Switching power amplifier with output harmonic suppression
A switching power amplifier with harmonic suppression including a polyphase converter and a power amplifier stage. The polyphase converter converts a frequency or phase modulated input signal into a 50% duty cycle rail-to-rail signal, a positive 25% duty cycle rail-to-rail signal that is centered with the 50% duty cycle signal when high, and a negative 25% duty cycle rail-to-rail signal that is centered with the 50% duty cycle signal when low. The power amplifier stage includes first and second branches coupled between upper and lower nodes, each including series-coupled P-channel and N-channel transistors forming an intermediate output node. The transistors of the first branch are controlled by the 50% duty cycle signal, and the transistors of the second branch are controlled by the positive and negative 25% duty cycle signals. The first and second branches generate output currents that are superimposed with each other to suppress third and fifth harmonics.
1. A switching power amplifier, comprising:
a polyphase converter that is configured to convert a frequency or phase modulated input signal into a plurality of rail-to-rail signals that each transition between a first rail and a second rail, including a 50% duty cycle rail-to-rail signal, a positive 25% duty cycle rail-to-rail signal that is high 25% of the time while being centered within the 50% duty cycle rail-to-rail signal when high, and a negative 25% duty cycle rail-to-rail signal that is low 25% of the time while being centered within the 50% duty cycle rail-to-rail signal when low; and
a power amplifier stage, comprising:
a first branch comprising a first P-channel transistor having current terminals coupled between an upper node and an intermediate output node and having a control terminal receiving the 50% duty cycle rail-to-rail signal, and a first N-channel transistor having current terminals coupled the intermediate output node and a lower node and having a control terminal receiving the 50% duty cycle rail-to-rail signal; and
a second branch comprising a second P-channel transistor having current terminals coupled between the upper node and the intermediate output node and having a control terminal receiving the negative 25% duty cycle rail-to-rail signal, and a second N-channel transistor having current terminals coupled between the intermediate output node and the lower node and having a control terminal receiving the positive 25% duty cycle rail-to-rail signal.
2. The switching power amplifier of claim 1 , wherein the polyphase converter is configured to transition the negative 25% duty cycle rail-to-rail signal to activate the second P-channel transistor for about 25% of each cycle, and wherein the polyphase converter is configured to transition the positive 25% duty cycle rail-to-rail signal to activate the second N-channel transistor for about 25% of each cycle.
3. The switching power amplifier of claim 1 , wherein the polyphase converter comprises:
a polyphase filter that is configured to convert the input signal comprising a sinusoidal input signal into a positive 45 degree phase-shifted sinusoidal signal, a non-shifted amplitude adjusted sinusoidal signal, and a negative 45 degree phase-shifted sinusoidal signal;
limiting amplifier circuitry that is configured to convert the positive 45 degree phase-shifted sinusoidal signal, the non-shifted amplitude adjusted sinusoidal signal, and the negative 45 degree phase-shifted sinusoidal signal into a positive 45 degree phase-shifted rail-to-rail signal, a non-shifted rail-to-rail signal, and a negative 45 degree phase-shifted rail-to-rail signal, respectively; and
combinational circuitry that is configured to logically combine the positive and negative 45 degree phase-shifted rail-to-rail signals into the positive 25% duty cycle rail-to-rail signal and into the negative 25% duty cycle rail-to-rail signal, and that comprises a delay matching buffer that is configured to delay the non-shifted rail-to-rail signal to provide the 50% duty cycle rail-to-rail signal delay matched with the positive and negative 25% duty cycle rail-to-rail signals.
4. The switching power amplifier of claim 3 , wherein the polyphase filter comprises:
a high-pass capacitor-resistor filter having an input coupled to an input node for receiving the sinusoidal input signal and having an output providing the positive 45 degree phase-shifted sinusoidal signal;
a capacitive attenuating matching circuit having an input coupled to the input node and having an output providing the non-shifted amplitude adjusted sinusoidal signal; and
a low-pass resistor-capacitor filter having an input coupled to the input node and having an output providing the negative 45 degree phase-shifted sinusoidal signal.
5. The switching power amplifier of claim 4 , wherein the high-pass capacitor-resistor filter, the capacitive attenuating matching circuit, and the low-pass resistor-capacitor filter are each tuned based on a predetermined transmission frequency.
6. The switching power amplifier of claim 3 , wherein the limiting amplifier circuitry comprises a plurality of amplitude limiting buffers.
7. The switching power amplifier of claim 3 , wherein the combinational circuitry comprises:
a Boolean logic OR gate having inputs receiving the positive 45 degree phase-shifted rail-to-rail signal and the negative 45 degree phase-shifted rail-to-rail signal and having an output providing the negative 25% duty cycle rail-to-rail signal; and
a Boolean logic AND gate having inputs receiving the positive 45 degree phase-shifted rail-to-rail signal and the negative 45 degree phase-shifted rail-to-rail signal and having an output providing the positive 25% duty cycle rail-to-rail signal.
8. The switching power amplifier of claim 1 , wherein the polyphase converter comprises:
a delay-locked loop that is configured to convert the input signal comprising a rail-to-rail input signal into a positive 45 degree phase-shifted rail-to-rail signal, a non-shifted rail-to-rail signal, and a negative 45 degree phase-shifted rail-to-rail signal; and
combinational circuitry that is configured to logically combine the positive and negative 45 degree phase-shifted rail-to-rail signals into the positive 25% duty cycle rail-to-rail signal and into the negative 25% duty cycle rail-to-rail signal, and that comprises a delay matching buffer that is configured to delay the non-shifted rail-to-rail signal to provide the 50% duty cycle rail-to-rail signal delay matched with the positive and negative 25% duty cycle rail-to-rail signals.
9. The switching power amplifier of claim 8 , wherein the delay-locked loop comprises:
a phase detector having a first input receiving the rail-to-rail input signal, having a second input, and having an output;
a delay line comprising a plurality of series-coupled inverters including a first inverter having an input receiving the rail-to-rail input signal and including a last inverter having an output coupled to the second input of the phase detector;
a loop filter having an input coupled to the output of the phase detector and having an output; and
a controlled current source having a control input coupled to the output of the loop filter and having an output coupled to a supply input of each of the series-coupled inverters of the delay line;
wherein selected ones of outputs of the plurality of inverters are used to generate the 50% duty cycle rail-to-rail signal, the positive 25% duty cycle rail-to-rail signal, and the negative 25% duty cycle rail-to-rail signal.
10. The switching power amplifier of claim 8 , further comprising an amplitude limiting buffer that converts a sinusoidal input signal into the rail-to-rail input signal.
11. The switching power amplifier of claim 8 , further comprising a phase-locked loop that provides the frequency or phase modulated input signal.
12. The switching power amplifier of claim 1 , wherein the first branch is configured to generate a first output current having about a 50% duty cycle, and wherein the second branch is configured to generate a second output current having about a 25% duty cycle that is superimposed with the first output current.
13. The switching power amplifier of claim 1 , further comprising:
non-overlap generation circuitry having an input receiving the 50% duty cycle rail-to-rail signal, having a first output providing a P-type drive rail-to-rail signal and having second output providing an N-type drive rail-to-rail signal, wherein the P-type drive and N-type drive rail-to-rail signals have skewed transitions to prevent overlapping transitions with respect to each other; and
select circuitry that is configured to convey the 50% duty cycle rail-to-rail signal and the positive and negative 25% duty cycle rail-to-rail signals to the power amplifier stage when harmonic suppression is enabled, and when harmonic suppression is disabled, that is configured to convey the P-type drive rail-to-rail signal to the control terminals of the first and second P-channel transistors, and that conveys the N-type drive rail-to-rail signal to the control terminals of the first and second N-channel transistors.
14. A method of suppressing harmonics of a switching power amplifier, comprising:
converting a frequency or phase modulated input signal into a plurality of rail-to-rail signals that each transition between a first rail and a second rail, including a 50% duty cycle rail-to-rail signal, a positive 25% duty cycle rail-to-rail signal that is high 25% of the time while being centered within the 50% duty cycle rail-to-rail signal when high, and a negative 25% duty cycle rail-to-rail signal that is low 25% of the time while being centered within the 50% duty cycle rail-to-rail signal when low;
driving an upper P-channel transistor and a lower N-channel transistor of a first branch of a power amplifier circuit with the 50% duty cycle rail-to-rail signal;
driving an upper P-channel transistor of a second branch that is coupled in parallel with the first branch of the power amplifier circuit with the negative 25% duty cycle rail-to-rail signal; and
driving a lower N-channel transistor of the second branch of the power amplifier circuit with the positive 25% duty cycle rail-to-rail signal.
15. The method of claim 14 , wherein the converting a frequency or phase modulated input signal comprises:
shifting phase of a sinusoidal input signal forward by 45 degrees to provide a positive 45 degree phase-shifted sinusoidal signal;
shifting phase of the sinusoidal input signal backward by 45 degrees to provide a negative 45 degree phase-shifted sinusoidal signal;
adjusting amplitude of the sinusoidal input signal to provide a non-shifted amplitude adjusted sinusoidal signal;
converting amplitude of the positive 45 degree phase-shifted sinusoidal signal, the non-shifted amplitude adjusted sinusoidal signal, and the negative 45 degree phase-shifted sinusoidal signal to provide a positive 45 degree phase-shifted rail-to-rail signal, a non-shifted rail-to-rail signal, and a negative 45 degree phase-shifted rail-to-rail signal, respectively;
logically combining the positive 45 degree phase-shifted rail-to-rail signal and the negative 45 degree phase-shifted rail-to-rail signal to provide a positive 25% duty cycle rail-to-rail signal and a negative 25% duty cycle rail-to-rail signal; and
delaying the non-shifted rail-to-rail signal to provide a 50% duty cycle rail-to-rail signal that is delayed equally in time as a delay of the positive and negative 25% duty cycle rail-to-rail signals.
16. The method of claim 15 , wherein:
said shifting phase of a sinusoidal input signal forward by 45 degrees comprises high pass filtering the sinusoidal input signal to provide the positive 45 degree phase-shifted sinusoidal signal;
wherein said shifting phase of the sinusoidal input signal backward by 45 degrees comprises low pass filtering the sinusoidal input signal to provide the negative 45 degree phase-shifted sinusoidal signal; and
wherein said adjusting amplitude of the sinusoidal input signal comprises capacitively attenuating the sinusoidal input signal to provide the non-shifted amplitude adjusted sinusoidal signal.
17. The method of claim 15 , wherein said converting amplitude comprises passing the positive 45 degree phase-shifted sinusoidal signal, the non-shifted amplitude adjusted sinusoidal signal, and the negative 45 degree phase-shifted sinusoidal signal through first, second, and third amplitude limiting buffers, respectively, to provide the positive 45 degree phase-shifted rail-to-rail signal, the non-shifted rail-to-rail signal, and the negative 45 degree phase-shifted rail-to-rail signal, respectively.
18. The method of claim 15 , wherein the logically combining comprises logically OR'ing the positive 45 degree phase-shifted rail-to-rail signal and the negative 45 degree phase-shifted rail-to-rail signal to provide the negative 25% duty cycle rail-to-rail signal, and logically AND'ing the positive 45 degree phase-shifted rail-to-rail signal and the negative 45 degree phase-shifted rail-to-rail signal to provide the positive 25% duty cycle rail-to-rail signal.
19. The method of claim 14 , wherein the converting a frequency or phase modulated input signal comprises:
providing a rail-to-rail input signal to a delay-locked loop comprising a multiple phase delay line having selected outputs configured to provide a positive 45 degree phase-shifted rail-to-rail signal, a non-shifted rail-to-rail signal, and a negative 45 degree phase-shifted rail-to-rail signal;
logically combining the positive 45 degree phase-shifted rail-to-rail signal and the negative 45 degree phase-shifted rail-to-rail signal to provide a positive 25% duty cycle rail-to-rail signal and a negative 25% duty cycle rail-to-rail signal; and
delaying the non-shifted rail-to-rail signal to provide a 50% duty cycle rail-to-rail signal that is delayed equally in time as a delay of the positive and negative 25% duty cycle rail-to-rail signals.
20. The method of claim 14 , further comprising:
generating a P-type drive rail-to-rail signal by logically OR'ing the 50% duty cycle rail-to-rail signal with an N-type drive rail-to-rail signal;
generating the N-type drive rail-to-rail signal by logically AND'ing the 50% duty cycle rail-to-rail signal with the P-type drive rail-to-rail signal, wherein the P-type drive and N-type drive rail-to-rail signals are non-overlapping; and
disabling harmonic suppression by driving the upper P-channel transistors of the first and second branches of the power amplifier circuit with the P-type drive rail-to-rail signal and driving the lower N-channel transistors of the first and second branches of the power amplifier circuit with the N-type drive rail-to-rail signal.