DC-Coupled Oscilloscope Probe Offset Compensation
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Solution Overview
Problem
Existing oscilloscope probes with active impedance converters face challenges in minimizing direct-voltage offset errors and transfer distortions, particularly in achieving a balance between bandwidth, input capacitance, and offset voltage, which complicates the measurement of earth-referenced signals.
Innovation Solution
A DC-coupled transistor amplifier using bipolar or field-effect transistors in emitter-follower or source-follower circuits is employed, with circuit design that cancels out offset voltages, and additional compensation methods to manage residual offsets, including calibration and automatic earth connection for precise measurement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a composite amplifier with high-frequency path and low-frequency path is used, then bandwidth is improved, but transfer distortions and offset errors increase
Solution Approach 1:
The patent extracts the offset compensation function from the complex composite amplifier structure and implements it separately through a dedicated offset compensation circuit. This allows the high-frequency path to maintain its bandwidth advantage while the offset error is addressed through a separate, simplified compensation mechanism, thereby resolving the contradiction between bandwidth and offset error.
Solution Approach 2:
The patent introduces an intermediary offset compensation circuit that acts as a mediator between the high-frequency signal path and the ground. This intermediary circuit dynamically adjusts to cancel offset errors without interfering with the high-frequency signal transmission, enabling both high bandwidth and low offset error simultaneously.
2Measurement precision
If operational amplifier with smallest possible offset is used, then offset error is improved, but input capacitance and input current increase
Solution Approach 1:
The patent replaces the expensive and complex operational amplifier with a simpler transistor-based amplifier that has inherently lower input capacitance. While individual transistors have larger offset errors, the overall system achieves minimal offset error through careful circuit design and compensation, avoiding the harmful input capacitance associated with operational amplifiers.
Solution Approach 2:
The patent changes the fundamental parameters of the amplifier by using transistor-based circuits instead of operational amplifiers. This parameter change reduces input capacitance and input current while offset error is controlled through circuit configuration and compensation techniques, resolving the contradiction between offset error and input capacitance.
3Measurement precision
If DC-coupled transistor amplifier is used, then offset error is improved, but temperature and ageing dependence increases
Solution Approach 1:
The patent implements feedback mechanisms through automatic offset compensation circuits that continuously monitor and adjust the offset voltage. This feedback system compensates for temperature and ageing effects in real-time, maintaining stable performance despite the inherent temperature dependence of transistor-based circuits.
Solution Approach 2:
The patent uses parameter changes through temperature compensation circuits that adjust operating parameters dynamically. By monitoring temperature variations and adjusting circuit parameters accordingly, the system maintains stable offset error performance despite the temperature sensitivity of transistor-based amplifiers.
Data Source
AI summary
Disclosed is a probe for an oscilloscope comprising a multi-stage transistor amplifier that acts as an impedance transformer. Said amplifier is a d.c.-coupled emitter follower circuit that is composed of bipolar transistors or a d.c.-coupled source follower circuit which is composed of field effect transistors and the successive amplifier elements of which are dimensioned and tuned to each other in such a way that the resulting offset direct voltage between the input and the output is minimal.