Dynamic Current Sensing Compensation for Output Transistors
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Solution Overview
Problem
Current sensing in driver or amplifier circuits often faces inaccuracies due to signal-dependent variations in drain-source voltage between sense transistors and output devices, leading to incorrect current limiting and potential distortion in output signals.
Innovation Solution
A signal processing circuit with a current monitor and compensation controller that adjusts the operation of the current sensor based on the signal level, compensating for signal-dependent errors by varying the device scaling, source resistance, or gain of the sense transistor, ensuring accurate current monitoring without impacting the output range or noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If current sensing is performed using current mirroring techniques with sense transistors, then the current can be monitored without impacting the actual current path (headroom and noise), but signal-dependent variations in drain-source voltage cause inaccuracies in the sensed current
Solution Approach 1:
The patent dynamically adjusts the scaling ratio between the sense transistor and output transistor based on the signal level. At different signal levels, different scaling ratios are applied to compensate for the non-linear effects of drain-source voltage variations. This parameter change approach allows the current sensing to remain accurate across the full dynamic range of the amplifier, resolving the contradiction between maintaining headroom and achieving accurate current measurement.
Solution Approach 2:
The patent implements a dynamic current sensing system where the sense transistor scaling is adjusted in real-time based on the operating conditions. The system transitions from a static scaling ratio to a dynamic adjustment mechanism that responds to signal level changes. This dynamic approach ensures that the current sensing remains reliable and accurate regardless of the drain-source voltage variations that occur during different operating phases.
2Measurement precision
If a sense resistance is used for current sensing, then the current can be directly measured, but the headroom is reduced due to the voltage drop across the sense resistance
Solution Approach 1:
The patent extracts the current sensing function from the main current path by using current mirroring techniques. Instead of placing a sense resistance in series with the output devices, the system creates a separate sensing path that mirrors the current through a sense transistor. This extraction eliminates the voltage drop in the output path while maintaining the ability to measure current, thus preserving headroom while enabling current measurement.
Solution Approach 2:
The patent uses current copying through the sense transistor to replicate the output current for measurement purposes. The sense transistor is configured to copy the current flowing through the output transistor, and this copied current is then used for sensing and limiting decisions. This copying mechanism allows accurate current measurement without introducing any element into the main current path that would reduce the output voltage range.
3Ease of manufacture
If the sense transistor scaling is fixed, then the circuit is simple to implement, but the current sensing accuracy varies with signal level
Solution Approach 1:
The patent transitions from a fixed scaling ratio to a dynamic adjustment mechanism. The sense transistor scaling is no longer static but is adjusted based on the signal level and operating conditions. This dynamic approach maintains circuit simplicity while significantly improving measurement precision across the full operating range, resolving the contradiction between ease of manufacture and measurement accuracy.
Solution Approach 2:
The patent changes the scaling parameter of the sense transistor dynamically based on operating conditions. Instead of using a single fixed scaling ratio, the system adjusts the effective scaling ratio to compensate for non-linear effects at different signal levels. This parameter change approach maintains implementation simplicity while achieving accurate current sensing across all operating conditions.
Data Source
AI summary
This invention relates to current sensing, in particular for a signal processing circuit (500) for outputting an output signal (Sout) based on an input signal (Sin). An output stage (101) includes an output transistor (102) driven, in use, by a drive signal. A current monitor (501) is configured to monitor, in use, a first current through the output transistor, wherein the current monitor comprises a current sensor (105) having a sense transistor (106) configured to be driven based on the drive signal so as to generate a sense current related to the first current. A compensation controller (301) receives an indication of signal level of the input signal and controllably varies operation of the current monitor (501) so as to at least partially compensate for signal-dependent variation in a relationship between the first current and the first sense current.