Current-Mode Chopper Amplifier for High Gain With Low Noise
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Solution Overview
Problem
Chopper amplifiers face challenges in maintaining high gain while minimizing noise and offset, particularly when processing differential input signals with small values, as high-gain amplification introduces undesirable noise and offset, and existing chopping methods can reduce gain due to parasitic capacitance.
Innovation Solution
The implementation of a current mode chopping circuit in a chopper amplifier, which switches current paths based on a clock signal to alternate couplings between nodes, allowing the amplifier to operate more efficiently by maintaining a voltage path and reducing input referred offset voltage and noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If high-gain amplification is used to amplify small differential input signals, then signal gain is improved, but noise and offset increase
Solution Approach 1:
The patent applies periodic chopping action to modulate the differential input signal to an intermediate frequency. By periodically switching the input signal between different nodes using clocked switches, the signal is transformed from DC to AC domain, allowing amplification without directly amplifying the DC offset and low-frequency noise. This periodic modulation enables high gain amplification of the modulated signal while the original DC offset remains at DC and can be easily filtered.
2Object-generated harmful factors
If conventional voltage mode chopping is used to reduce noise and offset, then noise and offset are reduced, but gain is reduced due to parasitic capacitance
Solution Approach 1:
The patent replaces conventional voltage mode chopping with current mode chopping. Instead of using voltage switches that introduce parasitic capacitance and limit gain, the invention uses current mode switches (current mirrors and current controlled switches) to perform the chopping function. This substitution of switching mechanism eliminates the parasitic capacitance bottleneck, allowing high gain amplification while maintaining the noise and offset reduction benefits of chopping.
Solution Approach 2:
The patent changes the operating parameters of the chopping circuit from voltage mode to current mode. By transforming the chopping mechanism to operate in current mode with controlled current sources and current mirrors, the circuit achieves different performance characteristics: high output impedance, high gain capability, and reduced sensitivity to parasitic capacitance, while maintaining the signal modulation function.
3Measurement precision
If current mode chopping is implemented to maintain high gain, then gain is improved, but circuit complexity increases
Solution Approach 1:
The patent segments the chopper amplifier into distinct functional blocks: input chopper stage, differential amplifier stage, and output chopper stage. Each stage is independently designed and optimized. The current mode chopping function is further segmented into separate current mirror circuits and switching networks, making the complex current mode operation manageable and implementable through modular design.
Solution Approach 2:
The patent employs current mirror circuits that serve multiple functions: they act as active loads for differential amplifiers, provide current switching for chopping operation, and enable impedance transformation. This multi-functionality reduces the need for separate dedicated components, thereby managing circuit complexity while achieving high gain current mode chopping.
Data Source
AI summary
A circuit, comprising an input chopper configured to receive an input signal, a differential amplifier having an input coupled to an output of the input chopper, a current mode chopping circuit coupled to an output of the differential amplifier, and a first current mirror bias transistor pair coupled between a voltage supply and the current mode chopping circuit.