Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – Nonlinear amplifying circuit
Reexamination Certificate
1998-09-17
2001-06-19
Wells, Kenneth B. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
Nonlinear amplifying circuit
C327S065000, C330S252000
Reexamination Certificate
active
06249178
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an output circuit device suitable for driving a low-impedance load (e.g., a laser diode) at high speed by supplying current thereto.
A circuit used for driving a laser diode for high-speed optical communication of 10 Gbps class converts a high-speed signal, whose rising time and falling time are short, into a large current output, and supplies this current to a laser diode. Therefore, the circuit requires a large-power output-stage transistor, which is large size. To attain high speed, the output stage of this circuit is formed of a differential circuit made up of a pair of transistors, and is connected to a laser diode wherein the collector of a transistor serves as a load. In other words, the circuit is of an open-collector type. The differential circuit of the output stage is driven by supplying a signal from the preceding stage thereto by way of a buffer circuit.
The paired differential transistors constituting the output-stage differential circuit switches from one to the other in response to a variation in base voltage, which is of the order of tens of mV depending upon the input signals. In order to secure a desirable noise margin, the circuit is designed such that the amplitude of an internal signal is of the order of hundreds of mV. At the time of high-speed switching, therefore, a variation in base voltage causes a voltage variation of hundreds of mV at the common emitters of the paired differential transistors of the output-stage differential circuit. When the voltage variation acts on the parasitic capacitor of the transistors of the large-sized output-stage differential circuit, transient current is produced, resulting in unstable operations.
The voltage variation at the common emitters of the paired differential transistors is fed back to the base. However, since the output impedance viewed from the base to the preceding-stage circuit is of a finite value, the voltage variation fed back to the base is amplified by the paired differential transistors once again. This phenomenon is an oscillation phenomenon occurring in a differential circuit and is generally referred to as a ringing phenomenon. The ringing phenomenon adversely affects the waveforms of the output current.
In order to cope with the ringing phenomenon, the conventional art employs an integrator circuit or the like so as to blunt the waveform of the signal supplied to the base of the paired differential transistors of the output-stage differential circuit. In other words, the rising and falling times of the signal are lengthened, and oscillation is prevented thereby. Alternatively, the output impedance of the circuit that precedes the output-stage differential circuit is set to be as low as possible, so as to suppress the ringing phenomenon.
However, the former method inevitably affects the high-speed characteristic, and the original function of the output circuit device (i.e., the function of generating a current output in response to input of a high-speed signal) is degraded. On the other hand, the latter method requires a large amount of current in the circuit, leading to increases in power consumption, the heat generation, etc.
BRIEF SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide an output circuit device capable of driving a low-impedance load, such as a laser diode, in a stable way, without affecting the high-speed characteristic and giving rise to an increase in power consumption.
To attain the above object, the present invention provides an output circuit device comprising: a pair of first transistors (a pair of output-stage transistors) having control electrodes connected to an input terminal and first main electrodes connected to an output terminal; and a pair of second transistors (a pair of bypass transistors) having first main electrodes connected to the control electrodes of the output-stage transistors and second main electrodes connected to the control electrodes of the output-stage transistors, a base bias of the bypass transistors being determined such that when one of the output-state transistors is ON, a corresponding one of the bypass transistors is turned on.
The present invention also provides an output circuit device comprising: a pair of differential transistors made up of two output-stage transistors whose second main electrodes are connected in common; an output-stage differential circuit including a current source which is connected to the second main electrodes of the two output-stage transistors of the differential transistors; and two bypass transistors including first and second main electrodes which are connected to the control electrode and first main electrode of the output-stage transistors, a base bias of the bypass transistor being determined such that a transistor which is one of the two bypass transistors and whose first and second main electrodes are connected to the control electrode and first main electrode of an ON-state output-stage transistor is turned on.
The output circuit device is designed such that when an output-stage transistor is on, the voltage applied between the control electrode and the first main electrode of that output-stage transistor and the voltage applied between the control electrode and the second main electrode of the same output-stage transistor are of the same polarity.
In the output circuit device of the present invention, the control electrode of the bypass transistor is connected to a constant-potential point, the potential at which is adjustable.
The output circuit device of the present invention further comprises potential adjusting means for adjusting at least one of the voltage applied to the control electrode of an output-stage transistor and the voltage applied to the first main electrode of the output-stage transistor.
The transistors employed in the present output circuit device may be either bipolar transistors or field effect transistors (FETs). In the case of a bipolar transistor, the base corresponds to a control electrode, the collector to a first main electrode, and the emitter to a second main electrode. In the case of an FET, the gate corresponds to a control electrode, the drain to a first main electrode, and the source to a second main electrode.
In the output circuit device of the present invention, noise is fed back from the emitter to the base when the output-stage transistors are switched from one to the other. A transient current, which is thereby caused at the collector, flows through the bypass transistors; it does not flow into the load connected to the output terminal. Accordingly, the ringing phenomenon is prevented.
This operation will be explained, referring to the case where the output-stage transistors are made of bipolar transistors. In this case, the collector and emitter of a bypass transistor are connected to the base and collector of an output-stage transistor, respectively. Due to this circuit configuration, the output transistor is driven in such a manner that the base-emitter transistor voltage Vbe and the base-collector voltage Vbc are of the same polarity when the output transistor is in the ON state. For example, when the output-stage transistor is an NPN transistor, the conventional output circuit device drives this transistor such that voltage Vbe is of a positive polarity and voltage Vbe is of a negative polarity. The output circuit device of the present invention, by contrast, drives the transistor such that both voltages Vbe and Vbc are both of a positive polarity.
In the case where the base bias of the bypass transistor connected to an ON-state output-stage transistor is so determined as to turn on the bypass transistor simultaneously, noise is fed back to the base from the emitter at the time of the switching between the output-stage transistors. Part of a transient current, which is caused thereby, flows through the collector-emitter path of the bypass transistor and thus reaches the collector of the output-stage transistor.
Assuming that the noise voltage fed back to the base of
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Kabushiki Kaisha Toshiba
Wells Kenneth B.
LandOfFree
High-speed output circuit device with a switch circuit does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with High-speed output circuit device with a switch circuit, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and High-speed output circuit device with a switch circuit will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2439674