Linear-Region MOS Attenuator for Low-Distortion AM RF Gain Control
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Solution Overview
Problem
Conventional RF attenuators in AM radio receivers face challenges in minimizing distortion and cost due to external PIN diodes, which require significant DC current and are not integrated into the circuitry, while direct bias current adjustments within the LNA introduce additional distortion.
Innovation Solution
A transistor-based attenuation circuit configured as a voltage-controlled variable resistor, operating in the linear region, is used to attenuate RF signals within the AM radio receiver, employing a scaling circuit and DC bias signal to adjust resistance and minimize distortion, with multiple stages of transistors in parallel to increase attenuation range.
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 power consumption increases and cost increases due to external components
Solution Approach 1:
The patent merges the attenuator function with the LNA by integrating a transistor directly into the LNA circuitry. The transistor's source terminal connects to the LNA input, gate terminal receives the RF signal, and drain terminal connects to the LNA output, eliminating the need for external PIN diodes and reducing both component count and power consumption.
Solution Approach 2:
The patent extracts the attenuation function from external components and implements it within the LNA using a transistor operating in the linear region. This extraction eliminates the need for separate external attenuator components while maintaining the attenuation capability through the transistor's variable resistance characteristic.
2Power
If a variable resistance PIN diode is used as a signal attenuator, then signal attenuation is achieved, but device complexity increases due to external components
Solution Approach 1:
The patent combines the attenuator and LNA into a single integrated circuit block. The transistor is fully integrated within the LNA, with its source connecting to the LNA input stage, gate serving as the signal input, and drain connecting to the LNA output, thereby simplifying the overall device structure.
3Ease of operation
If bias current in the LNA is changed to vary gain, then gain adjustment is achieved, but distortion increases
Solution Approach 1:
The patent uses a dynamically controllable transistor operating point that adjusts the transistor's resistance characteristics without changing the LNA's bias current. By controlling the transistor's gate voltage, the circuit achieves gain adjustment while maintaining the LNA in its optimal linear operating region, thereby avoiding distortion.
4Ease of manufacture
If a transistor is used as a variable resistor for signal attenuation, then integration and power consumption are improved, but distortion may increase due to non-linear resistance characteristics
Solution Approach 1:
The patent changes the transistor's operating parameters by biasing it in the linear region rather than using it in saturation or cutoff. This parameter change linearizes the transistor's resistance characteristics, allowing it to function as a low-distortion variable resistor that can be integrated into the LNA while maintaining signal fidelity.
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 solution effectively reduces distortion and increases the attenuation range while minimizing power consumption and cost by using integrated transistor-based attenuators, ensuring the RF signal is scaled without overloading the LNA, maintaining signal integrity across the desired frequency range.
Implementation Method 1
The transistor functions in the linear region to linearize the transistor resistance characteristics used for signal attenuation
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.