Simultaneous bandwidth extension at high gain and peaking reduction at minimum gain for wideband, variable gain, linear optical receivers

An optical communication system, a linear optical receiver, and an Integrated Circuit (IC) chip are disclosed, among other things. One example of the disclosed IC chip includes a transimpedance amplifier that receives an input electrical signal from a photodiode and provides an amplified version of the input electrical signal as an output, at least one variable gain amplifier that receives the amplified electrical signal output by the transimpedance amplifier and a bandwidth control mechanism that extends a bandwidth of the second amplified output at a maximum gain of the second amplification phase and also reduces a peaking of the second amplified output at a minimum gain of the second amplification phase.

FIELD OF THE DISCLOSURE

The present disclosure is generally directed toward amplifiers and their use in various types of systems such as optical communication systems.

BACKGROUND

There exists an ever increasing demand for larger data rates, especially in optical communication systems. This demand is being pushed mostly by rapidly growing data centers and the Cloud. The technical difficulty in design and fabrication of wideband photodiodes and laser diodes for both 850 nm Short Reach (SR), and 1.3 μm Long Reach (LR) applications, however, suggest utilizing more bandwidth efficient modulation techniques such as Pulse Amplitude Modulation (PAM) (e.g., PAM-4) and Quadrature Phase Shift Keying (QPSK) for heterodyne receivers, in which a linear gain/phase Trans-Impedance Amplifier (TIA) is required. Many of these techniques inherently have variations in the incoming light power shined on the photodiode resulting from different losses in the fiber link as well as the fluctuation of the laser output power. Because of this, a Variable Gain Amplifier (VGA) is required in such linear receivers to keep the peak-to-peak output voltage substantially constant. Depending on link budget for different applications, the gain of the receiver might change by 30 dBΩ, resulting in significant bandwidth limitation at high gain, and large peaking at minimum gain. This results in a significant detriment in the quality of the received eye.

DETAILED DESCRIPTION

Various aspects of the present disclosure will be described herein with reference to drawings that are schematic illustrations of idealized configurations. As such, variations from the shapes of the illustrations as a result, for example, circuit configurations, manufacturing techniques and/or tolerances, are to be expected. Thus, the various aspects of the present disclosure presented throughout this document should not be construed as limited to the particular circuit elements illustrated and described herein but are to include deviations in circuits and functionally-equivalent circuit components.

With reference now toFIGS. 1 and 2, an illustrative communication system100in which embodiments of the present disclosure may be utilized will be described. The system100is shown to include one or more transceivers104a,104b, each having a transmitter108and a receiver112. The transceivers104a,104bare shown to communicate with one another via one or more communication channels116that connect a transmitter108with a receiver112. It should be appreciated that embodiments of the present disclosure may also be implemented in a communication system having dedicated transmitters108and receivers112instead of a combination of a transmitter108and receiver112being implemented in a transceiver104.

In a specific, but non-limiting example of the communication system, signals carried between the transmitter108and receiver112are transmitted at a relatively high bit rate (e.g., 10 Gbps, 25 Gbps, or greater) using a modulation technique that doubles the achievable data rate for a given link bandwidth. More specific types of modulation techniques that may be used in the communication system100include, without limitation, a Pulse Amplitude Modulation (PAM)-4 modulation scheme (or a QAM-16 or QAM-64 modulation scheme).

The receiver112, as further depicted inFIG. 2, is configured to receive an input optical signal and produce an output electrical signal using the combined functionality of the optical component(s)208and Analog Front End (AFE)204. Although certain components are depicted as being included in the AFE204, it should be appreciated that embodiments of the present disclosure are not limited to the depicted configuration of components.

Although embodiments of the present disclosure will be described in connection with circuitry for an optical communication system having a variable gain amplifier and transimpedance amplifier, it should be appreciated that embodiments of the present disclosure are not so limited. To the contrary, any circuit element exhibiting a behavior that can benefit from some type of distortion-improvement scheme disclosed herein could utilize embodiments of the present disclosure. Furthermore, embodiments of the present disclosure are not limited to communication systems or optoelectronic components of communication systems. Instead, embodiments of the present disclosure can be used in a wide variety of environments including computing applications, server applications, data centers, etc.

In some embodiments, some or all of the components included in the receiver112or the AFE204may be provided on an Integrated Circuit (IC) chip or a collection of IC chips. The IC chip implementing the various components of the AFE204may correspond to an Application Specific IC (ASIC) chip or the like. In some embodiments, the TIA(s)212, other amplifiers216, and other circuitry220may be entirely implemented in an IC chip whereas the optical component(s) are not or are implemented on a different IC chip. In other embodiments, the TIA(s)212and other amplifier(s)216may be implemented on an IC chip whereas the other circuitry220may be implemented on a separate IC chip or as discrete circuit components connected to a Printed Circuit Board (PCB).

As will be discussed in further detail herein, an approach is presented herein in which the TIA's212bandwidth and linearity are greatly improved. With reference now toFIG. 3, additional details of a circuit300which may be included as part of the AFE204will be described in accordance with at least some embodiments of the present disclosure. The circuit300is shown to include a photodiode D, a first amplifier308, a plurality of second amplifiers316a-c, an output driver320, a gain control loop304, an amplifier control circuit312, and a circuit output324. In this figure we have to add the VGA as one of the amplifier with the control circuits.

The photodiode D may correspond to one example of an optical component208. The photodiode D may be biased by input voltage Vcc connected to the photodiode D through a supply resistor Rs.

The first amplifier308may correspond to an example of the TIA212and may further correspond to a first amplification stage of the circuit300. In addition to including the amplifier itself, the TIA212may also include a feedback resistor Rf connected between an input and an output of the first amplifier308. The feedback resistor Rf may include a static resistance or a controllable feedback network that is controlled by control circuit312. In some embodiments, the control circuit312may comprise one or more control elements that adjust the feedback resistor Rf or other values of feedback components in the feedback loop of the first amplifier308.

The output of the first amplifier308is provided to the series of second amplifiers316a,316b,316c, which may correspond to examples of other amplifiers216. Some or all of the second amplifiers316a,316b,316cmay also be considered a second amplification stage of the circuit300, either individually or collectively. In some embodiments, the amplifiers316a-ccomprise variable gain amplifiers that are each controlled with a gain control voltage336output by an integrator332and peak detector328in the gain control loop304. Specifically, the variable gain amplifiers316a-cmay have their control voltage adjusted as the output signal324changes over time. The change in the control voltage336may be implemented by the peak detector328detecting peaks and/or valleys in the output signal324and then provided information about such detected peaks and valleys to the integrator332. The integrator332may integrate the output of the peak detector328with a reference voltage Vref, which may correspond to a predetermined reference or threshold voltage. In other words, if the peak detector328detects peaks of the output signal324to exceed the reference voltage Vref, then the integrator332may adjust the control voltage336, thereby altering the amount of gain applied by the variable gain amplifiers316a-c.

AlthoughFIG. 3shows a series of three variable gain amplifiers, it should be appreciated that a greater or lesser number of variable gain amplifiers316a-ccan be incorporated into the circuit300without departing from the scope of the present disclosure. Furthermore, the configuration of variable gain amplifiers316a-cmay be the same or they may be different from one another without departing from the scope of the present disclosure.

The output driver320may correspond to an example of other circuitry220. In some embodiments, the output driver320receives the output from the plurality of variable gain amplifiers316a-cand produces the output signal324. The output driver320may include a 50 ohm output driver having two pairs of differential transistors connected to one another in a known fashion.

As discussed herein above, a conventional circuit300used in a communication system, such as the optical communication system100may have to be configured to accommodate fluctuations in laser/light source output power. In particular, when a signal is received at the photodiode D having a relatively high power, there is usually a need for minimum gain Gmin as shown inFIG. 4. On the other hand, when the received signal has a relatively low power, then there is a need for using an amount of gain closer to the maximum gain Gmax. As can be seen inFIG. 4, however, the reality of the circuits300frequency response when operating between the extremes of the minimum gain Gmin and maximum gain Gmax is that there is undesired peaking (at minimum gain Gmin) and bandwidth losses (at maximum gain Gmax).

It is, therefore, one aspect of the present disclosure to provide a solution that simultaneously mitigates both negative effects. In other words, a solution is provided herein that extends the bandwidth of the receiver at high/maximum gain conditions while also reducing peaking at low/minimum gain conditions.

In particular, and with reference now toFIGS. 5A-D, various configurations of a circuit504(or circuit portion) used to simultaneously combat the negative effects at high and low gain conditions for a linear optical receiver will be described in accordance with at least some embodiments of the present disclosure. With reference initially toFIG. 5A, a first configuration of the circuit504is shown whereby a feedback capacitance Cf is introduced into a positive feedback loop of one of the variable gain amplifiers316. It should also be appreciated that the feedback capacitance Cf may be introduced across the VGA without departing from the scope of the present disclosure. Although the feedback capacitance Cf is depicted as being provided by a single capacitor, it should be appreciated that any combination of a number of capacitors can be used to create the feedback capacitance Cf. One advantage of implementing the feedback capacitor Cf is that the dynamic range of the receiver112can be maintained at a desirable amount (e.g., 30 dBΩ) even when operating between the extremes of the minimum gain Gmin and maximum gain Gmax.

When the gain is at or near the maximum gain Gmax, the input capacitance Cin introduced by the positive feedback loop composed of the variable gain amplifier316and the feedback capacitance Cf to the previous stage (e.g., the TIA308), is given by the Miller theorem as: Cin=Cf(1−Gmax); where Gmax is assumed to be greater than 1. Because the maximum gain Gmax is larger than 1, the input capacitance Cin will be negative in this scenario, thereby reducing the total capacitance at the node between the TIA308and the variable gain amplifier316(e.g., at node A). Reducing the total capacitance at node A generates a peaking in the transfer function of that node A, thereby increasing the overall bandwidth of the variable gain amplifier316as can be seen inFIG. 6A. Another way to look at the input capacitance Cin in this scenario (where the input capacitance Cin is negative) is that the creation of the negative capacitance effectively creates an inductance for the positive feedback loop, which speeds up the operation of the circuit504.

On the other hand, when the gain is at or near the minimum gain Gmin, the similar equation described above can be used to determine the input capacitance Cin: Cin=Cf(1−Gmin); where Gmin is assumed to be less than or equal to 1. This creates a positive input capacitance Cin, which effectively adds a capacitor to the node A, thereby reducing the bandwidth at the node A and reducing the peaking as shown inFIG. 6B.

As can be seen from the above, by introducing the feedback capacitance Cf into the feedback loop of the variable gain amplifier316, the effective operating bandwidth of the circuit300can be extended and the deleterious effects of peaking (when operating at or near the minimum gain Gmin) can also be addressed. Utilization of the feedback capacitance Cf effectively helps extend the bandwidth of the receiver112between the minimum gain Gmin and the maximum gain Gmax.

FIGS. 5B-Ddepict other variations of the circuit504that can provided similar results to the circuit504depicted inFIG. 5A. In particular, the circuit504configuration depicted inFIG. 5Bshows that a feedback capacitance Cf can be applied across a plurality or all of the variable gain amplifiers316a-N in the second amplification stage. Alternatively or additionally, a feedback capacitance Cf may be applied across one of the variable gain amplifiers (e.g., the first variable gain amplifier316a) without being applied across others of the variable gain amplifiers in the second amplification stage as shown inFIG. 5C. Alternatively or additionally, individual feedback capacitances Cf may be applied across individual ones of the variable gain amplifiers316a-N in the second amplification stage as shown inFIG. 5D. In this particular configuration, the feedback capacitance Cf applied across one of the variable gain amplifiers may be the same as or different from the feedback capacitance Cf applied across another one of the variable gain amplifiers. In other words, the individual feedback capacitance Cf values may be the same as one another or different from one another without departing from the scope of the present disclosure.

Specific details were given in the description to provide a thorough understanding of the embodiments. However, it will be understood by one of ordinary skill in the art that the embodiments may be practiced without these specific details. In other instances, well-known circuits, processes, algorithms, structures, and techniques may be shown without unnecessary detail in order to avoid obscuring the embodiments.