Center tap level control for current mode differential driver

• US 6,603,348 B1
• Filed: 04/18/2002
• Issued: 08/05/2003
• Est. Priority Date: 04/18/2002
• Status: Expired due to Fees

• Current Assignee: International Business Machines Corporation
• Original Assignee: International Business Machines Corporation
• Inventors: Preuss, Curtis Walter, Hanson, Charles C.
• Primary Examiner(s): Cunningham, Terry D.
• Assistant Examiner(s): NGUYEN, LONG T
• Application Number:
  US10/124,966
• Time in Patent Office: 474 Days
• Field of Search: 327/67, 327/543, 327/108, 327/112, 327/170, 327/563, 327/65, 327/588, 327/423, 327/424, 330/260, 330/261, 326/83, 326/30, 326/86
• US Class Current: 327/563
• CPC Class Codes: H03F 2203/45008 the addition of two signals being made by a resistor addition circuit for producing the common mode signal
• CPC Class Codes: H03F 3/45179 using MOSFET transistors as the active amplifying circuit H03F3/45278 takes precedence
• CPC Class Codes: H04L 25/0274 Arrangements for ensuring balanced coupling
• CPC Class Codes: H04L 25/0282 Provision for current-mode coupling

Abstract

A feedback mechanism is provided to a current mode differential driver by connecting the center tap of a terminator of the output of the driver through feedback resistors to the gates of a positive and a negative current source connected to the driver. Connecting the center tap between the feedback resistors, the average common mode voltage at the output of the differential driver is substantially constant which avoids variations and reflective noise in high speed data transmission that can occur because of manufacturing tolerances.

Claims

1. A stabilized current mode differential driver, comprising:

(a) a pfet current source;

(b) an nfet current source;

(c) a current steering network connected to the pfet current source and the nfet current source;

(d) a center tap terminator connected to the current steering network;

(e) a feedback network connected to the center tap terminator to provide feedback to the pfet current source and the nfet current source to maintain a constant average common mode voltage, the feedback network comprising;

(i) a first feedback resistor connected to a gate of the pfet current source; and

(ii) a second feedback resistor connected to a gate of the nfet current source.

2. The current mode differential driver of claim 1, further comprising:

(a) a first pfet current mirror reference connected to the gate of the pfet current source; and

(b) a second nfet current mirror reference connected to the gate of the nfet current source;

the first feedback resistor also connected to the first pfet current mirror reference and the second feedback resistor also connected to the second nfet current mirror reference.

3. The current mode differential driver of claim 1, further comprising:

(a) a complementary signal driving the current steering network.

4. The current mode differential driver of claim 3, wherein the current steering network comprises:

(a) two pfets, the sources of each being connected to a drain of the pfet current source and the gates of each being connected to each of the complementary signal;

(b) two nfets, the sources of each being connected to the drain of the nfet current source and the gates of each being connected to each of the complementary signal and the drain of one nfet being connected to the drain of one pfet.

5. The current mode differential driver of claim 4, further comprising a differential output.

6. The current mode differential driver of claim 1, wherein the driver drives a transmission line having an impedance matching that of the center tap terminator.

7. A current mode differential driver, comprising:

(a) a first current mirror;

(b) a first current source having a first bias voltage provided by the first current mirror, the first current mirror and the first current source connected to a supply voltage;

(c) a second current mirror;

(d) a second current source having a second bias voltage provided by the second current mirror, the second current mirror and the second current source connected to a ground voltage;

(e) a first input connected to the gates of one leg of an H-bridge differential driver;

(f) a second input connected to the gates of a second leg of the H-bridge differential driver, the first and second input being complementary signals;

(g) a center tap terminator comprising a first terminating resistor and a second terminating resistor, the center tap terminator connected between a first complementary output at the center of the first leg and between a second complementary output at the center of the second leg,

(h) a resistor feedback network in which a center tap of the center tap terminator is connected between a first feedback resistor of the resistor feedback network and a second feedback resistor of the resistor feedback network, the first feedback resistor connected to the first bias voltage of first current source and the second feedback resistor connected to the second bias voltage of the second current source;

wherein the feedback resistor network is configured to maintain an average common mode voltage at the center tap.

8. A method to stabilize the output of a current mode differential driver, comprising the steps of:

(a) providing a first current to the current mode differential driver,

(b) providing a second current to the current mode differential driver;

(c) providing a center tap terminator at an output of the current mode differential driver;

(d) adjusting a voltage at a center tap of the center tap terminator to adjust the first current and/or the second current to maintain a constant average common mode voltage at the center tap by connecting the center tap to a first feedback resistor, and connecting the first feedback resistor to a gate of a first current source for providing the first current; and

connecting the center tap to a second feedback resistor, and connecting the second feedback resistor to a gate of a second current source for providing the second current, so that the center tap is connected between the first and second feedback resistors.

9. A stable H-bridge differential driver, comprising:

(a) means to provide current to the H-bridge differential driver;

(b) means to input a complementary signal to the H-bridge differential driver;

(c) means to direct the current through the H-bridge differential driver based on the complementary signal;

(d) means to maintain an average common mode voltage at a center tap of the H-bridge differential driver to be substantially constant, said maintaining means comprising a feedback resistor network connected between the center tap and the means to provide current.