Power amplifiers with adaptive bias for envelope tracking applications
Power amplifiers with adaptive bias for envelope tracking applications are provided herein. In certain embodiments, an envelope tracking system includes a power amplifier that amplifies a radio frequency (RF) signal and that receives power from a power amplifier supply voltage, and an envelope tracker that controls a voltage level of the power amplifier supply voltage based on an envelope of the RF signal. The power amplifier includes a current mirror having an input that receives a reference current, an output electrically connected to the power amplifier supply voltage, and a node that outputs a gate bias voltage. The power amplifier further includes a field-effect transistor that amplifies the radio frequency signal and a first depletion-mode transistor having a gate connected to the node of the current mirror and a source connected to a gate of the field-effect transistor.
1. A mobile device comprising:
a transceiver configured to generate a radio frequency signal;
a power management system including an envelope tracker configured to control a voltage level of a power amplifier supply voltage based on an envelope of the radio frequency signal; and
a front end system including a power amplifier configured to amplify the radio frequency signal and to receive power from the power amplifier supply voltage, the power amplifier including a current mirror electrically connected to the power amplifier supply voltage and having an input configured to receive a reference current and a node that outputs a gate bias voltage, the power amplifier further including a field-effect transistor configured to amplify the radio frequency signal and a first depletion-mode transistor having a gate connected to the node of the current mirror and a source connected to a gate of the field-effect transistor.
2. The mobile device of claim 1 wherein the power amplifier further includes a second depletion-mode transistor having a drain connected to the gate of the field-effect transistor.
3. The mobile device of claim 2 wherein a gate of the second depletion-mode transistor is connected to a source of the second depletion-mode transistor.
4. The mobile device of claim 3 wherein the gate of the second depletion-mode transistor and the source of the second depletion-mode transistor are connected to a ground voltage.
5. The mobile device of claim 1 further comprising a battery, the first depletion-mode transistor having a drain that receives a battery voltage from the battery.
6. The mobile device of claim 1 wherein the current mirror includes a first mirror transistor having a drain connected to the node, a second mirror transistor having a gate connected to a gate of the first mirror transistor, and a third mirror transistor having a drain connected to the input and a source connected to the node.
7. The mobile device of claim 6 wherein a gate of the third mirror transistor is connected to the input.
8. The mobile device of claim 6 wherein the current mirror further includes a fourth mirror transistor having a gate connected to a gate of the third mirror transistor and a drain connected to the power amplifier supply voltage.
9. The mobile device of claim 8 wherein a source of the fourth mirror transistor is connected to a drain of the second mirror transistor.
10. The mobile device of claim 9 wherein the drain of the second mirror transistor is connected to the gate of the second mirror transistor.
11. The mobile device of claim 6 wherein a source of the first mirror transistor and a source of the second mirror transistor are connected to a ground voltage.
12. The mobile device of claim 1 wherein the power amplifier further includes a current source configured to generate the reference current.
13. An envelope tracking system comprising:
an envelope tracker configured to control a voltage level of a power amplifier supply voltage based on an envelope of a radio frequency signal; and
a power amplifier configured to amplify the radio frequency signal and to receive power from the power amplifier supply voltage, the power amplifier including a current mirror electrically connected to the power amplifier supply voltage and having an input configured to receive a reference current and a node that outputs a gate bias voltage, the power amplifier further including a field-effect transistor configured to amplify the radio frequency signal and a first depletion-mode transistor having a gate connected to the node of the current mirror and a source connected to a gate of the field-effect transistor.
14. The mobile device of claim 13 wherein the power amplifier further includes a second depletion-mode transistor having a drain connected to the gate of the field-effect transistor.
15. The mobile device of claim 14 wherein a gate of the second depletion-mode transistor is connected to a source of the second depletion-mode transistor.
16. The mobile device of claim 15 wherein the gate of the second depletion-mode transistor and the source of the second depletion-mode transistor are connected to a ground voltage.
17. The mobile device of claim 13 further comprising a battery, the first depletion-mode transistor having a drain that receives a battery voltage from the battery.
18. The mobile device of claim 13 wherein the current mirror includes a first mirror transistor having a drain connected to the node, a second mirror transistor having a gate connected to a gate of the first mirror transistor, and a third mirror transistor having a drain connected to the input and a source connected to the node.
19. A method of radio frequency signal amplification in a mobile device, the method comprising:
controlling a voltage level of a power amplifier supply voltage based on an envelope of a radio frequency signal using an envelope tracker;
powering a power amplifier using the power amplifier supply voltage;
amplifying the radio frequency signal using a field-effect transistor of the power amplifier; and
biasing the field-effect transistor using a current mirror that is electrically connected to the power amplifier supply voltage, including receiving a reference current at an input of the current mirror, outputting a gate bias voltage from a node of the current mirror, and providing the gate bias voltage to the field-effect transistor using a first depletion-mode transistor having a gate connected to the node of the current mirror and a source connected to a gate of the field-effect transistor.
20. The method of claim 19 wherein biasing the field-effect transistor of the power amplifier further includes controlling the gate of the field-effect transistor using a second depletion-mode transistor having a drain connected to the gate of the field-effect transistor.