Power Amplifier Sense Circuits for 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 where high-frequency components and varying supply voltages complicate common mode rejection.
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 integratable 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 by reusing branch currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If measurement resistor is placed between amplifier outputs and load to measure current, then current measurement is achieved, but common mode rejection requirements become extremely challenging due to large common mode voltage swing
Solution Approach 1:
The patent introduces an intermediary circuit (difference amplifier or instrumentation amplifier) between the measurement resistor and the ADC to handle the common mode voltage swing. This intermediary circuit subtracts the common mode voltage from both sides of the measurement resistor, allowing accurate differential voltage measurement without requiring the ADC itself to have high common mode rejection capabilities.
Solution Approach 2:
The patent replaces the requirement for high common mode rejection in the ADC system with an analog preprocessing stage that actively cancels common mode voltages. This substitution moves the common mode rejection function from the digital domain to the analog domain, where it can be handled by dedicated difference amplification circuitry.
2Measurement precision
If two measurement resistors are placed in series with low-side output transistors to avoid common mode issues, then common mode rejection is improved, but chip area and current consumption increase
Solution Approach 1:
The patent extracts the common mode rejection function from the measurement resistors themselves and places it in a separate amplifier circuit. This allows the use of smaller measurement resistors without requiring them to inherently reject common mode voltages, thereby reducing the area occupied by the sensing network while maintaining measurement accuracy.
Solution Approach 2:
The patent makes the measurement resistors serve multiple functions: current sensing and differential voltage generation, while the common mode rejection is handled by a shared amplifier circuit that also processes other signal conditioning tasks. This multi-functionality reduces the total component count and chip area compared to having dedicated high-precision resistors for each function.
3Measurement precision
If high-side sense resistors are included to solve distortion issues with BD modulation, then measurement accuracy is improved, but noise and distortion increase due to four possible circuit states
Solution Approach 1:
The patent employs feedback mechanisms in the difference amplifier circuit to actively compensate for the distortion introduced by the switching of high-side sense resistors. The amplifier continuously adjusts its gain and offset to maintain accurate measurements across all four circuit states, reducing distortion and noise through closed-loop control.
4Measurement precision
If voltage divider circuits are used to measure voltage across load, then voltage measurement is achieved, but noise and distortion increase due to large common mode voltage swing
Solution Approach 1:
The patent introduces difference amplifiers as intermediary circuits between the voltage divider outputs and the ADC. These amplifiers subtract the common mode voltage component from the differential voltage signal, allowing accurate voltage measurement across the load without the noise and distortion that would result from directly ADC-ing the large common mode swing.
Data Source
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.