AC-Coupled Chopper Buffer for High-Impedance Wide-Swing Inputs
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Solution Overview
Problem
Conventional integrated circuits face challenges in implementing high impedance input amplifiers due to voltage offset and low frequency noise, which affect signal accuracy and transistor degradation, especially when dealing with large voltage swings that exceed the degradation voltage of low-voltage transistors.
Innovation Solution
A chopper-stabilized amplifier technique is employed, using a chopper clock signal with a peak-to-peak voltage less than the input signal range to chop the input signal, modulating a bias signal with an AC-coupled clock signal to generate a chopper clock signal, and biasing body terminals of low-voltage transistors in the input chopper circuit, allowing the use of low-voltage transistors beyond their degradation voltage range while maintaining stable operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If low-voltage transistors are used in the input chopper circuit, then the transistor switching characteristics are good and on-resistance is low, but the transistors cannot handle voltage swings exceeding their degradation voltage
Solution Approach 1:
An AC-coupled clock signal is introduced as an intermediary between the power supply and the chopper switches. This clock signal modulates the bias voltage applied to the low-voltage transistors, allowing their effective operating range to extend beyond their native degradation voltage limit. The AC coupling capacitor isolates the DC bias point while allowing the AC chopper signal to pass through, enabling the transistors to switch reliably even when the input signal voltage exceeds their maximum rated voltage.
Solution Approach 2:
The bias voltage parameter applied to the chopper switches is dynamically changed by AC coupling. Instead of using a fixed DC bias, the system uses an AC-coupled clock signal that varies the bias voltage in sync with the chopping frequency. This parameter change allows the transistors to operate in their optimal switching region during each half-cycle, maintaining low on-resistance and good switching characteristics across the full input voltage range.
2Adaptability or versatility
If high-voltage transistors are used in the input chopper circuit, then the transistors can handle large voltage swings, but the switching characteristics deteriorate and on-resistance increases
Solution Approach 1:
The AC-coupled clock signal acts as a mediator that allows low-voltage transistors to effectively handle high voltage swings. By superimposing the chopper switching action on top of the AC-coupled bias signal, the system enables low-voltage transistors to switch signals with amplitude exceeding their degradation voltage without requiring the transistors themselves to be rated for those high voltages.
3Power
If a conventional inverting amplifier topology is used, then the voltage gain can be controlled by resistor ratios, but the input impedance is limited by the input resistor value
Solution Approach 1:
The amplifier is segmented into multiple functional blocks: an input chopper circuit for impedance transformation, a buffer stage for voltage following, and a feedback network for gain control. This segmentation allows the input stage to present high impedance to the signal source while the feedback network independently controls the overall voltage gain, decoupling these two previously coupled parameters.
4Measurement precision
If chopper stabilization is applied to reduce DC offset and low frequency noise, then signal accuracy improves, but the chopper switches experience voltage swings that may exceed their degradation voltage
Solution Approach 1:
The AC-coupled clock signal serves as a protective intermediary that prevents the chopper switches from experiencing dangerous voltage differentials. By coupling the clock signal through a capacitor, the system allows the chopper operation to proceed at the required frequency for noise rejection while limiting the voltage stress on the transistor gate oxides to safe levels.
Data Source
AI summary
A technique for receiving a DC or low frequency input signal using a chopper-stabilized amplifier includes chopping an input signal using a chopper clock signal to generate a chopped input signal. The input signal has a first voltage range and the chopper clock signal has a second voltage range. The chopper clock signal has peak-to-peak voltage over a period of the chopper clock signal. The peak-to-peak voltage is less than the first voltage range and is less than the second voltage range. A frequency of the input signal is at least an order of magnitude less than a frequency of the chopper clock signal. The second voltage range may be greater than or equal to the first voltage range. The technique may include generating a bias signal based on a voltage reference signal and an output signal having the first voltage range.