Rail-to-Rail Amplifier Bias Control for Low THD Linearity
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional low-voltage rail-to-rail amplifiers often fail to provide sufficient linearity, leading to increased total harmonic distortion (THD) due to varying bias currents across input signal voltage swings.
Innovation Solution
The amplifier maintains a constant bias current across input signal voltage swings by generating a supply side current mirror gate voltage, using transconductance-cancelling transconductors with averaged source voltages, and employing digital-to-analog converters (DACs) to trim offsets, ensuring linearity across the rail-to-rail input common mode range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If conventional low-voltage rail-to-rail amplifiers are used, then the amplifier can operate across the full input common mode voltage range, but the linearity performance deteriorates and total harmonic distortion increases due to varying bias currents
Solution Approach 1:
The patent dynamically adjusts the bias current parameters based on the input common mode voltage level. Two separate bias current circuits are implemented: one for high input common mode voltages and another for low input common mode voltages. This parameter change approach ensures optimal linearity across the entire rail-to-rail input range by selecting appropriate bias conditions for each operating region.
Solution Approach 2:
The amplifier input stage is segmented into two distinct operational regions handled by different transconductor pairs. The first transconductor pair processes high input common mode voltages while the second pair handles low input common mode voltages. This segmentation allows each pair to be optimized for its specific voltage range, preventing the linearity degradation that occurs in conventional single-stage designs.
2Manufacturing precision
If bias current is increased to improve linearity, then linearity performance improves, but power consumption increases
Solution Approach 1:
The bias current is made dynamic rather than static. The patent implements voltage-dependent bias current control where the bias current automatically adjusts its magnitude based on the input common mode voltage level. This dynamic adjustment ensures sufficient bias current for linearity when needed (at high input voltages) while reducing bias current (and thus power consumption) when high linearity is not required (at low input voltages).
Data Source
AI summary
An amplifier includes input transconductors that receive an input signal, the input signal having a voltage swing. A supply side current mirror generates a gate voltage as a function of input signal voltage and current sources that provide a bias current of the input transconductors as a function of the gate voltage to maintain a constant bias current across the voltage swing of the input signal. Resistors average source voltages of the transconductance-cancelling transconductors to provide an average source voltage and apply the average source voltage to wells of input devices of the transconductance-cancelling transconductors to reduce back bias effect. The input devices are laid out in a same well and have a common centroid to cancel out process mismatches. A first I-DAC trims an offset of first transconductors, and a second I-DAC trims an offset of second transconductors to attain low offsets across a rail-to-rail input common mode range.