Hybrid voltage/current-mode transmission line driver

1. A method for transmitting signals through a communication channel, comprising:

receiving a sequence of bits for transmission through the communication channel;

determining if a given bit in the sequence of bits has the same state as the previously transmitted bit;

if so, using a voltage-mode driver to drive a signal for the given bit through the communication channel; and

otherwise, using a current source coupled to the voltage-mode driver to boost the drive-level of the voltage-mode driver while transmitting the given bit;

wherein a value of a resistor couple for driving the voltage-mode driver is set so that the impedance of the voltage-mode driver, substantially equals the impedance of the communication channel and wherein the current source supplies a current to the communication channel without changing the impedance of the voltage-mode driver, thereby compensating for frequency dependant losses in the communication channel without sacrificing impedance matching and without substantially increasing power consumption.

2. The method of claim 1 , wherein the current source is activated independently from the voltage-mode driver, thereby facilitating the optimization of the shape of the transmitted signal.

3. The method of claim 1 ,

wherein more than one current source is used to compensate for frequency dependant losses in the communication channel; and

wherein each current source is activated separately from the other current sources as well as the voltage-mode driver, thereby facilitating the optimization of the shape of the transmitted signal.

4. The method of claim 1 , wherein a sequence of previously transmitted bits is used to control the current source to compensate for the frequency dependant losses in the communication channel.

5. The method of claim 1 , wherein the voltage-mode driver is a differential driver.

6. An apparatus for transmitting signals through a communication channel, comprising:

a communication channel; and

a transmitter;

wherein the transmitter is configured to:

receive a sequence of bits for transmission through the communication channel;

determine if a given bit in the sequence of bits has the same state as the previously transmitted bit;

if so, to use a voltage-mode driver to drive a signal for the given bit through the communication channel; and

otherwise, to use a current source coupled to the voltage-mode driver to boost the drive-level of the voltage-mode driver while transmitting the given bit;

wherein a value of a resistor coupled for driving the voltage-mode driver is set so that the impedance of the voltage-mode driver substantially equals the impedance of the communication channel and wherein the current source applies a current to the communication channel without changing the impedance of the voltage-mode driver, thereby compensating for frequency dependant losses in the communication channel without sacrificing impedance matching and without substantially increasing power consumption.

7. The apparatus of claim 6 , wherein the current source is activated independently from the voltage-mode driver, thereby facilitating the optimization of the shape of the transmitted signal.

8. The apparatus of claim 6 ,

wherein more than one current source is used to compensate for frequency dependant losses in the communication channel; and

wherein each current source is activated separately from the other current sources as well as the voltage-mode driver, thereby facilitating the optimization of the shape of the transmitted signal.

9. The apparatus of claim 6 , wherein a sequence of previously transmitted bits is used to control the current source to compensate for the frequency dependant losses in the communication channel.

10. The apparatus of claim 6 , wherein the voltage-mode driver is a differential driver.

11. A computer system for transmitting signals through a communication channel, comprising:

a communication channel; and

a transmitter;

wherein the transmitter is configured to:

receive a sequence of bits for transmission through the communication channel;

determine if a given bit in a sequence of bits has the same state as the previously transmitted bit;

if so, to use a voltage-mode driver to drive a signal for the given Bit through the communication channel; and

otherwise, to use a current source coupled to the voltage-mode driver to boost the drive-level of the voltage-mode driver while transmitting the given bit;

wherein a value of a resistor coupled for driving the voltage-mode driver is set so that the impedance of the voltage-mode driver substantially equals the impedance of the communication channel and wherein the current source applies a current to the communication channel without changing the impedance of the voltage-mode driver, thereby compensating for frequency dependant losses in the communication channel without sacrificing impedance matching and without substantially increasing power consumption.

12. The computer system of claim 11 , wherein the current source is activated independently from the voltage-mode driver, thereby facilitating the optimization of the shape of the transmitted signal.

13. The computer system of claim 11 ,

wherein more than one current source is used to compensate for frequency dependant losses in the communication channel; and

wherein each current source is activated separately from the other current sources as well as the voltage-mode driver, thereby facilitating the optimization of the shape of the transmitted signal.

14. The computer system of claim 11 , wherein a sequence of previously transmitted bits is used to control the current source to compensate for the frequency dependant losses in the communication channel.

15. The computer system of claim 11 , wherein the voltage-mode driver is a differential driver.