Op-Amp Input Circuit With Current Mirror for DC Linearity
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Solution Overview
Problem
Operational amplifiers (op-amps) experience instability and unpredictability when a load is introduced, leading to distortion due to non-linear current levels, which existing designs attempt to address by increasing current gain but result in instability.
Innovation Solution
A circuit using two transistor elements, one coupled to each differential input of the op-amp, with the collectors' currents always equalized to eliminate base current changes caused by the load current, forming a compensated current mirror.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If more current gain is provided to the op-amp to accommodate load distortion, then the ability to handle load-induced non-linearity is improved, but the op-amp becomes unstable and unpredictable
Solution Approach 1:
A current mirror circuit is introduced as an intermediary between the load and the differential input stage. This current mirror acts as a mediator that senses the load current and replicates it to compensate for base current variations, thereby eliminating DC non-linearity without requiring increased current gain that would cause instability
Solution Approach 2:
The current mirror provides a feedback mechanism by continuously monitoring the load current and adjusting the base currents of the differential transistors accordingly. This feedback loop ensures that base current changes due to load variations are compensated, maintaining linear operation and stability without needing excessive current gain
2Adaptability or versatility
If the op-amp is designed to handle load currents, then the versatility is improved, but distortion results due to non-linear current levels
Solution Approach 1:
The current mirror serves as an intermediary that decouples the load from the differential input stage. It senses the load current and creates a compensating signal that prevents non-linear current levels from developing in the amplification path, allowing the op-amp to handle various loads without distortion
Solution Approach 2:
The circuit dynamically adjusts the base currents of the differential transistors based on load conditions. By changing the current parameters in response to load variations, the system maintains linear operation across different load scenarios, achieving versatility without distortion
Data Source
AI summary
A circuit for removing non-linearity produced when an amplifier includes a load that results in non-linear current levels is provided. The circuit includes a first transistor element being coupled to one of the differential inputs associated with the amplifier. A second transistor element is coupled to another of the differential inputs associated with the amplifier. The second transistor element is coupled to the current associated with the load. Current passing the collectors of the first and second transistors elements are arranged to be always equal so as to eliminate in the circuit the changes between the base currents of the second transistor element and first transistor element caused by the current associated with the load.