Multi-Stage Driver Circuit and Method of Distributed Driver Response Shaping Using Programmable Capacitors

Abstract

A multi-stage driver circuit has a transmission line coupled to an output of the multi-stage driver circuit. The transmission line has inductive elements and programmable capacitive elements selected to shape the transmitted data signal. The programmable capacitive elements have a first capacitor with a first terminal coupled to a first power supply conductor, and a first transistor with a first conduction terminal coupled to a second terminal of the first capacitor, and a second conduction terminal coupled to a second power supply conductor. The programmable capacitive elements have a register with a first output coupled to a control terminal of the first transistor. The programmable capacitive elements are selected to shape the transmitted data signal by observing operational dynamics of the multi-stage driver circuit.

Description

FIELD OF THE INVENTION

The present invention relates in general to electrical devices and, more particularly, to a multi-stage driver circuit and method of distributed driver response shaping using programmable capacitors.

BACKGROUND

Semiconductor and electrical devices are commonly found in modern electrical products. Electrical devices vary in the number and density of electrical components. Electrical devices perform a wide range of functions, such as signal processing, driving signal propagation, filtering, high-speed calculations, signal amplification, transmitting and receiving electrical signals, and controlling electrical devices. Electrical devices are found in the fields of automotive, communications, power conversion, networks, computers, and consumer products. Electrical devices are also found in military applications, aerospace, aviation, automotive, industrial controllers, and office equipment. A driver circuit is commonly used to transmit an electrical signal between two points or devices. The driver circuit should be properly tuned and calibrated to minimize transmission loss, return-loss, reflections, and other signal degradations, while maximizing bandwidth, transmission rate, and signal integrity.

A block diagram illustrating a conventional lumped driver is shown in FIG. 1. Amplifier 100 is a lumped stage driver with termination resistors 102 and 104 coupled to power supply conductor 106, operating at a positive voltage, and outputs 107 and 108. The bandwidth (BW) of the lumped driver, comprising a single stage, is dictated by the termination resistance (RT) 102 and 104, and the loading capacitance from the lumped stages. The lumped stage, and hence loading capacitance, can be quite large because of the large current required in the stage. The large current dictates the use of e.g., large MOSFET devices in the stage for a CMOS process. The large capacitance, from the driver stage devices and the associated routing parasitics, can be very restrictive and the benefit of moving to a distributed approach is that the parasitic loading is now reduced by the chosen number of stages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of a conventional lumped driver;

FIG. 2 illustrates a block diagram of a distributed driver architecture with selectable capacitors to shape the transmitted signal;

FIG. 3 illustrates an expanded block diagram of a distributed driver architecture with selectable capacitors to shape the transmitted signal;

FIG. 4 illustrates a selectable capacitor bank with a register to store the selected value;

FIG. 5 illustrates waveform plots of capacitive loading altered on all four stages; and

FIG. 6 illustrates waveform plots of capacitive loading altered on each of the four stages.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention is described in one or more embodiments in the following description with reference to the figures, in which like numerals represent the same or similar elements. While the invention is described in terms of the best mode for achieving the invention's objectives, it will be appreciated by those skilled in the art that it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims and their equivalents as supported by the following disclosure and drawings.

An exemplary 4-stage distributed driver architecture 116 is shown in FIG. 2. Distributed driver architecture 116 is chosen to address bandwidth and return-loss limitations of a conventional lumped driver approach, as described in FIG. 1. The number of stages is chosen to meet BW, power, and complexity concerns. For this example, each of the four stages of distributed amplifier 117, shown as amplifiers 118a-118d, would carry a quarter of the load current of the equivalent lumped stage.

The distributed driver incorporates input and output inductor-capacitor (LC) transmission lines. The inductive features of transmission line 119 are represented by inductive elements 120. Each inductive element 120 is coupled between two nodes of transmission line 119. For example, inductive element 120a is coupled between node 121 and node 123 of transmission line 119. The capacitive features of transmission line 119 are represented by capacitive elements 130. In particular, all capacitors 130 are selectable or programmable capacitors to shape to the data signal waveform. All programmable capacitors 130 are coupled to a node of transmission line 119 and referenced to power supply conductor 154. For example, programmable capacitor 130a is coupled between node 123 and power supply conductor 154 operating at ground potential. The output of distributed driver architecture 116 is terminals 136 and 138.

The transmission line L and C components are chosen to meet both impedance and delay requirements, thereby minimizing reflections from impedance mismatch, and ensuring in-phase addition of the distributed output stages. The impedance requirement is defined in equation (1) and the delay is defined in equation (2). The L and C product in equation (3) must also be chosen small enough so that the line approximates ideal behavior over the desired bandwidth.

Z=(L/C)^½  (1)

Tau=(L*C)^½  (2)

Cutoff Frequency=1/(π*(L/C)^½)   (3)

The driver output impedance of termination resistors (RT) 122 and 124 requirement can be restricted to values such as 25Ω and 50Ω depending on the final application e.g., a DML or EML laser driver. RT 122 and 124 are coupled to power supply conductor 126 operating at a positive voltage. Each segment of the output transmission line must ideally have an impedance (1) that matches that of the output impedance (RT). If this is not the case, then reflections will be evident on the driver response. The reflection can subtract from, or add to, a response of the driver depending on whether the impedance of the segment is above or below the target impedance. The reflection coefficient is defined in equation (4), where ZL is the load impedance and Z is the line segment impedance.

P=(ZL−Z)/(ZL+Z)   (4)

In FIG. 3, transmission line 119 functions as described in FIG. 2. FIG. 3 further shows the input of distributed driver architecture 116, including input signal at terminals 132 and 134, transmission line 139 represented by inductive elements 140 and capacitive elements 150, and termination resistor 142. For example, inductive element 140a is coupled between node 141 and node 143 of transmission line 119. Capacitor 150a is coupled between node 143 and power supply conductor 154 operating at ground potential. The multi-stage distributed amplifier 117, comprising amplifiers 118a-118d, is coupled between transmission line 119 and transmission line 139.

The driver response is the combination of a normal response (without any reflections) and the reflection component as dictated by the reflection coefficient multiplied by the normal response. The timing of this reflection perturbation is also impacted by the mismatched segment location in the transmission line.

The reflection perturbation can shape the response of the output driver. This offers the advantage of adjusting the driver response and this approach could also be used to filter out undesired reflections that arise from the driver integration on the end application. The application may include the integration of the driver with a channel media (such as PCB traces) and non-ideal loading, such as directly modulated laser diode (DML) or electro-absorption modulated laser (EML) loads. The channel media and the loading may not ideally match the output driver impedance. This would result in reflections that degrade the driver response. The impact of these undesired reflections can be attenuated by employing the programmable distributed driver segment impedance.

The segment impedance can be adjusted through, e.g., selectable or programmable capacitance loading. Each stage capacitor 130 can be selected or programmed individually. FIG. 4 illustrates one embodiment of programmable capacitor 130 with resistor or memory 160 capable of storing logic states or voltages. In this case, capacitor 130 is a programmable capacitor bank. A first terminal of capacitors 162, 164, 166, and 168 are coupled to the respective node of the transmission line. For example, capacitor bank 130a is coupled to node 144. A second terminal of capacitors 162-168 is coupled to the drains of transistors 170, 172, 174, and 176, respectively. The sources of transistors 170-176 are coupled to power supply conductor 154. Register 160 has four outputs respectively coupled to the gates of transistors 170-176. If the first output of register 160 is a positive voltage, transistor 170 turns on and connects capacitor 162 to ground. Capacitor 162 would be selected as part of capacitor bank 130. If the second output of register 160 is a positive voltage, transistor 172 turns on and connects capacitor 164 to ground. Capacitor 164 would be selected as part of capacitor bank 130. If the third output of register 160 is a positive voltage, transistor 174 turns on and connects capacitor 166 to ground. Capacitor 166 would be selected as part of capacitor bank 130. If the fourth output of register 160 is a positive voltage, transistor 176 turns on and connects capacitor 168 to ground. Capacitor 168 would be selected as part of capacitor bank 130.

Capacitors 162-168 may each have the same value or may have different values. The values of capacitors 162-168 and the selection of which capacitors to connect to ground through the respective transistor 170-176 determines the effective capacitance of capacitor bank 130. For example, if capacitor 162 is 10.0 picofarads (pf) and capacitor 164 is 10.0 pf, then turning on transistors 170 and 172 would provide 20.0 pf for capacitor bank 130. The selection of values for capacitor bank 130 can be done by observing driver 116 operational dynamics, such as bit error rate, or by observing the waveforms in FIGS. 5-6.

Programmable capacitors 130 facilitate a wide range of response shaping. Example differential voltage response shaping over time is shown in FIGS. 5 and 6. Lowering all stage loading in the same manner would result in a sharper emphasized response as shown in FIG. 5, while increasing the segment loading reduces the impedance and would result in a de-emphasized response that effectively slows down the response. The responses in FIG. 5 show the impact of a +/−50% increase in capacitance loading from the ideal on the example 4-stage driver, FIGS. 2-3. Adjusting the stage loading individually reduces the impact and facilitates a finer adjustment of the driver response. The responses derived from the +/−50% increase in capacitance loading, of each of the four individual stages, from the ideal, is shown in FIG. 6. The independent programmability of each stage in the driver facilitates a wide range of combinations that offers multiple response shaping options to best suit the end application. In this example, the magnitude and timing of the reflections, evident in the driver response, are influenced by the programmable segment load capacitance.

The stage tuning can help or replace existing TX side filtering (such as post-tap de-emphasis or pre-emphasis) and RX side filtering, such as forward feed equalizer (FFE). One benefit of this approach is that the settled TX output voltage value is not impacted by this shaping. This compares well to traditional post-tap de-emphasis where the settled TX output voltage is decreased with de-emphasis enabled. This new approach offers potential power consumption savings.

It should be noted that the segment delay in equation (2) also changes with the load capacitance. Care should be taken that this change should not be so significant to result in response degradation because the input and out transmission line segment delays are grossly mismatched. The corresponding input line segment delay can be adjusted in the same manner as the output line segment so that their delays track. In the design contained in the present invention, the change in the output delay segment was not so significant to cause a noticeable degradation in the output response. The desired response or shape can also be obtained by tuning the load RT.

An unmatched RT would generate reflections that can result in a sharper response for lower RT and a slower response for a larger RT. This does impact the driver output swing so more, or less, current may need to be delivered to the driver stages to compensate for the adjusted driver output voltage.

While one or more embodiments of the present invention have been illustrated in detail, the skilled artisan will appreciate that modifications and adaptations to those embodiments may be made without departing from the scope of the present invention as set forth in the following claims.

Claims

1. A multi-stage driver circuit, comprising: a first transmission line including an input receiving a data signal and further including a plurality of first inductive elements and a plurality of first programmable capacitive elements;a second transmission line including an output for the data signal and further including a plurality of second inductive elements and a plurality of second programmable capacitive elements; anda multi-stage distributed amplifier coupled between the first transmission line and second transmission line, wherein the second programmable capacitive elements are selected to shape the data signal.

2. The multi-stage driver circuit of claim 1, wherein a first inductive element of the plurality of first inductive elements is coupled between a first node and a second node of the first transmission line, and a first capacitive element of the plurality of first capacitive element is coupled between the first node and a power supply conductor.

3. The multi-stage driver circuit of claim 1, wherein a first inductive element of the plurality of second inductive elements is coupled between a first node and a second node of the second transmission line, and a first capacitive element of the plurality of second programmable capacitive element is coupled between the first node and a power supply conductor.

4. The multi-stage driver circuit of claim 1, wherein the second programmable capacitive elements include: a first capacitor comprising a first terminal coupled to a first power supply conductor; anda first transistor comprising a first conduction terminal coupled to a second terminal of the first capacitor, and a second conduction terminal coupled to a second power supply conductor.

5. The multi-stage driver circuit of claim 4, wherein the second programmable capacitive elements further include a register comprising a first output coupled to a control terminal of the first transistor.

6. The multi-stage driver circuit of claim 1, wherein the second programmable capacitive elements are selected to shape the transmitted data signal by observing operational dynamics of the multi-stage driver circuit, wherein a number of capacitive elements is chosen to meet the resolution of the response adjustment.

7. A multi-stage driver circuit, comprising a first transmission line coupled to an output of the multi-stage driver circuit, the transmission line including a plurality of first inductive elements and a plurality of first programmable capacitive elements selected to shape a data signal.

8. The multi-stage driver circuit of claim 7, further including: a second transmission line including an input receiving the data signal and further including a plurality of second inductive elements and a plurality of second capacitive elements; anda multi-stage distributed amplifier coupled between the first transmission line and second transmission line.

9. The multi-stage driver circuit of claim 8, wherein a first inductive element of the plurality of second inductive elements is coupled between a first node and a second node of the second transmission line, and a first capacitive element of the plurality of second capacitive element is coupled between the first node and a power supply conductor.

10. The multi-stage driver circuit of claim 7, wherein a first inductive element of the plurality of first inductive elements is coupled between a first node and a second node of the first transmission line, and a first capacitive element of the plurality of first programmable capacitive element is coupled between the first node and a power supply conductor.

11. The multi-stage driver circuit of claim 7, wherein the first programmable capacitive elements include: a first capacitor comprising a first terminal coupled to a first power supply conductor; anda first transistor comprising a first conduction terminal coupled to a second terminal of the first capacitor, and a second conduction terminal coupled to a second power supply conductor.

12. The multi-stage driver circuit of claim 11, wherein the second programmable capacitive elements further include a register comprising a first output coupled to a control terminal of the first transistor.

13. The multi-stage driver circuit of claim 7, wherein the second programmable capacitive elements are selected to shape the transmitted data signal by observing operational dynamics of the multi-stage driver circuit, wherein a number of capacitive elements is chosen to meet the resolution of the response adjustment.

14. A method of shaping a data signal in a multi-stage driver circuit, comprising: providing a first transmission line coupled to an output of the multi-stage driver circuit, the transmission line including a plurality of first inductive elements and a plurality of first programmable capacitive elements; andselecting values for the first programmable capacitive elements to shape the data signal.

15. The method of claim 14, further including: providing a second transmission line including an input receiving the data signal and further including a plurality of second inductive elements and a plurality of second capacitive elements; andproviding a multi-stage distributed amplifier coupled between the first transmission line and second transmission line.

16. The method of claim 15, wherein a first inductive element of the plurality of second inductive elements is coupled between a first node and a second node of the second transmission line, and a first capacitive element of the plurality of second capacitive element is coupled between the first node and a power supply conductor.

17. The method of claim 14, wherein a first inductive element of the plurality of first inductive elements is coupled between a first node and a second node of the first transmission line, and a first capacitive element of the plurality of first programmable capacitive element is coupled between the first node and a power supply conductor.

18. The method of claim 14, wherein the first programmable capacitive elements include: providing a first capacitor comprising a first terminal coupled to a first power supply conductor; andproviding a first transistor comprising a first conduction terminal coupled to a second terminal of the first capacitor, and a second conduction terminal coupled to a second power supply conductor.

19. The method of claim 18, wherein the second programmable capacitive elements further include providing a register comprising a first output coupled to a control terminal of the first transistor.

20. The method of claim 14, wherein the second programmable capacitive elements are selected to shape the transmitted data signal by observing operational dynamics of the multi-stage driver circuit, wherein a number of capacitive elements is chosen to meet the resolution of the response adjustment.