Power Amplifier Load Sensing With Common-Mode Rejection
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Solution Overview
Problem
Existing voltage and current sense circuits for power amplifiers face challenges in efficiently measuring load impedance due to common mode voltage swings, leading to noise, distortion, and increased power consumption, especially in class-D operation modes.
Innovation Solution
The proposed solution involves a voltage sense circuit with a driver circuit that rejects common mode variations by converting differential voltage into a current with a suitable amplitude and common mode voltage level, directly integrable by an ADC, and a current sense circuit with a common mode loop that replicates the output common mode voltage to high-side sense resistors, reducing noise and distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If measurement resistors are placed between the outputs of the amplifier and the load, then current measurement is achieved, but common mode rejection requirements become extremely challenging, increasing chip area and/or current consumption
Solution Approach 1:
The patent introduces an intermediary circuit (differential amplifier with common mode rejection) between the measurement resistor and the ADC to handle the common mode voltage swing. This mediator converts the differential voltage signal while rejecting the common mode component, allowing accurate current measurement without directly exposing the ADC to large common mode swings.
Solution Approach 2:
The patent replaces the direct voltage measurement approach with a current measurement approach using measurement resistors, and further substitutes the need for high-precision voltage differential amplification with a current-mode measurement system that is inherently more tolerant of common mode variations.
2Object-affected harmful factors
If two measurement resistors are placed in series with the sources of the low-side output transistors, then common mode issues are tackled, but distortion increases due to incomplete sensing in certain circuit states
Solution Approach 1:
The patent segments the measurement function across multiple resistors (high-side and low-side sense resistors) and multiple amplification stages. By dividing the sensing function, the system can accurately measure current in all circuit states including push-pull and class-B operation modes, where single-resistor approaches fail.
Solution Approach 2:
The patent implements dynamic switching of measurement paths based on the circuit state. The measurement system adapts its configuration to handle different operating modes (push-pull, class-B, etc.), ensuring accurate current sensing regardless of which transistors are conducting at any given moment.
3Measurement precision
If high-side sense resistors are included in the measurement, then distortion is reduced, but device complexity and power consumption increase
Solution Approach 1:
The patent merges the high-side and low-side sensing functions into a unified measurement system. By combining the voltage drops across both high-side and low-side resistors, the system achieves accurate full-range current measurement while sharing common circuit resources such as the ADC and reference voltages, thereby reducing overall complexity.
4Use of energy by moving object
If the measurement resistor impedance is made small (1% of load or smaller) for power efficiency, then power consumption is reduced, but the differential signal to be observed becomes much smaller, challenging measurement precision
Solution Approach 1:
The patent creates an amplified copy of the small differential voltage signal through operational amplifiers or transimpedance amplifiers. The measurement amplifier copies and amplifies the tiny voltage drop across the low-impedance sense resistor, making it suitable for ADC conversion while maintaining the power-efficient low-resistance sensing approach.
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 approach enables efficient measurement of load impedance with reduced noise and distortion, allowing for identical ADC circuits to handle both current and voltage, thereby improving power efficiency and chip area utilization.
Implementation Method 1
a first voltage divider circuit comprising an input coupled to the first input terminal and an output coupled to the first output terminal, a second voltage divider circuit comprising an input coupled to the second input terminal and an output coupled to the second output terminal
Implementation Method 2
a driver circuit comprising a first input configured to receive a reference voltage, a second input configured to receive a common mode signal of the first and the second voltage divider circuits, and an output configured to drive an output common mode voltage of the first and the second voltage divider circuits with the reference voltage
Implementation Method 3
a current sense circuit with a common mode loop that replicates the output common mode voltage to high-side sense resistors, reducing noise and distortion
Data Source
Figure 1A~1B
Figure 1C~1D
Figure 1E
AI summary
Voltage sense circuit for measuring a load connected to a power amplifier, the load configured to receive a first and second voltage in opposite phase, the voltage sense circuit comprises a first input terminal coupled to the first voltage, a second input terminal coupled to the second voltage, a first voltage divider circuit comprising an input coupled to the first input terminal and an output coupled to the first output terminal, a second voltage divider circuit comprising an input coupled to the second input terminal and an output coupled to the second output terminal and a driver circuit comprising a first input configured to receive a reference voltage, a second input configured to receive a common mode signal of first and second voltage divider circuits, and an output to drive an output common mode voltage of the first and the second voltage divider circuits with the reference voltage