Auxiliary Bipolar Trim Circuit for Amplifier Offset Current
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Solution Overview
Problem
Conventional circuits face difficulties in independently trimming offset currents in amplifiers, even when offset voltages are reduced to near zero, due to mismatches in base currents of bipolar transistors, which can significantly affect the precision and accuracy of signal measurement in high noise gain and high input impedance applications.
Innovation Solution
An offset current trim circuit is implemented using auxiliary bipolar transistors connected in parallel with the bases of input bipolar transistors, with auxiliary current sources biasing these transistors to compensate for mismatches in base currents, ensuring equal combined base currents and minimizing the impact on voltage noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional offset trimming circuits are used to reduce offset voltage, then offset voltage is trimmed close to zero, but offset current remains present and cannot be trimmed independently
Solution Approach 1:
The offset trimming function is segmented into two independent parts: offset voltage trimming (handled by conventional circuits) and offset current trimming (handled by the auxiliary transistor circuit). The auxiliary transistors Q3 and Q4 are separately controlled by current sources I2 and I3, allowing independent adjustment of base currents to compensate for offset current while leaving offset voltage trimming unaffected.
Solution Approach 2:
Auxiliary bipolar transistors Q3 and Q4 are introduced as intermediary elements to mediate the offset current compensation. These auxiliary transistors are connected in parallel with the bases of input transistors Q1 and Q2, and their base currents (controlled by I2 and I3) serve as compensating currents that counterbalance the mismatch in base currents of the input transistors, thereby eliminating offset current without interfering with offset voltage trimming.
2Measurement precision
If amplifier input impedance is increased to improve signal measurement, then measurement sensitivity is improved, but offset current impact is amplified
Solution Approach 1:
The circuit implements feedback compensation by continuously monitoring the base current mismatch of the input transistors and using auxiliary current sources to provide compensating currents. The auxiliary current sources I2 and I3 are adjusted to provide base currents that feedback compensate for the offset current, ensuring that the high input impedance amplifier operates with minimal offset current impact.
Data Source
AI summary
Apparatus and methods are disclosed related to trimming an input offset current of an amplifier. One such apparatus can include auxiliary bipolar transistors connected in parallel with bases of respective bipolar transistors of an input stage of an amplifier. The auxiliary bipolar transistors can be biased such that the base currents of the auxiliary bipolar transistors compensate for a mismatch in base currents of the bipolar transistors of the input stage of an amplifier. The offset current at an input of an amplifier can be reduced independent of an offset voltage at the input of the amplifier.