Variable-Gain Amplifier RC Gate Control for Low-Distortion Switching
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Solution Overview
Problem
Conventional variable-gain amplifiers (VGAs) face challenges in CMOS technology, particularly with poor linearity, small input signal handling capability, and increased noise due to upconverted 1/f noise and Vth matching issues, which are not adequately addressed by existing methods like Gilbert's signal tapping and Huang's attenuator-based approaches.
Innovation Solution
The proposed VGA employs a tapped-attenuator ladder with RC circuitry that couples drain, gate, and source terminals of transistors to control signals, allowing the gate terminal to float and reducing distortion by using a two-step transition control sequence and RC enabling circuitry to minimize loading, while also incorporating gate clamping circuits for improved signal handling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If Gilbert's signal tapping method with gm interpolation control is used, then the VGA can achieve continuous gain control, but it suffers from upconverted 1/f noise and poor linearity due to gm errors in CMOS technology
Solution Approach 1:
The patent replaces the Gilbert cell's transconductance-based control mechanism with a switched-capacitor based voltage control mechanism. Instead of using gm interpolation that is sensitive to MOSFET Vth matching errors, the invention uses capacitive coupling to transfer control voltages to the attenuator nodes, eliminating the upconverted 1/f noise and improving linearity while maintaining continuous gain control capability
Solution Approach 2:
The patent changes the control parameter from transconductance (gm) to capacitance (C). By using switched capacitors to couple control voltages to the attenuator nodes instead of using transconductance amplifiers, the system achieves better linearity and noise performance while maintaining the continuous gain control function
2Loss of energy
If Huang's attenuator-based VGA with series and parallel MOS devices is used, then DC bias currents are eliminated, but the control circuit becomes more difficult and gain control shows significant deviations from ideal linear-in-dB gain law
Solution Approach 1:
The patent segments the control function across multiple switched-capacitor circuits, each controlling a specific attenuator node. This segmentation allows independent control of each node without requiring complex interdependent control circuits, simplifying the overall control architecture while eliminating DC bias currents through capacitive coupling
Solution Approach 2:
The patent introduces switched capacitors as intermediary elements between the control voltage sources and the attenuator nodes. These capacitors act as mediators that transfer control signals without requiring DC bias currents, simplifying the control circuit while achieving the desired gain control function
3Adaptability or versatility
If transistors are used as attenuator nodes in the VGA, then CMOS compatibility is achieved, but distortion increases during transistor transitions between on and off states
Solution Approach 1:
The patent applies preliminary action by pre-charging the gate of the transitioning transistor to the voltage level of the incoming node before the actual switching occurs. This preparatory charging action, controlled by the RC circuitry, ensures that the gate voltage follows a smooth trajectory during transitions, minimizing distortion while maintaining CMOS compatibility
Solution Approach 2:
The patent provides beforehand cushioning by using RC circuitry to control the rate of change of the gate voltage during transitions. The capacitor in the RC circuit acts as a cushion that prevents abrupt voltage changes at the gate, reducing distortion during the transition between on and off states while maintaining CMOS compatibility
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
This design enhances the VGA's linearity and input signal handling capability, reduces distortion, and maintains stability across the gain control range, effectively addressing the limitations of previous technologies in CMOS technology.
Implementation Method 1
the RC circuitry for each attenuator node is configured to pass a control signal to the gate terminal of the transistor of the attenuator node in accordance with a first time scale and permit the gate terminal to float in accordance with a second time scale
Implementation Method 2
Each attenuator node comprises a transistor and an RC circuitry that couples drain, gate, and source terminals of the transistor to a control signal for the attenuator node
Data Source
AI summary
Method for a variable-gain amplifier (VGA). A plurality of attenuator nodes is serially connected via a first set of resistors between adjacent attenuator nodes to form an attenuator ladder and coupled to an AC input of the variable-gain amplifier. Each of the attenuator nodes includes a transistor and an RC circuitry that couples drain, gate, and source terminals of the transistor to a control signal for the attenuator node. The VGA also includes an amplifier that has an output produced based on an input to the amplifier connected to a plurality of coupled terminals, each of which is respectively from one of the plurality of attenuator nodes. The RC circuitry for each attenuator node is configured to pass a control signal to the gate terminal of the transistor of the attenuator node in accordance with a first time scale and permit the gate terminal to float in accordance with a second time scale so as to yield a reduction of distortion contributed by the transistor while the transistor is transitioning between on and off states.