Patent Description:
Certain communication devices transmit and receive signals over a wired communication medium, such as a telephone or cable line. These communication devices can include a transmitter and a receiver (sometimes referred to collectively as a transceiver) coupled to the communication medium through a line interface. Among other things, the line interface generally acts as the electrical interface between the transceiver and the line.

Transmitted signals can be reflected back to the receiver so as to constitute part of the received signals. These reflected signals are often referred to as "echoes. " To improve the dynamic range of the receiver, it can be desirable to monitor and cancel these echoes at the transmitter.

<CIT> discloses an apparatus and a method for predistortion of a radio frequency signal. At least one predistortion unit is described that is arranged to receive an analog baseband signal before amplification and to generate an analog predistortion signal based on said analog baseband signal.

<CIT> discloses a linear FET feedback amplifier that includes a Darlington transistor pair. The circuit has a frequency bias feedback network communicatively coupled between the output transistor and the input node for providing biasing to the Darlington transistor pair as well as for adjusting a phase and amplitude of the input signal.

The present inventor has recognized a need to improve the headroom and power consumption of an analog-to-digital converter (ADC) in a wired communication network. The present inventor has recognized that the distortion signal of the output power amplifier circuit (also referred to as a "doubler") can be resolved from the intended output signal of the output power amplifier circuit and can be fed back to a lower power and lower cost ADC for signal processing in an echo cancellation scheme.

In a first aspect, this disclosure is directed to a power amplifier circuit as defined in claim <NUM>.

In a second aspect, this disclosure is directed to a method of reducing an amount of distortion in an output signal of a power amplifier circuit as defined in claim <NUM>.

Power amplifier circuits can generate distortion signals as a result of nonlinearities in the components of the power amplifier. One issue with monitoring a reflected signal of a full duplex system for echo cancellation is that the reflected signal often includes both the desired fundamental signal and the distortion signal of the power amplifier. A result can be a need for a very high-performance analog-to-digital converter (ADC) to capture both the desired fundamental signal and the distortion signal of the reflected signal. The output of the ADC can be used by a digital signal processor to correct for any echo in the communications system, e.g., broadband communications systems such as cable television.

The present inventor has recognized a need to improve the headroom and the power consumption of the ADC in the system. The present inventor has recognized that the intermodulation distortion terms of the output power amplifier circuit (also referred to as a "doubler") can be resolved from the intended output signal of the output power amplifier circuit and can be fed back to a combiner circuit and combined with the input signal to reduce or null the amount of distortion in the transmit signal.

This disclosure describes techniques for monitoring a distortion signal of a power amplifier circuit of a broadband communications system, where the output of a distortion monitoring circuit includes little or no fundamental signal and closely represents the actual distortion of the amplifier circuit of a wired communications system. Using various techniques of this disclosure, the power amplifier circuit can generate a distortion feedback signal that does not affect the power amplifier's output power capability, e.g., no inherent loss in the fundamental output of the amplifier. That is, using a distortion monitor circuit, the power amplifier circuit can resolve a distortion feedback signal from the intended output signal of the output power amplifier circuit. These techniques contrast with other approaches in which a power amplifier's RF output is sampled, which will reduce the strength of the amplifier output and result in power loss.

This disclosure describes, among other things, two monitoring approaches that can be used either separately or together to monitor the distortion of the power amplifier circuit in the system: a partial (e.g., bottom stage transconductance) monitoring approach and a full output monitoring approach. The partial monitoring approach and the full output monitoring approach both can utilize "virtual ground" potential (also referred to as simply "virtual ground" in this disclosure) techniques. These approaches can provide adequate cancellation of the fundamental signal, resulting in a distortion monitoring output signal that predominantly includes the intermodulation distortion signal terms, e.g., the third order intermodulation distortion terms.

<FIG> is an example of a power amplifier circuit including distortion monitoring circuitry that can be used for data communication over a path in a wired communication network. The power amplifier circuit <NUM> (also referred to in this disclosure as "amplifier circuit <NUM>"), e.g., a push-pull amplifier configuration, can include a balun <NUM> coupled to an input <NUM> of the amplifier circuit <NUM>, a transformer <NUM> coupled to an output <NUM> of the amplifier circuit <NUM>, a first pair of transistors 110A, 110B (collectively referred to as "transistors <NUM>") coupled to the input <NUM> to convert an input voltage to a current, and a second pair of transistors 112A, 112B (collectively referred to as "transistors <NUM>") coupled to the output <NUM> of the amplifier circuit <NUM> to convert the current to an output voltage. The first pair of transistors <NUM> and the second pair of transistors <NUM> can include field-effect transistors.

In some example configurations, the first pair of transistors <NUM> and the second pair of transistors <NUM> can be different types of transistors. For example, the first pair of transistors <NUM> can include high-electron-mobility (HEMT) transistors, e.g., pseudomorphic HEMT transistors, and the second pair of transistors <NUM> can include gallium nitride (GaN) transistors.

In the partial (e.g., bottom stage transconductance) monitoring approach mentioned above, the first pair of transistors <NUM> can be cross-coupled such that a terminal of one transistor, e.g., gate terminal, is coupled to a terminal of the other transistor, e.g., source terminal. A distortion monitoring circuit can be included that has a first resistor network <NUM> and a second resistor network <NUM>. The first resistor network <NUM> can be coupled between a control terminal <NUM> of the transistor 110A, e.g., a gate terminal, and a second terminal <NUM> of the transistor 110B, e.g., a source terminal. The first resistor network <NUM> can include two resistors <NUM>, <NUM>.

Similarly, the second resistor network <NUM> can be coupled between a control terminal <NUM> of the transistor 110B, e.g., a gate terminal, and a second terminal <NUM> of the transistor 110A, e.g., a source terminal. The second resistor network <NUM> can include two resistors <NUM>, <NUM>.

The resistance values of the two resistors <NUM>, <NUM> of the first resistor network <NUM> are sized to minimize or null a fundamental signal at the node <NUM> (labeled "NULL1"), resulting in a virtual ground at node <NUM> and a signal having predominantly intermodulation distortion terms. Similarly, the resistance values of the two resistors <NUM>, <NUM> of the second resistor network <NUM> are sized to minimize or null a fundamental signal at the node <NUM> (labeled "NULL2"), resulting in a virtual ground at node <NUM> and a second distortion signal having predominantly intermodulation distortion terms. Because of the phase inverting nature of the amplifier, the virtual ground structure can help ensure that the fundamental signal is cancelled. However, the distortion signals generated in the amplifier may not be within a virtual ground structure and hence may not cancel at "NULL1" and "NULL2".

The signals at node <NUM> and node <NUM> represent the distortion generated by the transistors 110A, 110B. Node <NUM> and node <NUM>, e.g., high impedance nodes, can provide monitoring points for the distortion caused by transistors 110A, 110B, which can be the dominant sources of distortion in the amplifier circuit <NUM>. In this manner, the power amplifier circuit <NUM> can resolve distortion signals at nodes <NUM>, <NUM> from the intended output signal of the power amplifier circuit <NUM> rather than sampling the output signal itself from the output <NUM>, which would reduce the strength of the amplifier output and result in power loss. As such, the distortion signals at nodes <NUM>, <NUM> do not affect the power amplifier's output power capability, e.g., no inherent loss in the fundamental output of the amplifier.

One or both the first and second distortion signals at node <NUM> and node <NUM> can be coupled to an output of the power amplifier circuit to provide a distortion feedback signal that can be fed back to a combiner circuit and combined with the input signal to reduce or null the amount of distortion in the transmit signal, as shown and described below with respect to <FIG>, <FIG>, and <FIG>.

In some example configurations, the first node <NUM> and the second node <NUM> can be coupled to inputs of a differential amplifier (not depicted), and an output of the differential amplifier can be coupled to an output of the power amplifier circuit to provide the distortion feedback signal. The distortion feedback signal can correspond with the distortion of the amplifier circuit <NUM>. The distortion feedback signal can be fed back to a combiner circuit and combined with the input signal to reduce or null the amount of distortion in the transmit signal, as shown and described below with respect to <FIG>, <FIG>, and <FIG>.

In the full output monitoring approach mentioned above, a distortion monitoring circuit can be included that has a third resistor network <NUM> and a fourth resistor network <NUM>. The third resistor network <NUM> can be coupled in a first feedback path between the input <NUM> and the output <NUM> of the power amplifier circuit <NUM>. Similarly, the fourth resistor network <NUM> can be coupled in a second feedback path between the input <NUM> and the output <NUM> of the power amplifier circuit <NUM>.

The resistance values of the two resistors <NUM>, <NUM> of the third resistor network <NUM> can be sized to minimize or null a fundamental signal at the node <NUM> (labeled "NULL_OUT1"), resulting in a virtual ground at node <NUM> and a third distortion signal having predominantly intermodulation distortion terms, e.g., third order intermodulation distortion terms. Similarly, the resistance values of the two resistors <NUM>, <NUM> of the fourth resistor network <NUM> can be sized to minimize or null a fundamental signal at the node <NUM> (labeled "NULL_OUT2"), resulting in a virtual ground at node <NUM> and a fourth distortion signal having predominantly intermodulation distortion terms, e.g., third order intermodulation distortion terms.

The signals at node <NUM> and node <NUM> can represent the distortion generated by the entire amplifier circuit <NUM>, not just the transistors 110A, 110B as in the partial monitoring approach. Node <NUM> and node <NUM>, e.g., high impedance nodes, can provide monitoring points for all the distortion terms generated by the circuit <NUM>. In this manner, the power amplifier circuit <NUM> can resolve distortion signals at nodes <NUM>, <NUM> from the intended output signal of the power amplifier circuit <NUM> rather than sampling the output signal itself from the output <NUM>, which would reduce the strength of the amplifier output and result in power loss. As such, the distortion signals at nodes <NUM>, <NUM> do not affect the power amplifier's output power capability, e.g., no inherent loss in the fundamental output of the amplifier.

One or both node <NUM> and node <NUM> can be coupled to an output of the power amplifier circuit to provide a distortion feedback signal that can be fed back to a combiner circuit and combined with the input signal to reduce or null the amount of distortion in the transmit signal, as shown and described below with respect to <FIG>, <FIG>, and <FIG>.

In some example configurations, the third node <NUM> and the fourth node <NUM> can be coupled to inputs of a differential amplifier (not depicted), and an output of the differential amplifier can be coupled to an output of the power amplifier circuit to provide the distortion feedback signal. The distortion feedback signal can correspond with the distortion of the amplifier circuit. The distortion feedback signal can be fed back to a combiner circuit and combined with the input signal to reduce or null the amount of distortion in the transmit signal, as shown and described below with respect to <FIG>.

In some example configurations, the partial monitoring approach and the full monitoring approach can be used together such that the distortion signals produced by both approaches can be fed back to a combiner circuit and combined with the input signal to reduce or null the amount of distortion in the transmit signal.

The partial monitoring approach and the full monitoring approach can be sensitive to changes in source impedance. To minimize the effects of source impedance variation, a buffer circuit (not depicted) can be included before the amplifier circuit <NUM>.

<FIG> shows an example of a graph of linear responses of an output signal at a first output port of an amplifier circuit and a distortion feedback signal at a second output port of the amplifier circuit, for different frequencies. The x-axis represents frequency (GHz) and the y-axis represents gain (dB).

The line <NUM> depicts the forward gain of the amplifier circuit <NUM> of <FIG>, at approximately 25dB at <NUM>. The line <NUM> depicts the power of the fundamental signal produced at the output nulling ports "NULL_OUT1" and "NULL_OUT2" in <FIG> using the full monitoring approach described above, at approximately -18dB at <NUM>. The line <NUM> shows that the power of the fundamental signal has been substantially reduced using the full monitoring approach.

<FIG> shows an example of a circuit in a data communication over a path in a communication network that can implement various techniques of this disclosure. The circuit <NUM> can include the power amplifier circuit <NUM> of <FIG> to generate an amplified output signal and the distortion feedback signal. The amplifier circuit <NUM> can include an input <NUM>, a first output <NUM> to provide the amplified output signal, and a second output <NUM> to provide the distortion feedback signal from one or more of node <NUM>, node <NUM>, node <NUM>, and node <NUM> of <FIG>.

The circuit <NUM> can include a combiner circuit <NUM> to receive an input signal <NUM> at a first input <NUM>, e.g., including the data of interest for transmission, and receive at a second input <NUM> the distortion signal <NUM> fed back from the amplifier circuit <NUM>, e.g., with a phase change. The combiner circuit <NUM> can combine the input signal <NUM> and the distortion feedback signal <NUM> and generate a combined signal <NUM> at an output <NUM> of the combiner circuit <NUM>. For example, the combiner circuit <NUM> can add an inverted representation of the distortion feedback signal to the input signal.

The circuit <NUM> can include a scaling circuit <NUM> coupled between the output <NUM> of the power amplifier circuit <NUM> and the second input <NUM> of the combiner circuit <NUM>. The scaling circuit <NUM> can adjust a magnitude and phase of the distortion feedback signal <NUM> to correspond with a magnitude and phase of the input signal <NUM> to improve the distortion nulling effect of the combination.

<FIG> shows an example of a graph of nulled output third order intercept points ("OIP3") of an amplifier circuit utilizing various techniques of this disclosure. The x-axis represents power (dBm) and the y-axis represents OIP3 power (dBm).

The line <NUM> represents the output of the amplifier circuit <NUM> of <FIG>, without the distortion signal being fed back via a nulling loop to the combiner circuit <NUM> of <FIG>. The lines shown generally at <NUM> represent the output of the amplifier circuit <NUM> of <FIG> with the distortion signal fed back to the combiner circuit <NUM> of <FIG> with the difference in the various lines at <NUM> being the result of different scaling magnitudes used by a scaling circuit, e.g., scaling circuit <NUM> of <FIG>. As seen in <FIG>, the lines shown generally at <NUM> represent about a <NUM> dBm improvement in OIP3, and a <NUM> dBm intermodulation improvement.

<FIG> is an example of a method <NUM> of transmitting data over a path in a wired communication network using various techniques of this disclosure. At block <NUM>, the method <NUM> can include generating a distortion feedback signal at a first output of a power amplifier circuit of a transmission circuit in the communication network. For example, the power amplifier circuit <NUM> of <FIG> can generate a distortion feedback signal at the output <NUM> of <FIG>, e.g., using the partial (e.g., bottom stage transconductance) monitoring approach and/or the full output monitoring approach described above.

At block <NUM>, the method <NUM> can include receiving an input signal at a first input of a combiner circuit of the transmission circuit. At block <NUM>, the method <NUM> can include receiving the distortion feedback signal at a second input of the combiner circuit. For example, the combiner circuit <NUM> of <FIG> can receive an input signal <NUM> at input <NUM> and the distortion feedback signal <NUM> at input <NUM>.

At block <NUM>, the method <NUM> can include combining the input signal and the distortion feedback signal and generating a combined signal at an output of the combiner circuit, e.g., output <NUM> of the combiner circuit <NUM> in <FIG>.

At block <NUM>, the method <NUM> can include receiving the combined signal at a second input of the power amplifier circuit and generating an amplified output signal for transmission. For example, the power amplifier circuit <NUM> of <FIG> can receive the combined signal at input <NUM> and generate at output <NUM> an amplified output signal <NUM> for transmission.

<FIG> is simplified version of a portion of the power amplifier circuit of <FIG>. In the circuit <NUM> of <FIG>, resistors RFB1 and RFB2 can represent the two resistors <NUM>, <NUM> of the third resistor network <NUM> of <FIG>, and the "monitor" node can represent the node <NUM> (labeled "NULL_OUT1") or the node <NUM> of <FIG>. The present inventor has recognized that the monitor node of <FIG>, e.g., nodes <NUM> or <NUM> of <FIG> and similarly node <NUM> ("NULL_OUT2") or node <NUM> of <FIG>, may not differentiate between forward and reverse responses if it does not include directional capability.

In addition, the present inventor has recognized that the circuit of <FIG> can have a sensitivity to impedances. For example, the input node of <FIG> can be driven from differing input impedances from customer networks, which can impact the virtual ground balance and the monitor nulling of the circuit. Ideally, when balanced, only a distortion signal appears at the "monitor" output.

In addition, input impedance variations can impact the virtual ground balance of the circuit. Further, the output node of <FIG> can drive into uncertain output impedances, e.g., of a coaxial network, which can lead to unwanted reflections as customers change their networks.

To overcome or mitigate any sensitivity to impedances, the present inventor has recognized that a solution can include an addition of a directional device on the output of the amplifier. Various directional coupler implementations are shown and described below with respect to <FIG>.

<FIG> is a schematic diagram of an example of a power amplifier circuit including distortion monitoring circuitry modified to include directional coupler circuitry. The present inventor has recognized that adding a directional coupler circuit C1 on the amplifier output node can prevent load mismatches from influencing the nulling circuitry of the power amplifier circuit. By including the directional coupler C1 in the circuit <NUM>, the monitor circuit can respond primarily or only to a forward-going wave and not a reverse-going wave by inhibiting, e.g., preventing or mitigating, the reverse-going wave from being transmitted from the output port to the monitor port. In some configurations, the coupler C1 can have low loss between port <NUM> and port <NUM> and high loss between port <NUM> and port <NUM>. Ideally, a signal entering the output node, e.g., from a customer network, would not appear at the monitor node.

The coupler C1 can have various coupling ratios. For example, the resistance values of resistors R1 and R2 can be adjusted such that the fundamental response to the "monitor" port is near zero. <FIG> depict various directional coupler configurations.

<FIG> is a schematic diagram of another example of a power amplifier circuit including distortion monitoring circuitry modified to include directional coupler circuitry. In the circuit <NUM> of <FIG>, the directional coupler circuit can include a directional resistive bridge circuit formed by R1-R3 in which an outgoing signal wave can be sensed as a voltage across resistors R1 and R3. Any reflected waves do not appear across this node if the ratio of R1/R2 is approximately the same as R3/Zout, where Zout is the output impedance, e.g., about <NUM> ohms to about <NUM> ohms. That is, if a reflected wave is received at the output node, e.g., from a customer network, the differential voltage applied to a differential buffer circuit is zero. Thus, the reflected wave is not present, and the forward-going wave appears as part of the monitoring.

A first stage <NUM> of a differential buffer circuit is shown as a first voltage-controlled voltage source (VCVS) and a second stage <NUM> of the differential buffer circuit is shown as a second voltage-controlled voltage source (VCVS). An input signal from the input node can be coupled to resistor RA, and the output of the first stage <NUM> of the differential buffer circuit can be coupled to resistor RB. The first stage <NUM> of the differential buffer circuit can drive the nulling resistor pair formed by resistors RA and RB. The second stage <NUM> of the differential buffer circuit can be coupled between resistors RA and RB and can control the monitor output signal at the "monitor" node.

In some example configurations, it can be desirable to reduce the large signal level present on the directional resistor network R1-R3 of <FIG>. An example configuration that divides the voltages down is shown in <FIG>.

<FIG> is a schematic diagram of another example of a power amplifier circuit including distortion monitoring circuitry modified to include directional coupler circuitry. In contrast to the example shown in <FIG>, the circuit <NUM> shown in <FIG> can divide the signal level at the output node down to lower levels while maintaining the desired bridge impedances for directivity. For example, a voltage divider network including resistors RC and RD can be coupled between resistors R1 and R2 of the directional resistor network to decrease the signal level, as seen in <FIG>.

A directional buffer circuit <NUM>, shown as a VCVS in <FIG>, can be coupled to the node between resistors RC and RD to sense the output signal and applied to pin <NUM> of the differential buffer <NUM>. In addition, feedback resistors RFB <NUM> and RFB2 can be used to decrease the signal level and applied to pin <NUM> of the differential buffer <NUM>.

In some example configurations, feedback resistors RFB <NUM> and RFB2 can perform both the negative feedback and nulling functions while reducing the signal level.

<FIG> is a schematic diagram of another example of a power amplifier circuit including distortion monitoring circuitry modified to include directional coupler circuitry. The circuit <NUM> in <FIG> can use the resistor combination of RFB1 and RFB1 to set a virtual ground and can use resistors R1-R3 as a directional bridge.

In some example configurations, resistors RC and RD can be much larger than resistor R2 and provide signal attenuation so that the fundamental signal input to the differential buffer is approximately zero.

Further, in contrast to the configuration shown in <FIG> in which resistors R2 and RD are connected to ground, the resistors R2 and RD in <FIG> can be connected to the input to enable virtual operation.

In addition to the directional coupler configurations shown above, the present inventor has also recognized that a solution can include a mirrored monitor circuit, as shown in <FIG>. By integrating a mirrored version of the main stage, a virtual ground monitor can be achieved.

<FIG> is a schematic diagram of an example of a power amplifier circuit including distortion monitoring circuitry modified to include mirrored circuitry. The circuit <NUM> of <FIG> can include a main circuit (also referred to as a main stage) <NUM> and a mirrored circuit (also referred to as a mirrored stage) <NUM> that is a replica of the main circuit, e.g., a smaller scaled version of the main circuit. In some example configurations, the mirrored stage <NUM> can be a fraction of the size of the main stage <NUM>, such as about one percent of the size of main stage <NUM>. In the circuit <NUM>, the resistors R1 and R2 can form the virtual ground feedback network. Advantageously, the mirrored output labeled "monitor" can be isolated from the main output node labeled "output".

The main stage <NUM> can be a simplified version of the circuit shown in <FIG>, e.g., an amplifier circuit including a cascode transistor configuration with feedback. The mirrored stage <NUM> can include transistors <NUM>, <NUM>, which can be scaled replicas of the transistors <NUM>, <NUM>, of the main stage <NUM>. The resistances of various resistors of the mirrored stage <NUM> can be chosen such the mirrored stage <NUM> can have a load line similar to a load line of the main stage <NUM>. In addition, the mirrored stage <NUM> can be biased so that the output impedance of the mirrored stage <NUM> can be similar to the output impedance of the main stage <NUM>.

In some example configurations, the circuit <NUM> of <FIG> can include buffer circuitry that can include transistor Qbuf and resistor R4, for example, to further isolate the monitor node. The buffer circuitry can help prevent any signals through the feedback resistor RFB in the main stage <NUM> from coupling through to the mirrored stage <NUM>. Because the monitor node of the mirrored stage <NUM> can be isolated from the output node of the main stage <NUM>, any reverse-going wave appearing at the output node does not appear in the mirrored stage <NUM>. In this manner, any reflections from a customer network coupled to the amplifier circuit will not be seen in the mirror stage.

<FIG> shows another example of a circuit in a data communication over a path in a communication network that can implement various techniques of this disclosure. The circuit <NUM> can include the power amplifier circuit <NUM> of <FIG> to generate an amplified output signal <NUM> and a distortion feedback signal <NUM> (or distortion monitor output). The amplifier circuit <NUM> can include an input <NUM>, a first output <NUM> to provide the amplified output signal, and a second output <NUM> to provide the distortion feedback signal from one or more of node <NUM>, node <NUM>, node <NUM>, and node <NUM> of <FIG>.

In <FIG>, a digital input signal <NUM> can be applied to a combiner circuit <NUM>, e.g., a summation circuit, and an output of the combiner circuit <NUM> is applied to an input of a digital-to-analog converter (DAC) circuit <NUM>, which converts the signal into the analog domain. In some example configurations, a preamplifier circuit <NUM> can optionally be included to boost the signal level into the output amplifier <NUM>. The output amplifier circuit <NUM> can output the distortion feedback signal <NUM> with correlation to the distortion leaving the output amplifier <NUM>. Since the distortion monitor can cancel most of the fundamental response of the amplifier <NUM>, a low cost and low power analog-to-digital converter (ADC) circuit <NUM>, can convert the distortion monitor signal into the digital domain.

The output of the ADC circuit <NUM> can be applied to a first digital filter circuit <NUM> to further cancel out any remaining leakage of fundamental signal present on the distortion feedback signal. In some example implementations, the output of the first digital filter circuit <NUM> can be applied to a second digital filter circuit <NUM> to shape the signal in such a way that distortion from the RF output <NUM> is minimized. Then, an output of the second digital filter circuit <NUM> can be applied to the combiner circuit <NUM> and combined with the digital input <NUM> to reduce or null the amount of distortion in the transmit signal <NUM>.

In <FIG>, a digital input signal <NUM> is applied to a DAC circuit <NUM>, which converts the signal into the analog domain. In some example configurations, a preamplifier circuit <NUM> can optionally be included to boost the signal level into the output amplifier <NUM>. The output amplifier circuit <NUM> can output the distortion feedback signal <NUM> with correlation to the distortion leaving the output amplifier <NUM>. Since the distortion monitor cancels most of the fundamental response of the amplifier <NUM>, an ADC circuit <NUM>, e.g., a low cost and low power ADC, can convert the distortion monitor signal into the digital domain.

The output of the ADC circuit <NUM> can be applied to a first digital filter circuit <NUM> to cancel out any leakage of fundamental signal present on the distortion feedback signal. In some example implementations, the output of the first digital filter circuit <NUM> can be observed by a monitor that allows both any remaining fundamental leakage and the distortion from the RF output <NUM> to be minimized. Then, an output of the second digital filter circuit <NUM> can be applied to a combiner circuit <NUM>. The digital input <NUM> can be applied to a third digital filter circuit <NUM> and combined with the output of the second digital filter circuit <NUM> using the combiner circuit <NUM>.

A directional coupler <NUM> can be used on the RF output <NUM> to receive an intended signal coming from customer premises. Another ADC circuit <NUM> can digitize the return signal <NUM>. The combiner circuit <NUM>, e.g., a summation circuit, can combine: (<NUM>) the filtered input signal <NUM> to cancel reflections of the fundamental downstream signal occurring in the coax plant, (<NUM>) digitally filtered signal representing the distortion <NUM> of this downstream signal, and (<NUM>) the intended return path signal <NUM>. The combiner circuit <NUM> can output a corrected return path signal <NUM> that can remove both the reflected fundamental and the undesirable reflected distortion of the amplifier.

The drawings show, by way of illustration, specific embodiments in which the invention may be practiced. " Such examples may include elements in addition to those shown or described.

Method examples described herein may be machine or computerimplemented at least in part. Some examples may include a computer-readable medium or machine-readable medium encoded with instructions operable to configure an electronic device to perform methods as described in the above examples. An implementation of such methods may include code, such as microcode, assembly language code, a higher-level language code, or the like. Such code may include computer readable instructions for performing various methods. Further, in an example, the code may be tangibly stored on one or more volatile, non-transitory, or nonvolatile tangible computer-readable media, such as during execution or at other times. Examples of these tangible computer-readable media may include, but are not limited to, hard disks, removable magnetic disks, removable optical disks (e.g., compact discs and digital video discs), magnetic cassettes, memory cards or sticks, random access memories (RAMs), read only memories (ROMs), and the like.

The above description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more aspects thereof) may be used in combination with each other.

Also, in the above Detailed Description, various features may be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter may lie in less than all features of a particular disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description as examples or embodiments, with each claim standing on its own as a separate embodiment.

Claim 1:
A power amplifier circuit (<NUM>) configured to reduce an amount of distortion in an output signal in a wired communication network, the circuit comprising:
an input (<NUM>) to receive an input signal;
a distortion monitoring circuit configured to generate and output a distortion feedback signal at a first output (<NUM>, <NUM>) of the power amplifier circuit (<NUM>),
wherein the distortion monitoring circuit includes a first resistor network (<NUM>), the first resistor network (<NUM>) coupled in a first feedback path between the input (<NUM>) and a second output (<NUM>) of the power amplifier circuit (<NUM>), the first resistor network (<NUM>) having a first node (<NUM>), wherein resistance values of resistors in the first resistor network (<NUM>) are sized to minimize a fundamental signal at the first node (<NUM>) in the first resistor network (<NUM>),
wherein the distortion monitoring circuit includes a second resistor network (<NUM>), the second resistor network (<NUM>) coupled in a second feedback path between the input (<NUM>) and the second output (<NUM>) of the power amplifier circuit (<NUM>), the second resistor network (<NUM>) having a second node (<NUM>), wherein the resistance values of the resistors in the second resistor network (<NUM>) are sized to minimize a fundamental signal at a second node (<NUM>) in the second resistor network (<NUM>),
wherein at least one of the first node (<NUM>) in the first resistor network (<NUM>) and the second node (<NUM>) in the second resistor network (<NUM>) are coupled to the first output of the power amplifier circuit (<NUM>) to provide the distortion feedback signal; and
wherein the second output (<NUM>) provides the output signal.