of the existence and absence of light emission and because the driving current I
101
.
Second, the work voltage in the output terminal is limited because two transmitters are series in the differential output stage. Therefore, the circuit of
FIG. 1
does not work when the output work voltage is lower than V
DD
. The efficiency of the current is reduced and the application of the circuit becomes more complex.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a driving control circuit for a light-emitting device that has better efficiency and is able to increase the work voltage in the output terminal.
Another object of the present invention is to provide a driving control circuit for a light-emitting device that automatically adjusts a gate voltage of the output transistor according to a predetermined bias voltage, and a modulated current to control the gate voltage precisely, thereby enabling a decrease in the power consumption and an increase in the range of the work voltage in the output terminal.
The inventive circuit comprises an adjustable gain circuit, a current output circuit, and a level control circuit. The adjustable gain circuit receives a digital signal and generates an output voltage controlled by a first control signal and a second control signal. The current output circuit comprises a first transistor. A gate of the first transistor is coupled to the adjustable gain circuit. The adjustable gain circuit adjusts a range of the gate voltage to generate the high-speed modulated current. The level control circuit sets the voltages of the first control signal and the second control signal.
The level control circuit comprises a duplicate circuit, a first negative feedback current and a second negative feedback current. The duplicate circuit receives a high level signal and a low level signal and generates a first output current and a second output current. The first negative feedback current sets the first current equal to a first predetermined current and outputs a voltage to set a voltage of the first control signal. The second negative feedback current sets the second current equal to a second predetermined current and outputs a voltage to set a voltage of the second control signal. The high level signal raises the gate voltage of the first transistor above a high level voltage. The low level signal lowers the gate voltage of the first transistor below a low level voltage. The adjustable range of the gate voltage is between the high level voltage and the low level voltage.
Furthermore, the invention also provides another driving control circuit for a light-emitting device to generate a high-speed modulated current to drive the light-emitting device. The circuit comprises an adjustable gain circuit, a current output circuit and a level control circuit. The adjustable gain circuit receives a digital signal and generates an output voltage controlled by a first control signal and a second control signal. The current output circuit comprises a first transistor. A gate of the first transistor is coupled to the adjustable gain circuit. The adjustable gain circuit adjusts a range of the gate voltage to generate the high-speed modulated current. The level control circuit sets the voltages of the first control signal and the second control signal.
The level control circuit comprises a level detect circuit, a first negative feedback current and a second negative feedback current. The level detect circuit is coupled to the adjustable gain circuit and generates a first output current and a second output current. The first negative feedback current sets the first current equal to a first predetermined current and outputs a voltage to set a voltage of the first control signal. The second negative feedback current sets the second current equal to a second predetermined current and outputs a voltage to set a voltage of the second control signal. The adjustable range of the gate voltage is between a high level voltage and a low level voltage.
REFERENCES:
patent: 5563898 (1996-10-01), Ikeuchi et a
Birch & Stewart Kolasch & Birch, LLP
Industrial Technology Research Institute
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