MOS RF Attenuator Staging for Low-Distortion AM Receiver Gain Control
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Solution Overview
Problem
Conventional AM radio receiver front-ends face challenges in minimizing distortion when attenuating RF signals, as existing solutions like PIN diodes introduce cost and power consumption issues and can drive LNAs into non-linear regions, causing signal distortion.
Innovation Solution
A transistor configured as a voltage-controlled variable resistor is used in an attenuation circuit, with a scaling circuit and DC bias signal to adjust resistance, allowing for linear signal attenuation and minimizing distortion across a range of frequencies from zero Hz to 30 MHz, and multiple stages are cascaded to increase attenuation range while maintaining linearity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a variable resistance PIN diode is used as a signal attenuator, then signal attenuation is achieved, but cost and power consumption increase
Solution Approach 1:
The patent replaces the PIN diode (a semiconductor device requiring significant DC current) with a MOS transistor operating in the linear region. The MOS transistor uses voltage control at the gate terminal to adjust resistance, eliminating the need for high DC current through the signal path. This substitution reduces power consumption while maintaining signal attenuation functionality.
Solution Approach 2:
The patent extracts the variable resistance function from external components (PIN diodes) and integrates it directly into the receiver front-end using MOS transistors. This integration eliminates external components, reduces power consumption, and allows the transistor to operate in its linear region for low distortion attenuation.
2Adaptability or versatility
If the LNA gain is varied by changing bias current, then gain adjustment is achieved, but distortion is introduced
Solution Approach 1:
The patent introduces a MOS transistor as an intermediary component between the antenna and the LNA. This transistor acts as a voltage-controlled variable resistor that attenuates the signal before it reaches the LNA, allowing the LNA to operate at fixed bias current while still achieving gain adjustment. This intermediary approach avoids the distortion that would result from varying the LNA's bias current.
Solution Approach 2:
The patent segments the gain control function into two independent parts: signal attenuation (performed by the MOS transistor before the LNA) and LNA amplification (performed by the LNA at fixed bias). This segmentation allows each component to operate optimally - the transistor provides variable attenuation without distortion, and the LNA provides stable amplification without the distortion that would result from bias current variation.
3Reliability
If signal strength is increased to improve reception, then weak signals are amplified, but strong signals overload the LNA and cause distortion
Solution Approach 1:
The patent applies preliminary attenuation using the MOS transistor before the signal enters the LNA. By controlling the transistor's gate voltage, the signal is pre-attenuated to an appropriate level that prevents LNA overload. This preliminary action ensures that both weak and strong signals are conditioned to suitable levels for the LNA, maintaining linear operation and preventing distortion across the full dynamic range of input signal strengths.
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
The solution effectively reduces distortion and increases the range of attenuation, allowing for efficient signal scaling without overloading LNAs, thereby maintaining signal quality and reducing operational costs by integrating components within the radio receiver circuitry.
Implementation Method 1
The transistor functions in the linear region to linearize the transistor resistance characteristics used for signal attenuation
Implementation Method 2
A scaled version of the RF signal is supplied to the gate of the transistor. The scaled RF signal reduces the magnitude of distortion products generated by the transistor
Data Source
AI summary
An attenuation circuit uses a voltage controlled variable resistance transistor as a signal attenuator for receivers operating in the zero Hz to about 30 MHz range. The transistor functions in the linear region to linearize the transistor resistance characteristics used for signal attenuation. In an exemplary application, the attenuation circuit is used as an RF attenuator for AM radio broadcast receivers and amplifiers with automatic gain control. Multiple attenuation circuits can be coupled in parallel, each attenuation circuit having a different sized variable resistance transistor, to form sequentially activated stages that increase the range of attenuation while minimizing distortion.