Magnetic field cancellation in radio frequency power amplifier envelope trackers
Aspects of this disclosure relate to generating a bias signal with magnetic field cancellation. A voltage modulator circuit can generate a bias voltage that tracks an envelope of a radio frequency signal. For example, the bias signal can track the envelope of the radio frequency signal that is amplified by a power amplifier on a symbol-by-symbol basis. The voltage modulator circuit includes one or more pairs of switches with magnetic field cancellation.
1 . A power amplifier system with magnetic field cancellation in envelope tracking, the power amplifier system comprising:
a voltage modulator circuit configured to generate a bias voltage that tracks an envelope of a radio frequency signal, the voltage modulator circuit comprising a first pair of switches with magnetic field cancellation, wherein the first pair of switches includes a first switch and a second switch, the first switch is in a first current loop that also includes a first bypass capacitor, the second switch is in a second current loop that also includes a second bypass capacitor, and the first current loop and the second current loop are configured to generate magnetic fields having opposite directions; and
a power amplifier configured to receive the bias voltage and amplify the radio frequency signal.
2 . The power amplifier system of claim 1 , wherein the voltage modulator circuit comprises a second pair of switches with magnetic field cancellation, the first pair of switches is configured to receive a first supply voltage, and the second pair of switches is configured to receive a second supply voltage.
3 . The power amplifier system of claim 2 , wherein the bias voltage tracks the envelope of the radio frequency signal on a symbol-by-symbol basis.
4 . The power amplifier system of claim 2 , wherein the voltage modulator circuit is configured to adjust the bias voltage corresponding to symbol boundaries of the radio frequency signal.
5 . The power amplifier system of claim 2 , wherein the bias voltage tracks the envelope of the radio frequency signal for a group of symbols.
6 . The power amplifier system of claim 2 , wherein the voltage modulator circuit further comprises a capacitor coupled between a first input node configured to receive the first supply voltage and a second input node configured to receive the second supply voltage.
7 . The power amplifier system of claim 2 , wherein the voltage modulator circuit comprises a third pair of switches configured to receive a third supply voltage and a fourth pair of switches configured to receive a fourth supply voltage.
8 . The power amplifier system of claim 2 , wherein the second pair of switches includes field effect transistors.
9 . The power amplifier system of claim 2 , wherein a switch of the second pair of switches comprises back-to-back field effect transistors in a common-source configuration.
10 . The power amplifier system of claim 2 , wherein a switch of the second pair of switches comprises back-to-back field effect transistors in a common-drain configuration.
11 . The power amplifier system of claim 1 , further comprising an antenna and a quarter-wavelength transmission line, the quarter-wavelength transmission line coupled between the power amplifier and an output of the voltage modulator circuit.
12 . A voltage multiplexer with magnetic field cancellation, the voltage multiplexer comprising:
a first pair of switches configured to receive a first supply voltage, the first pair of switches comprising a first switch of a first current loop and a second switch of a second current loop, wherein the first current loop and the second current loop are configured to generate magnetic fields having opposite directions; and
a second pair of switches configured to receive a second supply voltage, the second pair of switches comprising a third switch of a third current loop and a fourth switch of a fourth current loop, wherein the third current loop and the fourth current loop are configured to generate magnetic fields having opposite directions;
wherein the voltage multiplexer is configured to generate a bias voltage that tracks an envelope of a radio frequency signal provided to a power amplifier.
13 . The voltage multiplexer of claim 12 , wherein the bias voltage tracks the envelope of the radio frequency signal on a symbol-by-symbol basis.
14 . The voltage multiplexer of claim 12 , wherein the voltage multiplexer is configured to adjust the bias voltage corresponding to symbol boundaries of the radio frequency signal.
15 . The voltage multiplexer of claim 12 , further comprising one or more additional pairs of switches with magnetic field cancellation configured to receive one or more additional supply voltages.
16 . The voltage multiplexer of claim 12 , wherein the voltage multiplexer is configured to generate the bias voltage by controlling pairs of switches to selectively provide one of a plurality of supply voltages as the bias voltage, the pairs of switches comprising the first pair of switches and the second pair of switches, and the plurality of supply voltages comprising the first supply voltage and the second supply voltage.
17 . A method of generating a bias voltage with magnetic field cancellation, the method comprising:
receiving a plurality of supply voltages; and
controlling pairs of switches to selectively provide one of the plurality of supply voltages as a bias voltage, wherein the controlling adjusts the bias voltage at symbol boundaries of a radio frequency signal that is amplified by a power amplifier that receives the bias voltage,
wherein each of the pairs of switches comprise a first switch of a first current loop and a second switch of a second current loop, and wherein the first current loop and the second current loop generate magnetic fields having opposite directions.
18 . The method of claim 17 , wherein the bias voltage tracks an envelope of the radio frequency signal on a symbol-by-symbol basis.
19 . The method of claim 17 , wherein the controlling comprises make-before-break switching.
20 . The method of claim 17 , wherein at least one of the pairs of switches comprises a switch that includes two series field effect transistors in a junction isolation configuration.