# Patent

## Title
Method and system for split voltage domain receiver circuits

## Abstract

Methods and systems for split voltage domain receiver circuits are disclosed and may comprise amplifying complementary received signals in a plurality of partial voltage domains, and combining the amplified received signals, utilizing a stacked cascode amplifier for each domain, into a single differential signal in a single voltage domain. The stacked cascode amplifiers may comprise a feedback loop having a comparator which controls a current source in each domain. The complementary signals may be received from a photodiode, which may be integrated in the integrated circuit. The amplified received signals may be combined via stacked common source or common emitter amplifiers. The received signals via may be amplified by stacked inverters. The amplified received signals may be AC or DC coupled prior to the combining. The complementary received signals may be amplified and combined via cascode amplifiers. The voltage domains may be stacked and may be controlled by feedback loops.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE
This application is a continuation of application Ser. No. 12/208,668 filed on Sep. 11, 2008, which in turn makes reference to, claims priority to and claims the benefit of U.S. Provisional Patent Application No. 60/997,282 filed on Oct. 2, 2007.
This application also makes reference to:

    U.S. patent application Ser. No. 12/208,650 filed on even date herewith.


Each of the above stated applications is hereby incorporated herein by reference in its entirety.


FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[Not Applicable]
MICROFICHE/COPYRIGHT REFERENCE
[Not Applicable]
FIELD OF THE INVENTION
Certain embodiments of the invention relate to integrated circuit power control. More specifically, certain embodiments of the invention relate to a method and system for split voltage domain receiver circuits.
BACKGROUND OF THE INVENTION
Electronic circuits typically require a bias voltage for proper operation. The voltage level required by a circuit depends on the application. A circuit for transmitting electromagnetic radiation may require a higher voltage than a circuit used for processing data. The optimum voltage may be determined by the bias voltage requirements of the transistors, or other active devices, within the circuit.
A bipolar transistor circuit may require a higher voltage in amplifier applications to avoid saturation of the amplifier, as opposed to switching operations, for example. CMOS circuits may require a lower voltage to drive the MOSFETs in the circuit.
Furthermore, as device sizes continue to shrink for higher speed and lower power consumption, a high voltage may degrade performance and cause excessive leakage. With thinner gate oxides, gate leakage current may become significant using historical bias voltages, thus driving gate voltages lower. However, if a transmitter/receiver may be integrated in the same device, a higher bias voltage may also be required. Bias voltages are typically DC voltage, and may be supplied by a battery. However, there may be noise in the bias voltage, which may be mitigated by capacitive filters. The variable output voltage of batteries my affect operation of battery powered devices. Devices generally must be capable of operating over a large range of voltage due to the variable output voltage capability of batteries.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with the present invention as set forth in the remainder of the present application with reference to the drawings.
BRIEF SUMMARY OF THE INVENTION
A system and/or method for split voltage domain receiver circuits, substantially as shown in and/or described in connection with at least one of the figures, as set forth more completely in the claims.
Various advantages, aspects and novel features of the present invention, as well as details of an illustrated embodiment thereof, will be more fully understood from the following description and drawings.



BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS
FIG. 1A is a block diagram of a photonically enabled CMOS chip, in accordance with an embodiment of the invention.
FIG. 1B is a diagram illustrating an exemplary CMOS chip, in accordance with an embodiment of the invention.
FIG. 1C is a diagram illustrating an exemplary CMOS chip coupled to an optical fiber cable, in accordance with an embodiment of the invention.
FIG. 1D is a block diagram of an exemplary n-type field effect transistor circuit, in accordance with an embodiment of the invention.
FIG. 1E is a block diagram of an exemplary p-type field effect transistor circuit, in accordance with an embodiment of the invention.
FIG. 2 is a block diagram of an exemplary two-domain receiver circuit, in accordance with an embodiment of the invention.
FIG. 3 is a schematic of an exemplary transimpedance amplifier domain combiner, in accordance with an embodiment of the invention.
FIG. 4 is a flow chart illustrating exemplary steps in the operation of a split domain transimpedance amplifier with domain combiner, in accordance with an embodiment of the invention.



DETAILED DESCRIPTION OF THE INVENTION
Certain aspects of the invention may be found in a method and system for split voltage domain receiver circuits. Exemplary aspects of the invention may comprise amplifying complementary received signals in a plurality of partial voltage domains. The signals may be combined into a single differential signal in a single voltage domain. Each of the partial voltage domains may be offset by a DC voltage from the other partial voltage domains. The sum of the partial domains may be equal to a supply voltage of the integrated circuit. The complementary signals may be received from a photodiode. The amplified received signals may be amplified via stacked common source amplifiers, common emitter amplifiers, or stacked inverters. The amplified received signals may be DC coupled prior to combining. The complementary received signals may be amplified and combined via cascode amplifiers. The voltage domains may be stacked, and may be controlled via feedback loops. The photodetector may be integrated in the integrated circuit.
FIG. 1A is a block diagram of a photonically enabled CMOS chip, in accordance with an embodiment of the invention. Referring to FIG. 1A, there is shown optoelectronic devices on a CMOS chip 130 comprising high speed optical modulators 105A-105D, high-speed photodiodes 111A-111D, monitor photodiodes 113A-113H, and optical devices comprising taps 103A-103K, optical terminations 115A-115D, and grating couplers 117A-117H. There is also shown electrical device and circuits comprising transimpedance and limiting amplifiers (TIA/LAs) 107A-107D, analog and digital control circuits 109, and control sections 112A-112D. Optical signals are communicated between optical and optoelectronic devices via optical waveguides fabricated in the CMOS chip 130.
The high speed optical modulators 105A-105D comprise Mach-Zehnder or ring modulators, for example, and enable the modulation of the CW laser input signal. The high speed optical modulators 105A-105D are controlled by the control sections 112A-112D, and the outputs of the modulators are optically coupled via waveguides to the grating couplers 117E-117H. The taps 103D-103K comprise four-port optical couplers, for example, and are utilized to sample the optical signals generated by the high speed optical modulators 105A-105D, with the sampled signals being measured by the monitor photodiodes 113A-113H. The unused branches of the taps 103D-103K are terminated by optical terminations 115A-115D to avoid back reflections of unwanted signals.
The grating couplers 117A-117H comprise optical gratings that enable coupling of light into and out of the CMOS chip 130. The grating couplers 117A-117D are utilized to couple light received from optical fibers into the CMOS chip 130, and the grating couplers 117E-117H are utilized to couple light from the CMOS chip 130 into optical fibers. The optical fibers may be epoxied, for example, to the CMOS chip, and may be aligned at an angle from normal to the surface of the CMOS chip 130 to optimize coupling efficiency.
The high-speed photodiodes 111A-111D convert optical signals received from the grating couplers 117A-117D into electrical signals that are communicated to the TIA/LAs 107A-107D for processing. The electrical signals generated by the high-speed photodiodes 111A-111D may be sensed in differential mode and amplified in a plurality of voltage domains. The sum of the individual voltage domains may sum up to the supply voltage. In this manner, the voltage swing across each stage of the receiver circuitry, such as stacked inverters, may be halved, while still providing the same photogenerated current, thus doubling the gain, improving the signal to noise ratio and the efficiency. The signals may then be combined into a single voltage domain via stacked common source amplifiers, for example. The analog and digital control circuits 109 may control gain levels or other parameters in the operation of the TIA/LAs 107A-107D. The TIA/LAs 107A-107D then communicate electrical signals off the CMOS chip 130.
The control sections 112A-112D comprise electronic circuitry that enable modulation of the CW laser signal received from the splitters 103A-103C. The high speed optical modulators 105A-105D require high-speed electrical signals to modulate the refractive index in respective branches of a Mach-Zehnder interferometer (MZI), for example. The voltage swing required for driving the MZI is a significant power drain in the CMOS chip 130. Thus, if the electrical signal for driving the modulator may be split into domains with each domain traversing a lower voltage swing, power efficiency is increased. Photogenerated signals may be sensed at both terminals of a photodiode, increasing the power efficiency, and then amplified in separate voltage domains.
FIG. 1B is a diagram illustrating an exemplary CMOS chip, in accordance with an embodiment of the invention. Referring to FIG. 1B, there is shown the CMOS chip 130 comprising electronic devices/circuits 131, optical and optoelectronic devices 133, a light source interface 135, CMOS chip surface 137, an optical fiber interface 139, and CMOS guard ring 141.
The light source interface 135 and the optical fiber interface 139 comprise grating couplers that enable coupling of light signals via the CMOS chip surface 137, as opposed to the edges of the chip as with conventional edge-emitting devices. Coupling light signals via the CMOS chip surface 137 enables the use of the CMOS guard ring 141 which protects the chip mechanically and prevents the entry of contaminants via the chip edge.
The electronic devices/circuits 131 comprise circuitry such as the TIA/LAs 107A-107D and the analog and digital control circuits 109 described with respect to FIG. 1A, for example. The optical and optoelectronic devices 133 comprise devices such as the taps 103A-103K, optical terminations 115A-115D, grating couplers 117A-117H, high speed optical modulators 105A-105D, high-speed photodiodes 111A-111D, and monitor photodiodes 113A-113H.
FIG. 1C is a diagram illustrating an exemplary CMOS chip coupled to an optical fiber cable, in accordance with an embodiment of the invention. Referring to FIG. 1C, there is shown the CMOS chip 130 comprising the electronic devices/circuits 131, the optical and optoelectronic devices 133, the light source interface 135, the CMOS chip surface 137, and the CMOS guard ring 141. There is also shown a fiber to chip coupler 143, an optical fiber cable 145, and a light source module 147.
The CMOS chip 130 comprising the electronic devices/circuits 131, the optical and optoelectronic devices 133, the light source interface 135, the CMOS chip surface 137, and the CMOS guard ring 141 may be as described with respect to FIG. 1B.
In an embodiment of the invention, the optical fiber cable may be affixed, via epoxy for example, to the CMOS chip surface 137. The fiber chip coupler 143 enables the physical coupling of the optical fiber cable 145 to the CMOS chip 130.
The light source module 147 may be affixed, via epoxy or solder, for example, to the CMOS chip surface 137. In this manner a high power light source may be integrated with optoelectronic and electronic functionalities of one or more high-speed optoelectronic transceivers on a single CMOS chip.
The power requirements of optoelectronic transceivers is an important parameter. Minimizing voltage swings is one option for reducing power usage, and operating photodiodes in differential mode enable reduced power consumption with improved signal to noise ratio. Photogenerated signals may be sensed at both terminals of a photodiode and amplified in separate voltage domains. Following the amplification of the photogenerated signals, they may be combined into a single voltage domain prior to subsequent processing, such as digital processing, for example.
FIG. 1D is a block diagram of an exemplary n-type field effect transistor circuit, in accordance with an embodiment of the invention. Referring to FIG. 1D, there is shown a source follower circuit 55 comprising an n-channel field effect transistor (NFET) 50, a resistor 33, a high rail 20, and a low rail 10. There is also shown a circuit input 100 and a circuit output 200.
The source follower circuit 55 has two power rails, comprising the high rail 20 biased at a voltage Vf, or full voltage, and the low rail 10, marked with the customary symbol of “ground”. The circuit has an input 100 on the gate of the NFET 50, while the circuit output 200 is on the NFET source side, or simply source. The NFET 50 drain side, or drain, is connected to the high rail 20. The resistor 33 is coupled between the source terminal of the NFET 50 and the low rail 10, completing an electrical path between the high 20 and low 10 rails.
In operation, an input signal is applied to the input 100. The source follower circuit 55 may be utilized to lower the impedance level in the signal path, drive resistive loads, or to provide DC level shifting, since the gate-source DC voltage drop may be controllable by the bias current. The gain of the source follower circuit 55 may be near unity, resulting in an AC output signal at the circuit output 200, but with a configurable DC output level.
FIG. 1E is a block diagram of an exemplary p-type field effect transistor circuit, in accordance with an embodiment of the invention. Referring to FIG. 1E, there is shown a source follower circuit 65 comprising a p-channel field effect transistor (PFET) 60, a current source 33′, a high rail 20, and a low rail 10. There is also shown a circuit input 100′ and a circuit output 200′.
The PFET source follower has two power rails, comprising a high rail 20 at a voltage Vf, or full voltage, and a low rail 10, marked with the “ground” symbol. The circuit has an input 100′ on the PFET 60 gate, while the circuit output 200′ is on the PFET 60 source side, or simply source. The PFET 60 drain side, or drain, is connected to the low rail 10. The current source 33′ is coupled to the high rail 20 and the PFET 60 source, completing an electrical path between the high 20 and low 10 rails.
In operation, an input signal is applied to the input 100′. The source follower circuit 65 may be utilized to lower the impedance level in the signal path, drive resistive loads, or to provide DC level shifting, since the gate-source DC voltage drop may be controllable by the bias current. The gain of the source follower circuit 65 may be near unity, resulting in a similar AC output signal at the circuit output 200′, but with a configurable DC output level.
FIG. 2 is a block diagram of an exemplary two-domain receiver circuit, in accordance with an embodiment of the invention. Referring to FIG. 2, there is shown a stacked domain transimpedance amplifier (TIA) 250 comprising compensation resistors R1 and R2, feedback resistors R3 and R4, PMOS transistors M1 and M3, and NMOS transistors M2 and M4. There is also shown a photodiode PD1, bias voltage Vdd, and an output signal defined by Vout+ and Vout−. Although the circuit in FIG. 2 shows a ground connection at the source terminal of NMOS transistor M4 and one terminal of the compensation resistor R2, the invention is not so limited. Accordingly, a bias voltage may be applied instead of ground to increase or decrease the voltage swing as desired.
The interconnection of the transistor pairs M1/M2 and M3/M4 comprise a stacked inverter pair and enables the generation of a differential signal at Vout+ and Vout− from the electrical signal generated by the photodiode PD1 when subjected to an optical signal. In an exemplary embodiment, the coupling point, VB, at the source terminal of the NMOS transistor M2 and the PMOS transistor M3 is configured to be equal to Vdd/2, thus generating two voltage domains of equal magnitude. The tuning of this voltage may be enabled by the compensation resistors R1 and R2, with variations in the transistor devices, for example, as a source of imbalance. The feedback resistors R3 and R4 may flatten the gain curve of the amplifiers defined by M1/M2 and M3/M4, respectively, and result in increased bandwidth. In another embodiment of the invention, the stacked inverters may be cascoded for increased output impedance and stage gain.
In operation, an optical signal is applied to the photodiode PD1. The electrical signal generated by the photodiode PD1 may be amplified by the stacked inverters M1/M2 and M3/M4. By biasing VB at Vdd/2, and applying the photodiode PD1 signal to each of the inverters defined by M1/M2 and M3/M4, the photodiode PD1 is thus monitored in a complementary mode. In this manner, the gain may be double that of a single inverter but at the same photodiode current, thus increasing the signal to noise ratio and increasing power efficiency. This configuration has an advantage over conventional designs for generating differential signals, in that DC coupling is possible with the present invention, eliminating the low frequency corner issue from AC coupling capacitors at the outputs of a conventional amplifier.
FIG. 3 is a schematic of an exemplary transimpedance amplifier domain combiner, in accordance with an embodiment of the invention. Referring to FIG. 3, there is shown a TIA combiner 300 comprising PMOS transistors M5, M6, M9, and M10, NMOS transistors M7, M8, M11, and M12, current sources 301A and 301B, comparators 303A and 303B, and compensation resistors R5 and R6. There is also shown input terminals Vin+ and Vin− at the gate terminals of transistors M5 and M10, respectively, and output terminals Vout+ and Vout−.
The gate terminal of transistor M8 may be coupled to Vdd, the gate terminals of transistors M6 and M11 may be coupled to Vdd/2, and the gate terminal of transistor M9 may be coupled to ground. In this manner, the bias conditions for the stacked cascodes in a common source configuration may define two voltage domains centered around Vdd/2, and the signals communicated to the two inputs at Vin+ and Vin− may see the same loading and thus the same latency, but with a DC offset. In instances where the loading is not symmetric, jitter would result.
In operation, complementary input signals in separate voltage domains, as described with respect to FIG. 2, may be coupled to the input terminals Vin+ and Vin−. The input signals may then be amplified and combined into a common domain by the separate voltage domain cascodes comprising the transistors M5/M6/M7/M8 and M9/M10/M11/M12, respectively, resulting in a differential signal at the Vout+ and Vout− outputs. The interconnection of the common source amplifiers results in a combining of the separate domains. An increase in the magnitude of the received photogenerated differential signal, i.e. the voltage difference between Vin+ and Vin− is increasing, results in an increased potential at Vout+ due to transistor M8 being biased in an ON state and transistor M7 conducting more current with increasing gate voltage, pulling Vout+ toward Vdd. Similarly, with transistor M9 biased ON with its gate at ground and transistor M10 conducting more current with reduced gate voltage, Vout− is thus pulled towards ground. Conversely, a decrease in the magnitude of the received photogenerated signal results in an increase at Vout− with transistor M5 more conductive due to the reduced gate voltage, and a decrease at Vout+ with transistor M12 more conductive due to the increased gate voltage at Vin−.
The compensation resistors R5 and R6 may enable sensing of the DC voltage at the Vout+ and Vout− outputs, respectively, which may be compared to a voltage reference by the comparators 303A and 303B, which may then adjust the DC level of the differential signal to Vdd/2 by adjusting the current in the current sources 301A and 301B. The current sources 301A and 301B may comprise push-pull capability to source or sink current, thereby controlling the DC level to the midpoint, Vdd/2.
FIG. 4 is a flow chart illustrating exemplary steps in the operation of a split domain transimpedance amplifier with domain combiner, in accordance with an embodiment of the invention. In step 403, after start step 401, a photogenerated signal may be sensed at both the anode and cathode of a photodiode. In step 405, the two electrical signals may be amplified by different voltage domain circuitry to reduce the voltage swing in each domain. In step 407, the signal may be amplified and/or inverted and combined into a single voltage domain. In step 409, a Vdd/2 differential signal may be output, followed by end step 411.
In an embodiment of the invention, a method and system are disclosed for amplifying complementary received signals in a plurality of partial voltage domains. The signals may be combined into a single differential signal in a single voltage domain. Each of the partial voltage domains may be offset by a DC voltage from the other partial voltage domains. The sum of the partial domains may be equal to a supply voltage of the integrated circuit 130. The complementary signals may be received from a photodiode PD1. The amplified received signals may be amplified via stacked common source amplifiers, common emitter amplifiers, or stacked inverters M1/M2/M3/M4. The amplified received signals may be DC coupled prior to combining. The complementary received signals may be amplified and combined via cascode amplifiers M5/M6/M7/M8 and M9/M10/M11/M12. The voltage domains may be stacked, and may be controlled via feedback loops. The photodetector PD1 may be integrated in the integrated circuit 130.
While the invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the present invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present invention without departing from its scope. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed, but that the present invention will include all embodiments falling within the scope of the appended claims.

## Claims

1. A method for processing signals, the method comprising:
in an integrated circuit:
amplifying complementary received signals in a plurality of partial voltage domains; and
combining said amplified received signals, utilizing a stacked cascode amplifier for each domain, into a single differential signal in a single voltage domain, said stacked cascode amplifiers comprising a feedback loop having a comparator which controls a current source in each domain.

2. The method according to claim 1, comprising receiving said complementary signals from a photodiode.

3. The method according to claim 2, wherein said photodiode is integrated in said integrated circuit.

4. The method according to claim 1, comprising combining said amplified received signals via stacked common source amplifiers.

5. The method according to claim 1, comprising combining said amplified received signals via stacked common emitter amplifiers.

6. The method according to claim 1, comprising amplifying said received signals via stacked inverters.

7. The method according to claim 1, comprising DC coupling said amplified received signals prior to said combining.

8. The method according to claim 1, comprising AC coupling said amplified received signals prior to said combining.

9. The method according to claim 1, comprising amplifying and combining said complementary received signals via cascode amplifiers.

10. The method according to claim 1, wherein said voltage domains are stacked.

11. The method according to claim 10, comprising controlling said stacked voltage domains via feedback loops.

12. A system for processing signals, the system comprising:
in an integrated circuit, one or more circuits operable to amplify complementary received signals in a plurality of partial voltage domains; and
said one or more circuits being operable to combine said amplified received signals into a single differential signal, utilizing a stacked cascode amplifier for each domain, in a single voltage domain, said stacked cascode amplifiers comprising a feedback loop having a comparator which controls a current source in each domain.

13. The system according to claim 12, wherein said one or more circuits are operable to receive said complementary signals from a photodiode.

14. The system according to claim 13, wherein said photodiode is integrated in said integrated circuit.

15. The system according to claim 12, wherein said one or more circuits are operable to combine said amplified received signals via stacked common source amplifiers.

16. The system according to claim 12, wherein said one or more circuits combine said amplified received signals via stacked common emitter amplifiers.

17. The system according to claim 12, wherein said one or more circuits amplify said received signals via stacked inverters.

18. The system according to claim 12, wherein said one or more circuits are operable to DC couple said amplified received signals prior to said combining.

19. The system according to claim 12, wherein said one or more circuits are operable to AC couple said amplified received signals prior to said combining.

20. The system according to claim 12, wherein said one or more circuits are operable to amplify and combine said complementary received signals via cascode amplifiers.

21. The system according to claim 12, wherein said voltage domains are stacked.

22. The system according to claim 12 wherein said one or more circuits are operable to control said stacked voltage domains via feedback loops.