Auto-Zero Transimpedance Amplifier for Offset and Bias Compensation
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Solution Overview
Problem
Photodiode-transimpedance amplifier circuits in laser power control circuits for DVD applications face limitations in achieving high speed and precision due to parasitic capacitance, limited open-loop gain, and significant input offset voltages and bias currents in traditional voltage feedback amplifiers.
Innovation Solution
A circuit using a current-mode-feedback amplifier with an operational transimpedance amplifier, employing input and output acquisition loops with buffers and sampling capacitors, and switchable resistors to compensate for input offset voltage and bias current errors, allowing for high precision over a wide range of operating conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a voltage feedback amplifier is used, then the input offset voltage and bias current are low, but the speed is limited due to parasitic capacitance and the secondary pole formed by diode capacitance and amplifier input resistance
Solution Approach 1:
The patent divides the feedback mechanism into two separate loops: an input acquisition loop that samples and stores offset voltage and bias current effects on capacitor C1, and an output acquisition loop that samples and stores output offset voltage on capacitor C2. This segmentation allows the amplifier to operate at high speed without being constrained by offset compensation requirements, as the compensation occurs in separate phases.
Solution Approach 2:
The input acquisition loop performs preliminary sampling and storage of offset voltage and bias current effects during a first phase before the actual signal amplification occurs. By pre-capturing these error terms on capacitor C1, the system eliminates the need for the amplifier to maintain high precision during high-speed operation, thus resolving the speed-precision tradeoff.
2Measurement precision
If the open loop gain is increased to achieve high precision, then the transimpedance-setting resistor can be larger, but the secondary pole contributes excessive phase shift leading to instability
Solution Approach 1:
The patent extracts the offset compensation function from the main signal amplification path by using separate acquisition phases. The input acquisition loop captures offset effects on capacitor C1 during a first phase, and the output acquisition loop captures output offset on capacitor C2 during a second phase. This extraction allows the amplifier to operate with high open-loop gain for precision without the secondary pole causing instability, as the compensation occurs independently in dedicated phases.
3Speed
If a current-mode-feedback amplifier is used to increase speed, then the secondary pole is moved away from the bandwidth, but the input offset voltage and bias current become extremely high
Solution Approach 1:
The patent implements dual feedback loops: the input acquisition loop provides feedback to cancel input offset voltage and bias current effects by storing sampled values on capacitor C1, and the output acquisition loop provides feedback to cancel output offset voltage by storing sampled values on capacitor C2. This feedback mechanism enables the use of high-speed current-mode-feedback amplifiers while maintaining precision through active compensation of their inherent offset errors.
Data Source
AI summary
A circuit with an input acquisition loop and an output acquisition loop is used to compensate for the input offset voltage and bias current errors of an operational amplifier.