Apparatus and method for sensing and converting radio frequency to direct current
The apparatus and method thereof accurately sense and convert a radio frequency (RF) current signal to direct current (DC) independent of process variation and temperature, and without requiring high speed, high voltage amplifiers for its operation. The apparatus comprises an AC coupled circuit that couples the RF signal from the main device to a sense device with an N:M ratio, a low pass filter system that extracts the DC content of the RF current signal, and a negative feedback loop that forces the DC content of the main device and the sensed device to be equal. Exemplary embodiments include a current sensor that provides feedback to protect an RF power amplifier from over-current condition, and a RF power detection and control in a RF power amplifier (PA) that multiplies the sensed output current by the sensed output voltage to be used as a feedback to control the PA's bias.
1. Circuitry comprising:
an output stage of a power amplifier (PA), the output stage comprising a first transistor; and
sensing circuitry comprising a second transistor and filtering circuitry, the filtering circuitry adapted to separate a radio frequency (RF) signal flowing through the output stage into a direct current (DC) component and an alternating current (AC) component, and the second transistor adapted to use the DC component and the AC component to replicate the RF signal, wherein a first W-over-L ratio of the first transistor is larger than one and a second W-over-L ratio of the second transistor is equal to one.
2. The circuitry of claim 1 , wherein the sensing circuitry is designed to ensure that a gate-to-source voltage of the second transistor is equal to a gate-to-source voltage of the first transistor.
3. The circuitry of claim 1 , wherein the sensing circuitry is designed to ensure that a drain-to-source voltage of the second transistor is equal to a drain-to-source voltage of the first transistor.
4. The circuitry of claim 1 , wherein the filtering circuitry comprises a first low-pass filter (LPF) adapted to filter the DC component of a signal at a drain of the first transistor and a second LPF to filter the DC component of a signal at a drain of the second transistor.
5. The circuitry of claim 4 , wherein an output of the first LPF and an output of the second LPF are coupled to inputs of an amplifier that controls a MOS device that provides current for operation of the second transistor.
6. The circuitry of claim 5 , wherein the first LPF and the second LPF are one of: a zero order filter, an n th order filter, where n is an integer greater than one.
7. The circuitry of claim 5 , wherein the first LPF and the second LPF are one of: an active filter, a passive filter.
8. The circuitry of claim 5 , wherein the amplifier is one of: an operational amplifier, a common gate amplifier, a common source amplifier.
9. The circuitry of claim 1 , wherein a capacitor is connected between a drain of the first transistor and a drain of the second transistor to ensure that the AC component of the RF signal is equal for both.
10. A method of sensing and converting a radio frequency (RF) signal to direct current in an output stage of a power amplifier (PA), the output stage having a first transistor, comprising:
replicating in a second transistor having its source coupled to a source of the first transistor and having its gate coupled to a gate of the first transistor, a fraction of both direct current (DC) and alternating current (AC) components of the RF signal through the first transistor;
low pass filtering a drain voltage of the first transistor to provide a first DC filter output;
low pass filtering a drain voltage of the second transistor to provide a second DC filter output;
controlling the average current through the second transistor to cause the first and the second DC filter outputs to be equal to provide a drain source voltage on the second transistor that is equal to a drain source voltage on the first transistor; and
coupling a capacitor between a drain of the first transistor and a drain of the second transistor to ensure that the AC components of the RF signal are equal for both the first transistor and the second transistor
whereby the direct current in the second transistor is proportional to the direct current in the first transistor.
11. The method of claim 10 wherein the low pass filtering is done using one of: a zero order filter, an n th order filter, where n is an integer greater than one.
12. The method of claim 11 wherein the low pass filtering is done using one of: an active filter, a passive filter.
13. Circuitry comprising:
an output stage of a power amplifier (PA), the output stage comprising a first transistor; and
sensing circuitry comprising a second transistor and filtering circuitry, the filtering circuitry configured to separate a radio frequency (RF) signal flowing through the output stage into a direct current (DC) component and an alternating current (AC) component, deliver the AC component directly to the second transistor, and deliver the DC component to the second transistor through a feedback loop;
wherein the second transistor is configured to replicate the RF signal.
14. The circuitry of claim 13 wherein the feedback loop forces the DC component of the RF signal at the first transistor to be about equal to the DC component of the replicated RF signal at the second transistor.
15. The circuitry of claim 13 wherein the filtering circuitry is coupled between a drain contact of the first transistor and a drain contact of the second transistor.
16. A method of sensing and converting a radio frequency (RF) signal to direct current in an output stage of a power amplifier (PA), the output stage having a first transistor, the method comprising the steps of:
passing a direct current (DC) component of the RF signal at the output of the first transistor to an output of a second transistor through a feedback loop;
passing an alternating current (AC) component of the RF signal at the output of the first transistor directly to the output of the second transistor; and
replicating the RF signal at the output of the first transistor in the output of the second transistor using the DC component and the AC component.
17. The method of claim 16 wherein the feedback loop forces the DC component of the RF signal at the output of the first transistor to be about equal to the DC component of the replicated RF signal at the output of the second transistor.
18. The method of claim 16 wherein the output of the first transistor is a drain of the first transistor, and the output of the second transistor is a drain of the second transistor.