Output buffer circuit with control circuit for changing resistance of output resistor pair

1. An output buffer circuit equipped with a preemphasis function in which the amplitude of an output signal is emphasized when the logic of the output signal undergoes a transition, said output buffer circuit comprising:

a first differential circuit having a first differential pair for differentially receiving a data signal; and

a second differential circuit having a second differential pair for differentially receiving an emphasis data signal;

output pairs of said first and second differential circuits being connected in common, respective common connection nodes of said output pairs constituting first and second output nodes;

an output resistor pair arranged between the output pairs of the first and second differential circuits and a power supply; and

a control circuit for performing switching control of a resistance value of said output resistor pair, so that said output resistor pair has a relatively larger resistance value at the time of preemphasis.

2. The output buffer circuit according to claim 1 , wherein said control circuit performs switching control so that, at the time of deemphasis when, after said transition of said output signal, the logic of the output signal remains the same as the logic to which the output signal has transitioned, and the emphasized amplitude is to be reduced, said output resistor pair has a relatively smaller resistance value.

3. The output buffer circuit according to claim 1 , wherein the output buffer circuit comprises a current mode logic circuit.

4. An output buffer circuit comprising:

a first differential pair for differentially receiving a data signal; and

a second differential pair for differentially receiving an emphasis data signal;

output pairs of said first and second differential pairs being connected in common, respective common connection nodes of said output pairs constituting first and second output nodes;

first and second output resistor circuits connected between the first and second output nodes and a first power supply; said first and second output resistor circuits including first and second variable resistance elements, respectively; the resistance values of said first and second variable resistance elements being varied by a control signal received; and

a logic circuit for receiving the data signal and the emphasis data signal and for outputting the control signal, which is of a first value at the time of preemphasis when the amplitude of an output signal is to be emphasized as the logic of the output signal undergoes a transition, said control signal being of a second value otherwise; said control signal being supplied to said first and second variable resistance elements.

5. The output buffer circuit according to claim 4 , wherein switching control is made so that at the time of preemphasis, said first and second variable resistance elements have a relatively large resistance value, and so that at the time of deemphasis when, after said transition of said output signal, the logic of said output signal remains the same as the logic to which the output signal has transitioned and the emphasized amplitude is to be reduced, said first and second variable resistance elements have a relatively small resistance value.

6. The output buffer circuit according to claim 4 , wherein

said emphasis data signal is said data signal delayed a preset time and inverted.

7. The output buffer circuit according to claim 4 , wherein

said first output resistor circuit includes:

a first output resistor connected between said first output node and said first power supply; and

a first series circuit made up of a first resistance element and a first transistor, said first series being connected between said first output node and said first power supply in parallel with said first output resistor; wherein

said second output resistor circuit includes:

a second output resistor connected between said second output node and said first power supply, and

a second series circuit made up of a second resistance element and a second transistor, said second series circuit being connected between said second output node and said first power supply in parallel with said second output resistor; and wherein

said control signal from said logic circuit is supplied to control terminals of said first and second transistors.

8. The output buffer circuit according to claim 4 , wherein

said first output resistor circuit includes:

a first parallel circuit, made up of a first resistance element and a first transistor, and a first output resistor; said first parallel circuit and said first output resistor being connected in series with each other between said first output node and said first power supply; wherein

said second output resistor circuit includes:

a second parallel circuit, made up by a second resistance element and a second transistor, and a second output resistor; said second parallel circuit and said second output resistor being connected in series with each other between said second output node and said first power supply; and wherein

said control signal from said logic circuit is supplied to control terminals of said first and second transistors.

9. The output buffer circuit according to claim 4 , wherein

said first and second differential pairs are connected to a second power supply via first and second current sources adapted for supplying the currents to said first and second differential pairs, respectively.