Coherent light generators – Particular component circuitry – Having feedback circuitry
Reexamination Certificate
2002-05-06
2004-03-02
Ip, Paul (Department: 2828)
Coherent light generators
Particular component circuitry
Having feedback circuitry
C372S038020, C372S038100
Reexamination Certificate
active
06700909
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a driving circuit, especially to a light-emitting device driving circuit with an adaptive control, which implements a negative feedback circuit to control a tunable differential circuit for automatically compensating the output response changed by different operation conditions and obtaining an optimal modulation current output waveform.
2. Description of Related Art
FIG. 1
a
illustrates a light-emitting device driver including two cascaded differential stages. In
FIG. 1
a
, the circuit
10
includes a differential gain stage and a differential output stage. As shown in
FIG. 1
a
, the differential gain stage is formed by three FETs
16
,
18
,
20
and two load resistors
11
,
12
. The differential output stage is formed by a load resistor
13
, three FETs
22
,
24
,
26
and a laser diode
14
. In the differential gain stage, the gates of FETs
18
,
20
are respectively connected to the outputs PA, PB of the previous differential gain stage, the sources are connected to the drain of FET
16
, and the drains A, B are connected to a positive operating voltage source V
DD
through resistors
11
,
12
. Also, the drain A is connected to the gate of FET
26
and the drain B is connected to the gate of FET
24
. Thus, a differential output voltage V
DIFF
with the polarity opposite to the front is generated for driving the differential output stage to output a current output. The gate of FET
16
is connected to a control voltage U to control the gain output of the differential gain stage and the source of FET
16
is connected to a negative operating voltage source Vss. In the differential output stage, the drain of FET
24
is connected to the positive operating voltage source V
DD
through resistor
13
. The sources of FETs
24
,
26
are connected to the drain of FET
22
. The drain of FET
26
is connected to the positive operating voltage source V
DD
through a laser diode
14
. The source of FET
22
is connected to the negative operating voltage source Vss, the gate C is used to receive a control voltage C so as to control the desired output current I
LASER
through the laser diode
14
. The light output on the laser diode is controlled by the desired output current changed by the differential output voltage V
DIFF
. The curve of output current-differential voltage (I-V) is shown in
FIG. 1
b
. In the curve CASE
1
, the current I
LASER
and the voltage V
DIFF
present a proportional relationship and the voltage V
DIFF
is a constant as controlled by the input gate control voltage (for example, U) . However, problems arise with this circuit when the circuit must operate in a relatively low modulation current (for example, in the range of 10-20 mA), as shown in CASE
2
. That is, the large value of V
DIFF
supplied as the input to FEDs
24
,
26
of the output stage will overdrive these devices in the presence of the low current level supplied by FET
22
. As a result, the laser output will overshoot and generate duty cycle distortion.
FIG. 2
a
illustrates another light-emitting device driver including two cascaded differential stages. In
FIG. 2
a
, compared to
FIG. 1
a
, the circuit is the same as that of
FIG. 1
a
except for the gain output control and the output modulation control. As shown in
FIG. 2
a
, the gain output control and the output modulation control are externally connected to a same control voltage U′, different from different control voltages U and C, to control gates of FETs
50
and
56
and generate a dynamic gain control for the current output waveform. As such, as shown in
FIG. 2
b
, the operating point positioned at either CASE
1
or CASE
2
can adjust the V
DIFF
operating range depending on the modulation current in the range of R so as to avoid the overshoot. However, for such a dynamic gain control circuit, when the operating temperature and/or the processes are changed, the operating point (condition) is changed so as to create problems. Unfortunately, the above-mentioned circuit cannot respond to the variance flexibly. For example, a duty cycle distortion is required to obtain the optimal current output characteristics.
SUMMARY OF THE INVENTION
Accordingly, an object of the invention is to provide a light-emitting device driving circuit with an adaptive control, which implements a negative feedback circuit to control a tunable differential circuit for automatically compensating the output response changed by different operation conditions and obtaining an optimal modulation current output waveform.
The invention is a light-emitting device driving circuit, which implements a negative feedback circuit to make the differential gain stage auto-adjusted to the optimal gain and thus to obtain the optimal current output waveform at any operating condition, so as to eliminate the overshoot and duty cycle distortion. The light-emitting device driving circuit includes: a tunable gain-controlled differential amplifier; a first differential output stage and a negative feedback control circuit. The negative feedback control circuit further includes: a detection circuit, a comparison circuit and a second differential output stage. The detection circuit acquires a current gain level and output the current gain level to the second differential output stage to generate an output current. The comparison circuit compares the output current and a predetermined reference current and feeds the comparison result back to the detection circuit and the tunable differential gain stage. Thus, auto-adjustment of the gain is achieved to optimize the output waveform of the first differential output stage.
REFERENCES:
patent: 6037832 (2000-03-01), Kaminishi
patent: 6097159 (2000-08-01), Mogi et al.
Al-Nazer Leith
Birch & Stewart Kolasch & Birch, LLP
Industrial Technology Research Institute
Ip Paul
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