Linearity performance for multi-mode power amplifiers
Improved linearity performance for multi-mode power amplifiers. A power amplifier (PA) assembly can include a radio-frequency (RF) amplification path having a first stage and a second stage, with each stage including a transistor. The PA assembly can further include a biasing circuit having a first bias path between a supply node and the base of a corresponding transistor. The PA assembly can further include a linearizing circuit implemented as either or both of a second bias path and a coupling path relative to the first bias path. The second bias path can be configured to provide an additional base bias current to the base under a selected condition. The coupling path can be configured to improve linearity of the corresponding transistor operating in a first mode while allowing a ballast resistance to be sufficiently robust for the corresponding transistor operating in a second mode.
1. A power amplifier (PA) assembly comprising:
an amplification path configured to receive and amplify a radio-frequency (RF) signal, the amplification path including a driver stage and an output stage, each of the driver and output stages including a transistor with a base, a collector and an emitter;
a first bias path implemented between a supply node and the base of the driver stage;
a second bias path implemented to be electrically parallel with the first bias path between the supply node and the base of the driver stage and configured to provide an additional base bias current to the base under a selected condition;
a third bias path implemented to provide a bias signal to the base of the output stage through a common node, a ballast resistance, and an input node associated with the base of the output stage; and
a coupling path implemented between the input node of the output stage and the common node of the third bias path and configured to improve linearity of the output stage operating in a first mode while allowing the ballast resistance to be sufficiently robust for the output stage operating in a second mode.
2. The PA assembly of claim 1 wherein the second bias path includes an emitter follower having a collector, an emitter, and a base, the collector coupled to the supply node, the emitter coupled to the base of the driver stage.
3. The PA assembly of claim 2 wherein the base of the emitter follower is coupled to a node having a DC voltage.
4. The PA assembly of claim 3 wherein the emitter follower is configured so that its average emitter voltage increases with an increase in RF power at an RF input node.
5. The PA assembly of claim 4 wherein the emitter follower is further configured to conduct at the selected condition to thereby provide the additional base bias current to the base, the selected condition including a selected level of increased RF power at the input node.
6. The PA assembly of claim 5 wherein the selected level of increased RF power resulting in the conduction through the emitter follower results in a reversal of gain and phase droop associated with the driver stage.
7. The PA assembly of claim 6 wherein the DC voltage is selected such that the emitter follower is biased just below a turn-on level with a selected low RF power at the input node.
8. The PA assembly of claim 1 wherein the coupling path is configured to couple an RF signal between the input node and a base-emitter junction of the output stage transistor to provide rectification on the base-emitter junction and correct AM-AM distortion and thereby provide improved linearity.
9. The PA assembly of claim 8 wherein the first mode includes an EDGE (enhanced data rates for GSM evolution) mode, and the second mode includes a GPRS (general packet radio service) mode.
10. The PA assembly of claim 8 wherein the input node is part of an input path that includes a DC blocking capacitance implemented between the base of the output stage and the input node.
11. The PA assembly of claim 8 wherein the coupling path includes a capacitance.
12. The PA assembly of claim 11 wherein the coupling path further includes a resistance connected in series with the capacitance.
13. The PA assembly of claim 11 wherein the coupling path further includes an inductance connected in series with the capacitance.
14. A power-amplifier (PA) module comprising:
a packaging substrate configured to receive a plurality of components;
a power amplifier (PA) circuit formed on a die that is mounted on the packaging substrate, the PA circuit including an amplification path configured to receive and amplify a radio-frequency (RF) signal, the amplification path including a driver stage and an output stage, each of the driver and output stages including a transistor with a base, a collector and an emitter;
a first bias path implemented between a supply node and the base of the driver stage;
a second bias path implemented to be electrically parallel with the first bias path between the supply node and the base of the driver stage and configured to provide an additional base bias current to the base under a selected condition;
a third bias path implemented to provide a bias signal to the base of the output stage through a common node, a ballast resistance, and an input node associated with the base of the output stage; and
a coupling path implemented between the input node of the output stage and the common node of the third bias path and configured to improve linearity of the output stage operating in a first mode while allowing the ballast resistance to be sufficiently robust for the output stage operating in a second mode.
15. A method for operating a power-amplifier (PA), the method comprising:
receiving a radio-frequency (RF) signal through an amplification path that includes a driver stage and an output stage, each of the driver and output stages including a transistor with a base, a collector and an emitter;
providing a bias signal from a supply node, through a first bias path, to the base of the driver stage;
providing an additional base bias current to the base of the driver stage under a selected condition through a second bias path that is electrically parallel with the first bias path;
providing a bias signal to the base of the output stage through a third bias path having a common node, a ballast resistance, and an input node associated with the base of the output stage; and
coupling the input node of the output stage the common node of the third bias path to improve linearity of the output stage operating in a first mode while allowing the ballast resistance to be sufficiently robust for the output stage operating in a second mode.
16. The method of claim 15 wherein the first mode includes an EDGE (enhanced data rates for GSM evolution) mode, and the second mode includes a GPRS (general packet radio service) mode.
17. The method of claim 15 wherein the coupling of the input node and the common node includes path includes providing a capacitance.
18. The method of claim 17 wherein the coupling of the input node and the common node further includes providing a resistance in series with the capacitance.
19. The method of claim 17 wherein the coupling of the input node and the common node further includes providing an inductance in series with the capacitance.