# Patent

## Title
Output matching network for differential power amplifier

## Abstract

An output matching network for a differential power amplifier comprises an output transformer having a center tap and a low pass filter. The output transformer is configured to receive a first amplified signal from a first differential output stage amplifier of the differential power amplifier and provide a first output signal to the low pass filter. The output transformer is also configured to receive a second amplified signal from a second differential output stage amplifier of the differential power amplifier and provide a second output signal to the low pass filter. The low pass filter is configured to receive the first and second output signal from the output transformer and provide a filtered output signal.

## Background
RELATED APPLICATIONS
This application claims the benefit of U.S. provisional patent application No. 62/129,133, filed Mar. 6, 2015, entitled “DIFFERENTIAL INTERSTAGE POWER LIMITER,” the disclosure of which is hereby incorporated herein by reference in its entirety.
This application claims the benefit of U.S. provisional patent application No. 62/132,031, filed Mar. 12, 2015, entitled “COMPACT, HIGH PERFORMANCE, BROADBAND POWER AMPLIFIER ARCHITECTURE,” the disclosure of which is hereby incorporated herein by reference in its entirety.
This application claims the benefit of U.S. provisional patent application No. 62/133,529, filed Mar. 16, 2015, entitled “DIFFERENTIAL POWER AMPLIFIER FOR MOBILE CELLULAR ENVELOPE TRACKING,” the disclosure of which is hereby incorporated herein by reference in its entirety.
This application is also related to U.S. application Ser. No. 14/715,173, filed May 18, 2015, entitled “MULTISTAGE DIFFERENTIAL POWER AMPLIFIER HAVING INTERSTAGE POWER LIMITER,” the disclosure of which is hereby incorporated herein by reference in its entirety.
This application is also related to U.S. application Ser. No. 14/715,181, filed May 18, 2015, entitled “DIFFERENTIAL POWER AMPLIFIER FOR MOBILE CELLULAR ENVELOPE TRACKING,” the disclosure of which is hereby incorporated herein by reference in its entirety.


FIELD OF THE DISCLOSURE
The present disclosure relates to power amplifier circuitry, and specifically to differential power amplifiers for use in a mobile device.
BACKGROUND
The complexity of a cellular handset has increased dramatically over recent years because of increased band proliferation as the world aggressively moves to 4G (fourth generation) mobile telephone technology. The size of an RF component might be its most important feature now.
The number of supported bands in high-end smartphones now exceeds twenty (20). Most of these bands require their own duplex filter, which means the size of an RF component is one of the most (if not THE most) important feature. In most cases, RF component suppliers are competing for the same socket and are given the same footprint requirement. It is up to suppliers to determine how to budget the space they have available. If they use too much area on the power amplifier, they will need to use smaller, lower performance filters and surface mount devices (SMDs). If they choose to use premium, higher performance filters, the power amplifier area must be reduced. Thus, there remains a need to the ability to cover all of the cellular handset bands in a single power amplifier core.
The present disclosure relates to a power amplifier architecture design that covers all of the cellular handset bands from 698-915 MHz in a single power amplifier core. The disclosed power amplifier architecture is designed to simultaneously achieve broad bandwidth, high performance, compact size, and low cost with almost no compromise in any of these features. In spite of its small size and broad bandwidth, the power amplifier disclosed herein maintains very high performance and is extremely low in cost.
SUMMARY
An output matching network for a differential power amplifier comprises an output transformer having a center tap and a low pass filter. The output transformer is configured to receive a first amplified signal from a first differential output stage amplifier of the differential power amplifier and provide a first output signal to the low pass filter. The output transformer is also configured to receive a second amplified signal from a second differential output stage amplifier of the differential power amplifier and provide a second output signal to the low pass filter. The low pass filter is configured to receive the first and second output signal. In one embodiment, the low pass filter comprises an inductor and a capacitor and is configured to filter out higher order harmonics of the first and second amplified signals.
In another embodiment, circuitry is disclosed that includes a differential power amplifier having a differential output stage, an envelope tracker, and an output matching network. The circuitry comprises envelope tracking power supply circuitry configured to provide an envelope power supply signal to differential power amplifier circuitry and to an output matching network. The circuitry further comprises differential power amplifier circuitry. The differential power amplifier circuitry comprises input stage circuitry having at least one input and at least one output, wherein the input stage circuitry is configured to provide a differential signal having a first portion and a second portion. The differential power amplifier circuitry also comprises a first output stage amplifier configured to receive the first portion of the differential signal at a first output stage input and provide a first amplified signal at a first output stage output, and a second output stage amplifier configured to receive the second portion of the differential signal at a second output stage input and provide a second amplified signal at a second output stage output. The circuitry further comprises output matching network circuitry. The output matching network circuitry comprises an output transformer having a center tap, the output transformer configured to receive a first amplified signal from a first differential output stage amplifier of the differential power amplifier and provide a first output signal. The output transformer is also configured to receive a second amplified signal from a second differential output stage amplifier of the differential power amplifier and provide a second output signal. The output matching network circuitry also comprises a low pass filter configured to receive the first and second output signal from the output transformer and provide a filtered output signal.
Those skilled in the art will appreciate the scope of the present disclosure and realize additional aspects thereof after reading the following detailed description in association with the accompanying drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the disclosure, and together with the description serve to explain the principles of the disclosure.
FIG. 1 shows a generic power amplifier with a single-ended output stage.
FIG. 2 is a schematic of a differential multistage power amplifier having a differential output stage and an output matching network, according to one exemplary embodiment.
FIG. 3 is a schematic of a differential multistage power amplifier having a differential output stage, a connected envelope tracker, and an alternate output matching network, according to another exemplary embodiment.
FIG. 4 is a schematic of a differential multistage power amplifier having a differential output stage, a connected envelope tracker, and an output matching network, without a ferrite, according to another exemplary embodiment.
FIG. 5A shows simulation results of a differential power amplifier with an output matching network, according to an exemplary embodiment.
FIG. 5B shows a graph representing simulation results of a differential power amplifier with an output matching network, according to an exemplary embodiment.
FIG. 6 is a graph representing harmonic rejection of a differential power amplifier with an output matching network, according to an exemplary embodiment.
FIG. 7 shows simulation results of a differential power amplifier where the separate impedances of the transformer section and the low pass filter are indicated, according to an exemplary embodiment.
FIG. 8 is a graph that shows the system efficiency of a differential multistage power amplifier having a differential output stage and including a Tx band switch and an envelope tracker.
FIG. 9 is a graph that shows the system efficiency of a differential multistage power amplifier having a differential output stage and an envelope tracker without the Tx band switch loss.



DETAILED DESCRIPTION
The complexity of a cellular handset has increased dramatically over recent years because of increased band proliferation as the world aggressively moves to 4G. The number of supported bands in high-end smartphones now exceeds twenty (20). Most of these bands require their own duplex filter, which means the size of an RF component is one of the most (if not THE most) important feature. In most cases, RF component suppliers are competing for the same socket and are given the same footprint requirement. It is up to suppliers to determine how to budget the space they have available. If they use too much area on the power amplifier, they will need to use smaller, lower performance filters and SMDs. If they choose to use premium, higher performance filters, the power amplifier area must be reduced. The present disclosure relates to a power amplifier architecture designed to simultaneously achieve broad bandwidth, high performance, compact size, and low cost with almost no compromise in any of these features.
An output matching network for a differential power amplifier comprises an output transformer having a center tap and a low pass filter. The output transformer is configured to receive a first amplified signal from a first differential output stage amplifier of the differential power amplifier and provide a first output signal to the low pass filter. The output transformer is also configured to receive a second amplified signal from a second differential output stage amplifier of the differential power amplifier and provide a second output signal to the low pass filter. The low pass filter is configured to receive the first and second output signal. In one embodiment, the low pass filter comprises an inductor and a capacitor and is configured to filter out higher order harmonics of the first and second amplified signals.
In another embodiment, circuitry is disclosed that includes a differential power amplifier having a differential output stage, an envelope tracker, and an output matching network. The circuitry comprises envelope tracking power supply circuitry configured to provide an envelope power supply signal to differential power amplifier circuitry and to an output matching network. The circuitry further comprises differential power amplifier circuitry. The differential power amplifier circuitry comprises input stage circuitry having at least one input and at least one output, wherein the input stage circuitry is configured to provide a differential signal having a first portion and a second portion. The differential power amplifier circuitry also comprises a first output stage amplifier configured to receive the first portion of the differential signal at a first output stage input and provide a first amplified signal at a first output stage output, and a second output stage amplifier configured to receive the second portion of the differential signal at a second output stage input and provide a second amplified signal at a second output stage output. The circuitry further comprises output matching network circuitry. The output matching network circuitry comprises an output transformer having a center tap, the output transformer configured to receive a first amplified signal from a first differential output stage amplifier of the differential power amplifier and provide a first output signal. The output transformer is also configured to receive a second amplified signal from a second differential output stage amplifier of the differential power amplifier and provide a second output signal. The output matching network circuitry also comprises a low pass filter configured to receive the first and second output signal from the output transformer and provide a filtered output signal. This output matching network is very small, high performance, and supports broadband.
FIG. 1 shows a generic power amplifier with a single-ended output stage. A power amplifier 10 may be connected via its output to an output matching network 11. The power amplifier 10 in FIG. 1 has a first stage amplifier 12 and a second stage amplifier 15, each of which has a single input and a single output, making it a single-ended output. An inductor 13 and other processing circuitry 14 may be coupled to the output of the first stage amplifier 12. An inductor 16 may be coupled to the output of the second stage amplifier 15. Voltage supplies VCC1 17 and VCC2 18, coupled to capacitors 19(1) and 19(2), respectively, provide power to the power amplifier 10. This single-ended output stage design is vulnerable to RF feedback to the input stage, especially when tied to an envelope tracker, due to the fact that the envelope tracker can drive a limited amount of bypass capacitance.
Typically, a ferrite, such as ferrite 20 in FIG. 1, is required to help isolate the first and second stages of the power amplifier 10. However, the ferrite 20 has its own disadvantages. For example, the ferrite 20 causes spectral regrowth because of its “memory effect.”
A differential multistage power amplifier having a differential output stage and connected to a high performance output matching network, as disclosed herein, offers many advantages over the generic power amplifier with the ferrite 20 shown in FIG. 1. One attribute of the disclosed differential power amplifier design is that the output matching network is very small, high performance, and supports broadband. Another attribute of the design is that it uses a differential output stage, which gives it several advantages over a single-ended output stage, as discussed in U.S. application Ser. No. 14/715,181, filed May 18, 2015, entitled “Differential Power Amplifier for Mobile Cellular Envelope Tracking,” the disclosure of which is hereby incorporated herein by reference in its entirety. In addition, when the differential output stage is used with an envelope tracker, it is unique and has better performance than a single-ended power amplifier design.
FIG. 2 is a schematic of a differential multistage power amplifier having a differential output stage and an output matching network, according to an exemplary embodiment. The design in FIG. 2 is somewhat unique in that it has a differential output stage, which has several advantages over a single-ended output stage, as discussed more fully in co-owned related application U.S. application Ser. No. 14/715,181, filed May 18, 2015, entitled “Differential Power Amplifier for Mobile Cellular Envelope Tracking.” The differential power amplifier 10 includes an input stage 21, indicated by the dashed line. The portion of the differential power amplifier 10 outside the dotted line may be referred to as the output stage. The design shown in FIG. 2 has a differential output stage represented by amplifiers 42 and 44.
The output matching network 11 in FIG. 2 is very small, high performance, and supports broadband. The output matching network 11 receives power from a voltage supply VCC2 12. A capacitor 14 may be coupled to the voltage supply VCC2 12 in one embodiment. In one embodiment, the capacitor 14 may have a capacitance of thirty-three picofarads (33 pF). The output matching network 11 in FIG. 2 may comprise an output transformer 25 consisting of a pair of windings 16 and 18, cascaded with a low-pass filter (comprising inductor 50 and capacitor 27). The output transformer 25 is configured to receive a first amplified signal from the first output stage output 43 of the first differential output stage amplifier 42 of the differential power amplifier 10 and provide a first output signal to the cascaded low pass filter. The output transformer 25 is configured to receive a second amplified signal from the second output stage output 45 of the second differential output stage amplifier 44 of the differential power amplifier 10 and provide a second output signal to the cascaded low pass filter.
The output transformer 25 may be a laminate transformer in one embodiment. In other embodiments, the output transformer 25 may be implemented on a die. The output transformer 25 has a center tap 56. In addition, as seen in FIG. 2, a parasitic second harmonic trap comprising a capacitor 52 and a trap inductor 54 may also be formed in the first leg of the filter for enhanced second harmonic rejection. Higher order harmonics are sufficiently rejected with the low pass filter comprising Llpf inductor 50 and and Clpf capacitor 27. Harmonic rejection is critical for carrier aggregation support and for emissions into the global satellite positioning bands. This output matching network 11 of FIG. 2 has remarkable bandwidth, insertion loss, size, and harmonic rejection.
Going back to the input stage 21 and the differential output stage of the differential power amplifier 10, a first stage input 22 is fed into an input stage amplifier 24 that is grounded via inductor 26. The input stage output 28 is coupled via capacitance 32 and a transformer 35 consisting of windings 34 and 36 to first and second output stage inputs 38 and 40 of a plurality of output stage amplifiers 42 and 44, respectively. In this manner, the input stage amplifier 24 has an input stage output 28, which provides a differential signal having a first portion and a second portion to the first and second output stage inputs 38 and 40 of a plurality of output stage amplifiers 42 and 44, respectively. The first output stage amplifier 42 has a first output stage output 43. The second output stage amplifier 44 has a second output stage output 45. The first and second output stage outputs 43 and 45 of the first and second output stage amplifiers 42 and 44 are coupled to capacitance 46 and to the output transformer 25 of the output matching network 11.
FIG. 3 is a schematic of a differential multistage power amplifier having a differential output stage, a connected envelope tracker, and an output matching network, according to one exemplary embodiment. The differential power amplifier 10 is shown having multiple stages. The design in FIG. 3, like FIG. 2, also has a differential output stage. The design shown in FIG. 2 has a differential output stage represented by output stage amplifiers 42 and 44.
The differential power amplifier 10 is also coupled to an output matching network 11. The output matching network 11 in FIG. 3 is a simpler version than the one in FIG. 2. A power supply voltage VCC2 33 is connected to capacitor 14 and to an output transformer 25 consisting of a pair of windings 16 and 18, which are coupled to a capacitance 27. The output transformer 25 has a center tap 56. This output matching network 11 is very small, high performance, and broadband.
The differential power amplifier 10 includes an input stage 21, indicated by the dashed line. The portion of the differential power amplifier 10 outside the dotted line may be referred to as the output stage. A first stage input 22 is fed into an input stage amplifier 24 that is grounded via inductor 26. The differential power amplifier 10 also comprises power supply voltage VCC1 30. The input stage output 28 is coupled via capacitance 32 and a transformer 35 consisting of windings 34 and 36 to first and second output stage inputs 38 and 40 of a plurality of output stage amplifiers 42 and 44, respectively.
In this manner, the input stage amplifier 24 has an input stage output 28, which provides a differential signal having a first portion and a second portion to the first and second output stage inputs 38 and 40 of a plurality of output stage amplifiers 42 and 44, respectively. The first output stage amplifier 42 has a first output stage output 43. The second output stage amplifier 44 has a second output stage output 45. The first and second output stage outputs 43 and 45 of the first and second output stage amplifiers 42 and 44 are coupled to capacitance 46 and to the output transformer 25 of the output matching network 11.
Although FIG. 2 shows a single input stage amplifier 24 and a pair of output stage amplifiers 42 and 44, any number of stages and amplifiers may be used in a given power amplifier design. For example, the input stage amplifier 24 could itself be replaced by a multistage differential power amplifier, such as differential power amplifier 10 of FIG. 2. In other embodiments, a differential multistage power amplifier may have a number n of multiple stages, wherein each stage has a plurality of amplifiers. Any number of stages may be present between the first stage and a last, differential output stage, until one or more outputs of a given n−1 stage are coupled to inputs of a plurality of last differential output stage amplifiers.
Referring again to FIG. 3, an envelope tracker 48 is coupled to the differential power amplifier 10. The envelope tracker 48 is configured to provide an envelope power supply signal to the differential power amplifier 10. In particular, an output of the envelope tracker 48 may be coupled to an output of the input stage amplifier 24, prior to the differential signal being provided to the first and second output stage inputs 38 and 40 of the first and second output stage amplifiers 42 and 44, respectively. The envelope power supply signal provided by the envelope tracker 48 provides power for amplification. A capacitance 31 may also be coupled to the envelope tracker 48 in one embodiment. In one embodiment, the capacitance 31 may be forty-five picoFarads (45 pF). The combination of the power amplifier having a differential output stage and being connected to an envelope tracker provides a unique power amplifier design that has a compact size, low cost, and offers increased performance over a single-ended power amplifier design.
Many of the advantages of a differential amplifier over a single-ended design are well known (e.g., better power supply rejection, improved grounding, and even-order harmonic rejection). A less obvious advantage is that a differential architecture has more bandwidth because the output load line is approximately four ( ̃4) times higher than a single-ended design, which reduces the impedance transformation ratio of the output matching network. A lower transformation ratio results in higher bandwidth.
Another advantage of a differential power amplifier with a differential output stage is feedback stability. This is particularly a benefit when a power amplifier, such as differential power amplifier 10 in FIG. 3, is tied to an envelope tracker, such as envelope tracker 48 in FIG. 3. The envelope tracker 48 can drive a limited amount of bypass capacitance. This causes a single-ended output stage design to be vulnerable to RF feedback to the input stage, as discussed above with respect to FIG. 1. Typically, a ferrite, such as ferrite 20 in FIG. 1, is required to help isolate the input and output stages of the power amplifier. However, this ferrite causes spectral regrowth because of its “memory effect.” A differential output stage, as shown in FIG. 2, dramatically reduces the RF feedback.
This can be seen further in FIG. 4. FIG. 4 is a schematic of a differential multistage power amplifier having a differential output stage, a connected envelope tracker, and an alternate output matching network, without a ferrite, according to another exemplary embodiment. The differential power amplifier 10 in FIG. 4 is similar to the one in FIG. 2. A first stage input 22 is fed into an input stage amplifier 24 that is grounded via inductor 26. A common collector power supply VCC 33 provides power to the differential power amplifier 10 and to the output matching network 11. The input stage output 28 is coupled via capacitance 32 and a transformer 35 consisting of windings 34 and 36 to first and second output stage inputs 38 and 40 of a plurality of output stage amplifiers 42 and 44, respectively. In this manner, the input stage amplifier 24 has an input stage output 28, which provides a differential signal having a first portion and a second portion to the first and second output stage inputs 38 and 40 of a plurality of output stage amplifiers 42 and 44, respectively. The first output stage amplifier 42 has a first output stage output 43. The second output stage amplifier 44 has a second output stage output 45. The first and second output stage outputs 43 and 45 of the first and second output stage amplifiers 42 and 44 are coupled to capacitance 46 and to the output transformer 25 of the output matching network 11.
The output matching network 11 in FIG. 4 is like the one in FIG. 2. The output matching network 11 receives power from the common collector voltage supply VCC 33. A capacitor 14 may be coupled to the common collector voltage supply VCC 33 in one embodiment. In one embodiment, the capacitor 14 may have a capacitance of thirty-three picofarads (33 pF). The output matching network 11 in FIG. 4 may comprise an output transformer 25 consisting of a pair of windings 16 and 18, cascaded with a low-pass filter (comprising inductor 50 and capacitor 27). The output transformer 25 is configured to receive a first amplified signal 43 from the first differential output stage amplifier 42 of the differential power amplifier 10 and provide a first output signal to the cascaded low pass filter. The output transformer 25 is configured to receive a second amplified signal 45 from the second differential output stage amplifier 44 of the differential power amplifier and provide a second output signal to the cascaded low pass filter.
The output transformer 25 may be a laminate transformer in one embodiment. In other embodiments, the output transformer 25 may be implemented on a die. The output transformer 25 has a center tap 56. In addition, as seen in FIG. 4, a parasitic second harmonic trap comprising a capacitor 52 and a trap inductor 54 may also be formed in the first leg of the filter for enhanced second harmonic rejection. Higher order harmonics are sufficiently rejected with the low pass filter comprising Llpf inductor 50 and and Clpf capacitor 27. Harmonic rejection is critical for carrier aggregation support and for emissions into the global satellite positioning bands. This output matching network 11 of FIG. 4 has remarkable bandwidth, insertion loss, size, and harmonic rejection.
A differential output stage, such as shown in FIG. 4, dramatically reduces the RF feedback 58 because of the cancellation at the center-tap 56 of the output transformer 25, as shown in FIG. 4. The ferrite 20 is therefore no longer needed, which eliminates its memory effect and results in improved performance. The differential output stage reduces the RF feedback 58 to the voltage supply in the differential power amplifier 10, which allows both stages of the differential power amplifier 10 to operate with the envelope tracker 48. As mentioned above, the differential output stage also increases the loadline by approximately four times, which improves the RF bandwidth. This increased loadline also improves the output return loss, which reduces ripple into the filters, which improves the envelope tracker linearity. For example, in one embodiment, the optimal load line for the power amplifier design may be approximately seventeen ( ̃17) ohms. Better bandwidth and harmonic rejection may be achieved if all of the impedance transformation is done in the low-pass filter, though this is not necessary. This means the output transformer 25 may need a 1:1 turns ratio. However, a turns ratio of 2:2 can be achieved by stacking the turns vertically in the laminate for a laminate transformer. This reduces the size of the transformer dramatically (because the mutual inductance increases with the square of the turns). In one embodiment, the die and laminate output matching network section is approximately 1.5 mm×2.5 mm (including the bypass capacitors).
In one embodiment, the output transformer 25 may be a 2:2 transformer, as indicated in FIG. 4, with approximately two turns on the primary winding and approximately two turns on the secondary winding. However, in other embodiments, the turns ratio may vary between approximately two and approximately three turns on the primary winding 34 and between approximately one and a half turns and approximately three turns on the secondary winding 36. In one embodiment, the output transformer 25 may be a 2:1.5 transformer, with approximately two turns on the primary winding 34 and approximately one and a half turns on the secondary winding 36.
In addition, the on-chip capacitor 46 on the die of the differential power amplifier 10 also improves the efficiency of the envelope tracker 48. The on-chip capacitor 46 creates short, at odd harmonics, resulting in inverse Class F power amplifier operation, which improves envelope tracker 48 efficiency. Thus, the differential power amplifier 10 with the differential output stage when used in connection with an envelope tracker 48 provides many advantages over a single-ended output stage. In addition, adding the on-chip capacitor 46 on the die of the differential power amplifier 10 also improves the efficiency of the envelope tracker 48. This design may be used with any output matching network, including the ones shown in FIGS. 2-4. The differential power amplifier 10 with the differential output stage when used in connection with an envelope tracker 48 is itself a unique design and one that provides many advantages over a single-ended output stage. In addition, the differential power amplifier 10 with the differential output stage can be combined with the high performance output matching networks shown in FIGS. 2-4 to get even better efficiency.
Simulation results of the output matching network 11 shown in FIGS. 2 and 4 are shown in FIGS. 5A and 5B. The input return loss at the desired load line impedance, as indicated by the dotted line on the graph in FIG. 5B, is better than 25 dB from 698-915 MHz. The corresponding insertion loss, as indicated by the solid line on the graph in FIG. 5B, is 0.66-0.86 dB, which is excellent considering that the output matching network 11 includes a harmonic filter.
The harmonic rejection for the disclosed output matching network is shown in FIG. 6.
As mentioned previously, the differential architecture with the differential output stage is partially responsible for the excellent bandwidth. However, the topology of the output matching network is also a factor. This can be better illustrated if the output matching network simulation is broken into two pieces (the transformer section and the low-pass filter section). FIG. 7 shows the simulated impedances of each section and the cascaded results. The transformer only simulation is labeled XFMR and is represented by the dashed line. The low pass filter only simulation is labeled LPF and is presented by the dotted line. The cascaded results are labeled CASC and are represented by the solid line. This shows that the impedance variation of the low pass filter and the transformer sections of the output matching network are cancelling each other out by a significant amount, which accounts for much of excellent bandwidth.
As mentioned previously, the differential architecture with the differential output stage is partially responsible for the excellent bandwidth. The total system efficiency of the power amplifier, a Tx band switch, and the envelope tracker are shown in FIG. 8 with LTE modulation. FIG. 8 is a graph that shows the system efficiency of a differential multistage power amplifier having a differential output stage and including a Tx band switch and an envelope tracker. The insertion loss of the TX band switch is  ̃0.4 dB. This level of efficiency is considered very good for any power amplifier architecture of any size.
FIG. 9 is a graph that shows the system efficiency of a differential multistage power amplifier having a differential output stage and an envelope tracker without the Tx band switch loss. Once again, the efficiency of the differential multistage power amplifier with the differential output stage is at a level that is considered to be very good.
Those skilled in the art will recognize improvements and modifications to the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein.

## Claims

1. An output matching network for a differential power amplifier comprising:
an output transformer having a center tap, the output transformer configured to:
receive a first amplified signal from a first differential output stage amplifier of the differential power amplifier and provide a first output signal;
receive a second amplified signal from a second differential output stage amplifier of the differential power amplifier and provide a second output signal; and
a low pass filter configured to receive the first and second output signal from the output transformer and provide a filtered output signal,

wherein the output matching network is configured to reduce RF feedback to the differential power amplifier due to cancellation at the center tap of the output transformer.

2. The output matching network of claim 1, wherein the low pass filter comprises an inductor and a capacitor.

3. The output matching network of claim 2, wherein the low pass filter is configured to filter out higher order harmonics of the first and second amplified signals.

4. The output matching network of claim 1, further comprising a parasitic second harmonic trap comprising a capacitor and a trap inductor, wherein the parasitic second harmonic trap is configured to provide second harmonic rejection of at least one of the first and second output signals.

5. The output matching network of claim 1, wherein the output transformer comprises a primary winding and a secondary winding, with between approximately two and approximately three turns on the primary winding and between approximately one and a half turns and approximately three turns on the secondary winding.

6. The output matching network of claim 5, wherein the output transformer has approximately two turns on the primary winding and approximately two turns on the secondary winding.

7. The output matching network of claim 5, wherein the output transformer has approximately two turns on the primary winding and approximately one and a half turns on the secondary winding.

8. The output matching network of claim 1, wherein the output matching network is configured to receive an envelope power supply signal from an envelope tracking power supply that also provides an envelope power supply signal to the differential power amplifier.

9. The output matching network of claim 8, further comprising a capacitor coupled to the envelope power supply signal from the envelope tracking power supply.

10. The output matching network of claim 9, wherein a capacitance of the capacitor is thirty-three picoFarads (33 pF).

11. The output matching network of claim 1, wherein the output transformer is a laminate transformer.

12. The output matching network of claim 1, wherein the output transformer is formed on a die.

13. Circuitry comprising:
envelope tracking power supply circuitry configured to provide an envelope power supply signal to differential power amplifier circuitry and to an output matching network;
differential power amplifier circuitry comprising:
input stage circuitry having at least one input and at least one output, the input stage circuitry configured to provide a differential signal having a first portion and a second portion;
a first output stage amplifier configured to receive the first portion of the differential signal at a first output stage input and provide a first amplified signal at a first output stage output; and
a second output stage amplifier configured to receive the second portion of the differential signal at a second output stage input and provide a second amplified signal at a second output stage output; and
an output matching network comprising:
an output transformer having a center tap, the output transformer configured to:
receive the first amplified signal from a first differential output stage amplifier of the differential power amplifier circuitry and provide a first output signal;
receive the second amplified signal from a second differential output stage amplifier of the differential power amplifier circuitry and provide a second output signal; and a low pass filter configured to receive the first and second output signal from the output transformer and provide a filtered output signal.

14. The circuitry of claim 13, wherein the low pass filter comprises an inductor and a capacitor and is configured to filter out higher order harmonics of the first and second amplified signals.

15. The circuitry of claim 13, wherein the output matching network is configured to reduce RF feedback to the differential power amplifier circuitry due to cancellation at the center tap of the output transformer.

16. The circuitry of claim 13, wherein the output matching network further comprises a parasitic second harmonic trap comprising a capacitor and a trap inductor, wherein the parasitic second harmonic trap is configured to provide second harmonic rejection of at least one of the first and second output signals.

17. The circuitry of claim 13, wherein the output transformer comprises a primary winding and a secondary winding, with between two and three turns on the primary winding and between one and a half turns and three turns on the secondary winding.

18. The circuitry of claim 13, wherein the differential power amplifier circuitry further comprises a capacitance coupled to the first output stage output of the first output stage amplifier and to the second output stage output of the second output stage amplifier.

19. An output matching network for a differential power amplifier comprising:
an output transformer having a center tap, the output transformer configured to:
receive a first amplified signal from a first differential output stage amplifier of the differential power amplifier and provide a first output signal;
receive a second amplified signal from a second differential output stage amplifier of the differential power amplifier and provide a second output signal; and
a low pass filter configured to receive the first and second output signal from the output transformer and provide a filtered output signal,

wherein the output matching network is configured to receive an envelope power supply signal from an envelope tracking power supply that also provides an envelope power supply signal to the differential power amplifier.

20. The output matching network of claim 19, further comprising a capacitor coupled to the envelope power supply signal from the envelope tracking power supply.

21. The output matching network of claim 20, wherein a capacitance of the capacitor is thirty-three picoFarads (33 pF).