RF Current Sensing Circuit With AC Coupling and DC Extraction
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Solution Overview
Problem
Current sensing in RF power amplifiers at high frequencies is inefficient due to the need for high current consumption by analog circuits with feedback control loops and the use of external resistors, which degrade system efficiency and increase costs.
Innovation Solution
An AC coupled circuit with a low pass filter system and negative feedback loop that replicates the DC content of the RF current signal on-chip without a voltage regulator or sense resistor, using an N:M ratio to accurately sense and convert RF current to DC, thereby eliminating the need for high-speed amplifiers and external resistors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a voltage regulator is used to control the supply of RF power amplifier, then the DC current can be sensed through an amplifier, but the voltage regulator consumes large area and power, resulting in overall power amplifier efficiency degradation
Solution Approach 1:
The patent extracts the DC sensing function from the RF path by using a dedicated sense transistor (M3) that mirrors the drain current of the main RF transistor (M1). This separation allows DC sensing without requiring a voltage regulator, eliminating the associated power loss and area consumption while maintaining sensing accuracy through the current mirror relationship.
Solution Approach 2:
The patent creates a copy of the RF transistor's drain current through the sense transistor M3, which is biased in saturation region. This current copy (Isense) is then processed through filtering and amplification stages to extract DC components, enabling accurate DC sensing without interfering with the RF signal path or requiring additional power-consuming regulation circuitry.
2Measurement precision
If an off-chip external resistor is used to sense DC current, then the DC current can be sensed, but the external resistor is expensive, consumes large area, and compromises accuracy when voltage is brought back into chip
Solution Approach 1:
The patent merges the current sensing function directly into the chip by integrating the sense transistor M3, filtering capacitors (C1, C2), and amplification stages within the same semiconductor device. This integration eliminates the need for external sense resistors, reducing component count, area, and cost while improving accuracy by keeping the sensing voltage within the chip where it can be properly conditioned and processed.
Solution Approach 2:
The patent introduces an intermediary sense transistor M3 that acts as a current-to-voltage converter through its drain-source voltage drop. This intermediary device enables accurate DC sensing by converting the RF transistor's drain current into a proportional voltage signal that can be filtered and amplified, replacing the need for external sense resistors and their associated accuracy and area problems.
3Speed
If high frequency RF current is sensed directly, then the RF signal can be detected, but high-speed amplifiers are required which consume high current and reduce system efficiency
Solution Approach 1:
The patent performs preliminary filtering of the high-frequency RF signal using capacitors C1 and C2 in the filtering stage, which removes the AC components before the signal reaches the amplification stage. This preliminary action allows subsequent amplifiers to operate at lower speeds with reduced bandwidth requirements, significantly reducing their current consumption while still preserving the essential signal information needed for DC extraction and control.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution enhances efficiency and accuracy by reducing power consumption and area usage, improving current sensing without compromising accuracy, and enabling effective over-current protection and RF power detection within the RF power amplifier.
Implementation Method 1
a low pass filter system that extracts the DC content of the RF current signal
Implementation Method 2
an AC coupled circuit that couples the RF signal from the main device to a sense device
Data Source
AI summary
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.