Dynamic look up table measurements for transmitter with envelope tracking systems
The representative embodiments discussed in the present disclosure relate to techniques in which the operating characteristics (e.g., gain and/or efficiency) of a power amplifier in a transmitter may be regulated according to an operation mode of the transmitter. More specifically, in some embodiments, different look-up tables (LUTs) may be employed for each mode of operation to suitably adjust the supply voltage to the power amplifier and modulate its operating characteristics based on power input to the power amplifier. Further, in some embodiments, a method to calibrate a LUT for uplink carrier aggregation (ULCA) operation mode of the transmitter may be employed to populate a LUT used to suitably adjust the supply voltage during ULCA.
1. A transmitter, comprising:
a power amplifier configured to receive an input signal and amplify the input signal based on a transfer function characterizing the power amplifier; and
a control circuit operably coupled to the power amplifier, wherein the control circuit is configured to cause the power amplifier to generate an output signal while tracking an envelope of the input signal at least in part by:
applying a first set of variable supply voltages to the power amplifier to operate the power amplifier in a non-linear region of the transfer function; and
applying a second set of variable supply voltages to the power amplifier to operate the power amplifier in a linear region of the transfer function, wherein the second set of variable supply voltages vary over time to adjust the linear region of the transfer function to be more linear.
2. The transmitter of claim 1 , wherein applying the first set of variable supply voltages corresponds to a normal operation mode, and wherein applying the second set of variable supply voltages corresponds to an uplink carrier aggregation mode.
3. The transmitter of claim 2 , wherein, in the uplink carrier aggregation mode, the output signal comprises two to five baseband signals aggregated into a single baseband signal, wherein a total frequency of the two to five baseband signals is 100 (megahertz) MHz.
4. The transmitter of claim 1 , wherein the control circuit is configured to generate the first set of variable supply voltages, the second set of variable supply voltages, or both based in part on an envelope of an unprocessed signal in the transmitter, wherein the power amplifier is configured to generate the output signal based in part on the first set of variable supply voltages, the second set of variable supply voltages, or both.
5. The transmitter of claim 4 , wherein the control circuit comprises:
an envelope shaping circuit having a first look up table corresponding to a first mode of operation and a second look up table corresponding to a second mode of operation, the envelope shaping circuit being configured to:
receive the unprocessed signal;
in the first mode of operation, generate an unprocessed supply signal based in part on an envelope of the unprocessed signal and the first look up table; and
in the second mode of operation, generate the unprocessed supply signal based in part on the envelope of the unprocessed signal and the second look up table;
an envelope tracking digital-to-analog converter (DAC) operably coupled to the envelope shaping circuit, the envelope tracking DAC being configured to convert the unprocessed supply signal to an analog signal; and
an envelope modulator circuit operably coupled to the envelope tracking DAC, the envelope modulator circuit being configured to receive the analog signal and configured to generate the first set of variable supply voltages or the second set of variable supply voltages.
6. The transmitter of claim 4 , wherein the control circuit comprises:
a first look up table corresponding to a first mode of operation, the first look up table having a first mapping of data representative of the envelope to data representative of the first set of variable supply voltages; and
a second look up table corresponding to a second mode of operation, the second look up table having a second mapping of the data representative of the envelope to the data representative of the second set of variable supply voltages.
7. The transmitter of claim 6 , wherein, in the second mode of operation, the control circuit is configured to cause the power amplifier to generate the output signal by applying the constant gain to the input signal based at least in part on the second mapping of the data representative of the envelope to the data representative of the second set of variable supply voltages.
8. The transmitter of claim 1 , wherein the transmitter is configured to receive an unprocessed input signal and configured to generate the input signal based in part on the unprocessed input signal.
9. The transmitter of claim 8 , comprising a digital pre-distortion circuit configured to introduce distortion to the unprocessed input signal to generate a distorted signal, wherein the distortion offsets distortion generated by the power amplifier.
10. The transmitter of claim 9 , comprising a digital-to-analog converter (DAC) configured to convert the distorted signal from a digital signal to an analog signal to generate a distorted analog signal.
11. A method of operating a power amplifier of a transmitter having a normal operation mode and an uplink carrier aggregation mode to perform envelope tracking, comprising:
receiving an input signal at the power amplifier;
in the uplink carrier aggregation mode, generating an output signal having an iso-gain relative to the input signal by applying a first set of variable supply voltages to the power amplifier to adjust a linear portion of a transfer function of the power amplifier to be more linear; and
in the normal operation mode, generating the output signal having a variable gain relative to the input signal by applying a second set of variable supply voltages to the power amplifier to operate the power amplifier in a non-linear region of the transfer function.
12. The method of claim 11 , comprising:
in the uplink carrier aggregation mode:
receiving a supply signal at the power amplifier, wherein the supply signal is based in part on an envelope of the input signal; and
generating, via the power amplifier, the output signal based in part on the supply signal and the input signal.
13. The method of claim 11 , comprising:
in the uplink carrier aggregation mode:
generating, via the power amplifier, the output signal based in part on contents of a first look up table; and
in the normal operation mode:
generating, via the power amplifier, the output signal based in part on contents of a second look up table.
14. The method of claim 11 , wherein, in the uplink carrier aggregation mode, the output signal comprises a plurality of baseband signals, wherein each of the plurality of baseband signals comprise a frequency of 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, or 20 MHz.
15. The transmitter of claim 1 , wherein the control circuit is configured to calibrate the power amplifier at least in part by:
generating, via the power amplifier, a first output signal based on a first input signal and a supply signal;
generating, via the power amplifier, a second output signal based on a second input signal and the supply signal;
measuring a first change in power between a power level corresponding to the first input signal and a power level corresponding to the second input signal;
measuring a second change in power between a power level corresponding to the first output signal and a power level corresponding to the second output signal; and
adjusting the supply signal so that the first change in power corresponds to the second change in power.
16. The transmitter of claim 1 , wherein the control circuit is configured to be operated in a second mode of operation to apply the second set of variable supply voltages to the power amplifier.