Electronic digital logic circuitry – Interface – Current driving
Reexamination Certificate
2000-07-28
2001-10-23
Tokar, Michael (Department: 2819)
Electronic digital logic circuitry
Interface
Current driving
C326S030000, C326S034000, C326S115000
Reexamination Certificate
active
06307402
ABSTRACT:
The present invention relates to an output buffer for differentially driving a symmetrical transmission line, according to the preamble of claim
1
. An output buffer of this kind is known from U.S. Pat. No. 5,519,728.
Digital communication systems show a continuous increase in complexity and speed. This results in a strong demand for interfaces between different system sections which allow the transmission of digital data with high data rates. Such interfaces should be power efficient in order to keep the total power consumption of complex systems within reasonable limits.
From U.S. Pat. No. 5,519,728 it is known to adopt a differential signaling scheme for transmitting data with high speed and low power over a transmission line. The differential signaling scheme allows to keep the differential signal amplitude across the transmission line low, and hence the power required for transmitting the signals. From this document it is furthermore known to terminate the transmission line at the receiver side in order to improve the signal quality. This document furthermore provides means for keeping the common mode voltage on the symmetrical transmission line at a predetermined level. In this document, this is achieved by means of feeding a constant current into the bridge driving circuit at the transmitter side, and connecting the lower power supply terminal of the bridge driver circuit in series with a resistor connected with ground. The constant current through the bridge and through this resistor results in a constant voltage drop across the resistor which appears as a common mode voltage on the transmission line. The resistor must have a fairly high value if a power efficient design is envisaged.
If reflections occur on the transmission line for whatever reason, e.g. due to changes in the characteristic impedance in the course of the transmission line or improper termination at the receiver side, the output buffer known from this document will not be able to absorb such reflections. Rather, the reflections arriving at the output buffer will be reflected back to the receiver because the constant current source renders a very high output impedance of the output buffer. Such reflections back and forth are particularly disturbing at high bit rates and steep edges of the transmitted signal. This imposes limits on circuits of the kind known from U.S. Pat. No. 5,519,728 when operating at high bit rates.
It is the object of the present invention, to provide an output buffer circuit for differentially driving a symmetrical transmission line, which is able to operate in a power efficient manner at very high bit rates and which is able to keep the common mode voltage on the transmission line within predetermined narrow limits.
According to the present invention, this object is solved as defined in claim
1
.
According to the present invention, the output buffer circuit has a power supply circuit for supplying the bridge driver circuit with the power necessary for driving the transmission line, which power supply circuit behaves like a voltage source with a source impedance. The sum of the source impedance of the power supply circuit and of the impedance contributed by the respective diagonal in the ON state of the bridge driver circuit, is selected such that the output impedance of the output buffer circuit essentially matches the characteristic impedance of the transmission line. A voltage generator circuit biases the lower voltage source potential to a predetermined bias level in order to achieve a desired common mode level on the transmission line. Preferably, the voltage generator circuit has an impedance lower than a characteristic impedance, preferably the common mode characteristic impedance of the transmission line.
An output buffer circuit according to the present invention is advantageous in that it allows to match the characteristic impedance of the transmission line at the transmitter side of the transmission line while at the same time, the voltage generator circuit provides for the common mode level expected by the receiver on the transmission line. If the impedance of the generated voltage is sufficiently low, the output buffer circuit can match both the differential and the common mode characteristic impedance of the transmission line. The differential characteristic impedance appears between the two signal conductors of the symmetrical transmission line. The common mode characteristic impedance appears between the respective signal conductor and ground.
Advantageous embodiments are given in the dependent claims.
According to an embodiment of the present invention, the voltage source circuit of the power supply circuit of the output buffer circuit comprises a transistor connected as a source follower. At its control terminal the transistor receives a reference potential. The source of that transistor provides the upper power supply potential to the bridge driver circuit. The lower power supply potential is provided by the voltage generator circuit.
The voltage generator circuit preferably comprises a diode or a series connection of diodes, depending on the desired common mode level, or comprises a transistor connected to provide a constant drain source voltage. This can for instance be achieved by means of connecting gate and drain of that transistor together. If the bias level provided by said voltage generator circuit is the ground level, the voltage generator may simply consist of a resistor.
According to a preferred embodiment, the reference potential applied to the control terminal of the source follower transistor, is generated with reference to the voltage generated by the voltage generator circuit. However, a circuit can be provided for detecting the common mode potential on the transmission line and for controlling the voltage generator circuit such that the common mode level on the transmission approximates a desired level.
Advantageously, the transistors constituting the bridge driver circuit are selected such that the sum of the ON impedances of the transistors constituting a diagonal of the bridge, approximates or equals the differential characteristic line impedance. This structure is simple and requires little space on the surface of an integrated circuit. Preferably, the impedance of the voltage generated by the voltage generator circuit, is selected such that the impedance between at least one of the output terminals and ground of the buffer circuit approximates or equals the common mode characteristic impedance of the transmission line.
According to a preferred embodiment of the present invention, the bridge driver circuit receives its power supply from a supply amplifier the output stage of which is adapted to tolerate output currents not only flowing out of the output stage but also output currents flowing into the output stage. An output stage having this ability is called a push pull stage. The provision of a supply amplifier with this kind of output stage is advantageous in that it allows an even better approximation of the behavior of an ideal voltage source for supplying the bridge driver circuit with power. A resistor can be connected in series between the supply amplifier and the bridge driver circuit in order to adjust the output impedance of the bridge driver circuit.
Preferably, the voltage generator circuit is embodied as an additional supply amplifier with a push pull stage for supplying the lower power supply potential to the bridge driver circuit. In this way it can be achieved that the output buffer circuit behaves highly linear even if reflections arriving at the transmitter result in temporarily feeding power from the transmission line into the power supply circuit of the output buffer.
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patent: 5089723 (1992-02-01), Davis et al.
patent: 5266845 (1993-11-01), Sakaue
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patent: 5977796 (1999-1
Burns Doane Swecker & Mathis L.L.P.
Le Don Phv
Telefonaktiebolaget LM Ericsson
Tokar Michael
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