JPS63114285A - Laser-diode driving circuit - Google Patents

Abstract

PURPOSE:To obtain high-speed operation in a simple constitution, by inputting a voltage which is proportional to the current difference between a photovoltaic current generated in a optoelectric transducer means and a reference current set at a specified value, and controlling a driving transistor with said output. CONSTITUTION:A driven laser diode LD generates an optical output Po, which is proportional to a forward current If supplied from a driving transistor Q5. Meanwhile, a photovoltaic current Is, which is proportional to the optical output Po from the driven laser diode LD, is generated in a photodiode PD. In a differential amplifier 4, a reference voltage Vref=0 is set at an inverted input terminal. A voltage Ra.(Is-Iref), which is proportional to a current between the photovoltaic current Is generated in the photodiode PD and the reference current Iref, is inputted to a non-inverted input terminal. Therefore the driving transistor Q5 is controlled so that the photovoltaic current Is is proportional to the reference current Iref based on an output current I0 in the amplifier 4. Thus, high speed operation can be obtained in a simple circuit constitution.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a laser diode drive circuit that controls the optical output of a laser diode to a constant value.

(Prior Art) Generally, a laser diode drive circuit uses an APC (Automatic Power) that stabilizes the optical output by controlling the optical output of the laser diode to a constant value.
:, ontrol) circuit.

As such a conventional laser diode drive circuit,
For example, there are known ones described in the laser diode data book.

In this laser diode drive circuit, a photodiode for monitoring the optical output of a driven laser diode is arranged in parallel.

First, a forward voltage set to an appropriate voltage value is applied to the anode of the driven laser diode by a bias voltage setting circuit including a first operational amplifier.

On the other hand, a photovoltaic current detection circuit equipped with a second operational amplifier is connected to the photodiode.

The output terminal of the photovoltaic current detection circuit is connected to one input terminal of an arithmetic circuit including a third operational amplifier, and a predetermined reference voltage is set to the other input terminal of the arithmetic circuit.

The output terminal of the arithmetic circuit is connected to the base of a driving transistor connected to a forward current circuit of a driven laser diode.

Then, in the arithmetic circuit, the voltage proportional to the photovoltaic current monitored by the photodiode is compared with the base voltage, and the comparison output controls the conduction of the driving transistor, thereby increasing the optical output of the driven laser diode. is controlled to a constant value corresponding to the reference voltage.

(Problems to be Solved by the Invention) Laser diodes are often used as light sources for optical information processing, optical communication, etc., so a drive circuit for driving them must have high-speed operation and be cost-reduced. is required.

However, the above laser diode drive circuit
~The third three operational amplifiers are provided, and the circuit configuration is quite complicated, which increases the cost.Also, among the three operational amplifiers mentioned above, the second and third operational amplifiers are used as feedback loops for optical output. Since the light output is disposed within the interior, there is a problem in that once the optical output changes, the response time required to stabilize it at a constant value is relatively long, resulting in a lack of high-speed operation.

The present invention has been made based on the above circumstances, and provides a laser diode drive circuit that can stably control optical output to a constant value with a relatively simple circuit configuration and can realize high-speed operation. The purpose is to

[Structure of the Invention] (Means for Solving the Problems) In order to solve the above problems, the present invention provides a driving transistor for controlling the forward current of the driven laser diode, and a driving transistor for controlling the forward current of the driven laser diode. A photoelectric conversion means for generating a photovoltaic current proportional to the output, a reference current source to which a predetermined reference current is set, and a pair of transistors of a first polarity and a pair of transistors of a second polarity constituting a differential pair. A differential drive transistor that inputs a voltage proportional to the difference current between the photovoltaic current generated by the photoelectric conversion means and the reference current of the reference current source and controls the driving transistor so that the photovoltaic current is proportional to the reference current. The gist is to have an amplifier.

(Function) Optical output is generated from the driven laser diode by the forward current controlled by the driving transistor.

A photoelectric conversion means for monitoring generates a photovoltaic current proportional to its light output.

A voltage proportional to the difference current between this photovoltaic current and a reference current set to a predetermined value is input to a differential amplifier, and the drive transistor is controlled by its output so that the photovoltaic current is proportional to the reference current. .

The optical output of the driven laser diode is thus controlled to a constant value according to the reference current.

(Example) Embodiments of the present invention will be described below based on the drawings.

(A) and (B) of FIG. 1 and FIG. 2 are diagrams showing a first embodiment of the present invention.

Figure 1 is a circuit diagram, Figure 2 (A) is a characteristic diagram showing the relationship between the forward current of the driven laser diode and the optical output, and
<8) in the figure is a characteristic diagram showing the relationship between the optical output of the driven laser diode and the photovoltaic current of the photodiode (photoelectric conversion means).

First, to explain the configuration, in Fig. 1, LD is a driven laser diode, PD is a photodiode (photoelectric conversion means) for monitoring (, and the anode of the driven laser diode LD and the cathode of the photodiode PD are Commonly connected.

Here, the laser diode and photodiode for monitoring paired with it are built into one package, and the anode of the laser diode and the cathode of the photodiode are commonly connected to the package. There is a type in which a total of three terminal bins, a terminal, a cathode terminal of a laser diode, and an anode terminal of a photodiode, are taken out to the outside.

In this embodiment, a pair of such a type of laser diode and photodiode is applied.

Between the (+) power supply line 1 and the common connection point of the 7 nodes of the driven laser diode LD and the cathode of the photodiode PD, there is an npn type wire for controlling the forward current If of the driven laser diode D. A driving transistor Q5 is connected.

The cathode of the driven laser diode LD is grounded -C
There is.

2 is a reference current source, and a predetermined reference current 1 ref is set in the reference current source 2 as a reference for determining the optical output Po of the driven laser diode LD.

The reference current source 2 is connected between the anode of the photodiode PD and the low potential line 3, and its connection point m is connected to an inverting input terminal of a differential amplifier described below.

The differential amplifier 4 is first equipped with a pair of transistors Q1 and Q2 consisting of pno transistors of a first polarity.The collectors of the bare transistors Q+ and Q2 are as follows.
It is connected to the low potential line 3, and second polarity npn transistors Q3 and Q4 are connected to each emitter side.

pnp transistor Q1 and npn transistor Q3
An equivalent npn transistor is formed by the pnp transistor Q2 and another equivalent npn transistor is formed by the npn transistor Q4.

A constant current source 5.6 serving as a load for each equivalent npn transistor is connected between the collectors of the bare transistors Q3 and Q4 of the second polarity and the power supply line 1, respectively. The current of each constant current source 5.6 is set to rb.

An output current 1o is obtained from the connection point between the collector of the npn transistor Q3 and the constant current source 5. This connection point is connected to the base of the driving transistor 5.

As mentioned above, the first polarity pair transistors Q1, Q2
, a second transparent bare transistor Q3, Q4, and each constant current source 5.6.
The base of transistor Q2 in the polar bare transistor serves as a non-inverting input terminal, and the base of the other transistor Q1 serves as an inverting input terminal. A reference voltage Vrer is set to the inverting input terminal. In the example of FIG. 1, the inverting input terminal is grounded and the reference voltage is set to Vref=O.

The stationary amplifier 4 is a voltage input, current output type amplifier, and its transconductance G is defined as C as follows.

That is, the base-emitter voltages of the first polarity bare transistors Q+ and Q2 are Vbe1 and Vbe2, the base-emitter voltages of the second polarity bare transistors Q3 and Q4 are Vbe3 and Vbe4, and the input voltage (difference dynamic input voltage) is y r n, and the impedance at point m is R
If a, then the following equation holds true and is reversed.

V in+■be1+Vbe3=Vbe2+Vbe4- (1)V
in=Vref-Ra (Is-1ref) pnp transistors Q1, Q2 and npn transistors Q3
, Q4's base-emitter voltage Vbe can all be approximated by the following equation.

Vbe=Vt-Ωn(Ic/l5o) <2) Here, Vt is the thermal voltage (approximately 26 mV at room temperature>, IC is the collector current, and lso is the reverse bias saturation current.

Substituting equation (2) into equation (1) above, V i n+
2Vt ・+in ((Ib-1o)/l5o)=2Vt-p
n(Ib/l5o)...(3) The output current 1o of the differential amplifier (4) is calculated from the equation (3) as shown in the following equation.

1o = Ib (1-eXO(-V i n /2Vt)
)...(4) ■When in is a small signal, 1o = Vin-Ib/ (2Vt)...
・It can be approximated by (5). Transconductance G is expressed as the ratio of output current Io to input voltage Vin, so
From the above equation (5), it is defined as the following equation.

G-1b/(2Vt)
(6) Next, the operation of the laser diode drive circuit according to this embodiment will be explained using (A> and (B)) of FIG.

As shown in FIG. 2A, when the forward current If supplied from the driving transistor Q5 approaches the threshold current 1tfi, the driven laser diode LD oscillates and generates an optical output Po. From now on, the optical output PO is the forward current If
increases in proportion to Therefore, the optical output PO is approximated as follows.

po=o (I l'< I th
)Po=a(If-1th) (If>1th)・
(7) Here, a is the proportionality coefficient C of the current-light conversion of the driven laser diode LD.

On the other hand, the photovoltaic current IS generated in the photodiode PD is proportional to the optical output Po of the driven laser diode LD,
It is approximated by the following formula.

ls=b-po...
(8) Here, b is the proportionality coefficient of photo-current conversion of the photodiode PD.

The differential amplifier 4 has a reference voltage V ref= at its inverting input terminal.
o is set, and a photodiode PD is connected to the non-inverting input terminal.
Since a voltage Ra.(Is-Ircf) proportional to the difference current between the photovoltaic current Is generated in and the reference current 1ref is input, the output current 1o is expressed by the following equation.

1o=G-Vin Vin=Vref-Ra(Is-Iref)...(9
) The impedance Ra at point m, that is, the input impedance of the non-inverting input terminal of the differential amplifier 4, is Ra.

Since the photovoltaic current 1s of the photodiode PD is sufficiently smaller than the forward current if of the driven laser diode LD, T:L from the emitter of the driving transistor Q5
Most of the current flows to the coated UJ laser diode LD.

Therefore, the forward current) f is expressed by the following equation.

lf=β·IO (10) Here, β is the common emitter current amplification factor of the driving transistor Q5.

, by substituting equations (8) to (10) of L into equation (7>), the following equation is obtained.

po=a (βG<Vref+Ra−1ref)−It
h)/<1+abβG-Ra) ... (11) Since β and Ra are large and the second term in the denominator of equation (n) is sufficiently large compared to 1, if Vr″ef=Q, then (11
) is simplified as follows.

Po=! ref/b ・<12)
Thus, the photovoltaic current Is generated in the photodiode PD is proportional to the base t\lightning current ref through a feedback system consisting of an optical system between the driven laser diode LD and the photodiode PD. As shown, the optical output PO of the driven laser diode LD is equal to the reference current 1re.
It is controlled to a constant value according to f.

Next, by showing a numerical example, it will be briefly explained that the above-mentioned formula <0 holds true.

Typical characteristics of the driven laser diode LD are as follows.

itfi=70mA a=Q, 6mW/mA b=0.023mA'/mW Typical characteristics of transistors are as follows.

β=100 G=100mA/ (26mVx2) The impedance at point m is determined by the impedance of the reference current source 2 and has the following value.

From these values when Ra=10, abβG-Ra=2.6xlO6, and it can be seen that the approximation of the above equation (12) is sufficiently established.

Next, FIG. 3 shows a second embodiment of the present invention.

In this embodiment, the pace of the driving transistor Q5 is set to n
p n Connected to the collector side of transistor Q4, 1
1-2 The phase is reversed from that of the first embodiment, and the driving transistor Q5 is used as an inverting amplifier to perform the C operation.

In order to set the collector potential of npr+transistor Q4 to 2 vbe or higher, the above-mentioned inverting amplifier is constructed by connecting two npn transistors Qs and Qe in a Darlington connection. Resistor 8 is the collector of transistor Q5.
This is a resistor for setting the bias current between emitters.

A current source 7 is connected to the supply line for the forward current I' of the driven laser diode LD.

The driving transistor Q5 in the first embodiment is an emitter follower with respect to the driven laser diode LD, but in this embodiment, the Darlington-connected transistors Qs and Qe are collectively an inverting amplifier with a common emitter. It becomes.

In this embodiment, the forward current If of the driven laser diode LD is supplied from the current source 7;
I1g is Darlington) connected drive transistor Q
5, controlled by Qe.

Note that in each of the above embodiments, the driven laser diode L
In D and the first diode PD for monitoring that is paired with this, the anode of the driven laser diode LD and the cathode of the photodiode PD are commonly connected.

However, the laser diode and the photodiode paired with the laser diode may have various types of common connection bins in addition to the connection modes described above, or the laser diode and the photodiode may not have a common connection bin. Some have independent terminal bins.

The present invention can also be applied to driven laser diodes and photodiodes having various connection modes, etc. In this case, depending on the connection mode, etc.,
The applied polarity of the power supply or the polarity of each transistor constituting the differential pair is changed.

[Effects of the Invention] As explained above, according to the configuration of the present invention, the voltage proportional to the difference current between the photovoltaic current generated in the photoelectric conversion means and the reference current set to a predetermined value has the first polarity. The drive transistor is controlled such that the photovoltaic current is proportional to the reference current at its output, and the drive transistor is controlled so that the photovoltaic current is proportional to the reference current. Light output is controlled to a constant value. Therefore, the feedback loop for controlling the optical output is constructed relatively easily, and there is an advantage that response delay is extremely reduced, high-speed operation is achieved, and costs can be reduced.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a first embodiment of a laser diode drive circuit according to the present invention, and FIG. 2 shows the relationship between forward current and optical output of a driven laser diode used in the same embodiment. The characteristic diagram and FIG. 3 are circuit diagrams showing a second embodiment of the present invention. 2: Reference current source, 4: Differential amplifier, LD: Driven laser diode, PD: Photodiode (charging conversion means), (h, Q
2: Pair transistor of first polarity, Q3, Q4
: Pair transistor of second polarity, Q5 : Drive transistor. Yasuo Miyoshi, Patent Attorney Figure 2 (B) Figure 2 (A) Figure 3

Claims (1)

[Scope of Claims] A driving transistor that controls a forward current of a driven laser diode, a photoelectric conversion means that generates a photovoltaic current proportional to the optical output of the driven laser diode, and a predetermined reference current set. a reference current source, a pair of transistors of a first polarity and a pair of transistors of a second polarity forming a differential pair; a differential amplifier that controls the driving transistor so that the photovoltaic current is proportional to the reference current by inputting a voltage proportional to the reference current.