Note: Descriptions are shown in the official language in which they were submitted.
33~
PHT. 82.344
The invention relates to a circuit arrangement Eor driving a
current-controlled ccmponent in which the current-controlled component,
a controllable resistance and a current source are arranged in series.
Current-controlled components are, for example, lasers and
light-emitting diodes used as transmitting elements in the optical
message transmission comprising optical fibres and a transmission
medium. Therein, the light intensity of the laser or of the light-
emitting diode is m~dulated by the input signal. In the digital
optical message transmission the transmission of light correspon~s to,
for example, a logic 1, whereas a logic 0 is present when no light is
transmittedO
In The Bell System, Technical Journal, Vol. 57, No. 6,
July-August 1978 a la æ r transmitter for optical transmission systems
is disclosed on pages 1823 to 1833 in the article "Ga~lAs Laser Trans-
mitter for Lightwave Transmission Systems" by Shumate, Chen and Dorman.
Figure 2 on page 1826 shows a circuit arrangement for dri~ing a laser,
which is here shown in Figure 1. Figure 2 shows a first embodiment of
the invention and Figure 3 a second embodiment, while the characteris-
tic curve of a laser is shown in Figure 4.
In Figure 1, the input signal is applied via a transfer stage
comprising a transistor Q3 to one input of a differential amplifier
which comprises two transistors Ql and Q2. The other input of the
differential amplifier is connected to a reference voltage. A resis-
tor having a value of 10 ~ is included in the collector lead of the
transistor Ql' while the laser is arranged in the collector-lead of
the transistor Q2. m e emitters of -the two transistors Ql and Q2 are
connected to one output of a current source. When the laser is to
transmit light, the transistor Q2 is rendered conductive, whereas the
transistor Ql is cut off, so that the current source is connected to
the laser via the emitter-collector path of the transistor Q2. If no
light is to be transmitted, that is to say there is a pulse pause,
the transistor Q2 is cut-off, whereas transistor Ql is rendered con-
ductive, so that the c~ ent source is not operated in the no-load
m~de, but the current circui-t is closed vla the emitter-collector path
3~iD
PHT. 82.344 2
of the transistor Ql and the 10 ~ -resistor.
This prior art circuit arrangement has the disadvantage that
it has a comparati~rely high power dissipation, as during the pulse
pauses the current produced by the current source does not 1aw
through the laser, but through the 10 J~ - resistor, in which now
ohmic losses occur.
The invention has for its object to provide a circuit
arrangement with reduced power loss for driving a current-controlled
component.
According to the invention, this object is accomplished in
that a current branch comprising a first capacitor is provided in
parallel with the series arrangement formed by the controllable resis-
tan~e and the current-controlled component.
The sub-claims describe advantageous embodiments of the
invention.
In Figure 2, an input signal UE is applied vla a capacitor
Cl to the base of a transistor T, which is connected to ground via a
resistor Rl and to the supply voltage -Us via a resistor R2. The
emitter of the transistor T is connected to the supply voltage -UB
via a current source Q, whereas its collector is connected to ground
via a laser or a light-emitting diode L, denoted by IED in the
.
further course of the description. ~ capacitor C2 connects the
emitter of the transistor T to ground.
The resistors Rl and R2 are used to adjust the operating
point of the transistor T~ The digital input signal UE which is
applied to the base of the transistor T via the capacitor Cl, (a.c.
coupling) modulates the light intensity of the laser or the LED.
During the transmission ~f a light pulse by the laser or
the LED the transistor is ren~ered cond~ctive, whereas it is cut-off
3Q during a pulse pause, so that the current from the current source Q
flGws as a charging current into the capacitor C2. For a subsequent
tra~smission of a light pulse the transistor beccmes conductive again.
m e current for the laser or the LED are now produced jointly by the
current source Q and the char~ed capacitor C2. In the then subse-
quent pulse pause the capacitor C2 is again charged by the currentsource Q.
Whilst during the pulse pauses the current from the current
source does not pxoduce ohmic losses in a resistor, but only provides
~2Z33()~
PHT ~2.344 -3_ 25.11.1983
chargin~ of a capacitor and at each pulse subsequent to a pulse pause is
again appl-ed to t.he laser or to the LED, the current source needs only
t.o produce half of the current it must produce in the pr~or art circuit.
arrangement.. In other words, when the transistor ls operated as a switch
only half the power is consumed.
For a current.-controlled component. havtn~ a threshold value,
as, for example, in the case of a laser, it: may be wise not to
cut-off t.he transistor completely, but. to send a bias current t.hrou~h
the current-controlled component., as will be further described
hereafter wlth referencr? t.o t.he character sti.c curve of a laser which
is shown ln Flgure 4.
In order t.o oht.ain an adequatel~ hl~h li~ht int.ens~ty, it. is
necessary t.o operate t.he laser in t.he st.eep port.ion of the curve after
t.he hend. To that end An avera~e power PM is establ;shed as an operat.ing
point. around which t.he supplied li~ht power fluctuates. The point PM is
est.abl-shed by t.he quiescent current..IC. Half the pulse current 1/2 A IC
which s proportional to the fluct.uations ~ UE of the input. volta~e UF.
is now only superirnposed on the quiescent. current IC. During a pulse
pause in t.he lnput slgnal the capacitor C2 takes up half the pulse
current. 1/2 ~ IC, which it applles again t.o the laser during the
subsequent. pulse. At t.he moment. at. which the collector current. t.hrou~h
t.he trans-stor and also through the laser consequently assurnes the
values IC + 1/2 4 IC and IC - 1/2 ~IC, t.hen the laser transmits light
having t.he lntensities PM ~ 1/2 ~ PM and PM - 1/2 apM. Then t.he current
IC~ 1/2 ~ IC and the ll~ht. int.en~sit.y PM + 1/2 ~ PM corresponrl to a
pulse, whereas t.he current IC - 1/2 4IC and the light intensity
PM - 1/2 ~ PM represent. a pulse pause. The pulse current ~ IC depends on
t.he quiescent. current IC in accordance wit.h t.he followin~ formula:
IC
~ IC ~ UT .~UE
where UT i.s t.he ther~ovolta~e of t.he t.ransist.or.
The dependence of t.he pulsr? current. ~ IC on the quiescent
current IC can br.? reriuced by means of nr-?~ative feedback.
~ 11e embod~ment shown in Figure 3 differs t.opologically from
t.he embodiment. shown in Fl~ure 2 only hy a neFatlve feedback resistor R3
arran~ed brtween the current source Q and the emitt.er of t.he
~ ~ ~ 3 3 ~ ~
RHT (~2. 3lJ4 _4_ 25.11.1983
transistor T. The capacltor C2 is arran~ed in parallel with t,he ser~'es
arrangement formed by the laser L, the collect,or-em.itter path of
t,ranslst.or T and the ne~ative feedback resist,or R3. In thls case, t.he
pulse current ~ IC depends on t,he qu.~escent current IC in accordance
w~t,h the rollowi~ng formula:
4 IC = [~I ~ R3 4 UE
IC
The dependence of t.he pulse current. ~ IC on the quiescent. current IC ~s
consequently reduced by t.he factor - -R3-~rT
O 1+ IC
