Chopper Op Amp Input Current Trimming for Charge Injection Bias
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Solution Overview
Problem
Chopper-stabilized operational amplifiers face accuracy issues due to significant input bias current caused by charge injection and clock feed-through from input chopping switches, which existing solutions often complicate or require additional components.
Innovation Solution
A current control circuit using two variable voltage sources and a plurality of inverters to generate control signals that adjust the amplitudes of base currents at the input terminals, reducing the DC currents by controlling the gate drive voltages of transistor switches through level shifting capacitors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If input chopping switches are used in chopper operational amplifier, then offset voltage performance is improved, but input bias current increases significantly
Solution Approach 1:
The patent converts the harmful charge injection effect into a beneficial trimming mechanism. By intentionally injecting equal and opposite charges through control switches during a trimming phase, the harmful DC offset current is canceled out. The charge injection that was previously purely harmful is now harnessed to generate the trimming signal needed to eliminate the offset.
Solution Approach 2:
The patent introduces control switches and capacitors as intermediary elements between the input chopping switches and the output. These intermediaries capture the charge injection during the chopping operation and then release controlled amounts of charge to cancel the DC offset, mediating between the harmful effect and the desired low bias current state.
2Object-generated harmful factors
If existing solutions for controlling input bias current are implemented, then input bias current is reduced, but device complexity increases due to additional components
Solution Approach 1:
The control switches serve multiple functions: they act as input chopping switches during normal operation to achieve low offset voltage, and simultaneously serve as charge injection sources during the trimming phase to cancel DC offset current. This multi-functionality eliminates the need for separate trimming switches or additional control circuitry.
Solution Approach 2:
The patent merges the offset trimming function with the existing input chopping circuitry. The same switches and capacitors used for chopping are also used for trimming, combining two previously separate functions into a unified circuit structure that reduces overall complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Substantially reduces base currents to approximately zero, enhancing the accuracy of chopper operational amplifiers by minimizing charge injection-induced DC offsets.
Implementation Method 1
The charge injection that occurs at the input chopping switches of a chopper op amp results in a DC current component
Implementation Method 2
The input bias current of CMOS chopper op amps is significantly larger than that in CMOS standard op amps because of charge injection and clock feed-through induced from input chopping switches
Data Source
AI summary
In examples of a chopper operational amplifier, a current control circuit comprises a pair of voltage sources, each of which may be varied to generate a voltage signal of a particular value, and multiple inverters, each of which is configured to receive either a clock signal or its complement signal and one of the voltage signals. Based on these inputs, each inverter generates a control signal that is delivered to a corresponding switch in the input stage of the chopper operational amplifier to control the gate voltage of that switch. Based on the difference between the values of the voltage signals, the current control circuit operates to reduce the amplitudes of base currents induced by charge injection at the input terminals of the chopper operational amplifier.