MOS Attenuator Circuit With Linearized Resistance for Low Distortion
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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 functioning as a voltage-controlled variable resistor, with a scaling circuit and DC bias signal, is used to linearize resistance characteristics for signal attenuation, allowing multiple stages to be coupled in parallel to increase attenuation range while minimizing distortion, using MOSFETs and a DC coupled amplifier to handle frequencies from zero Hz to 30 MHz.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a PIN diode is used as a variable resistor for signal attenuation, then signal attenuation is achieved, but DC current consumption increases and device cost increases
Solution Approach 1:
The patent merges the attenuator function with the LNA by using the LNA's own transistor (M1) as the variable resistor. This eliminates the need for a separate PIN diode component, integrating the attenuation function directly into the amplifier circuitry. The transistor's channel resistance is controlled by its gate voltage, allowing it to serve dual purposes as both amplifier and attenuator.
Solution Approach 2:
The LNA transistor serves itself by providing both amplification and attenuation functions. The same transistor that amplifies the signal can be biased to provide attenuation when needed, eliminating the need for external attenuation components. The circuit uses its own internal resources (the transistor's channel resistance) to achieve the attenuation function.
2Adaptability or versatility
If direct bias current adjustment in LNA is used for gain control, then gain adjustment is achieved, but signal distortion increases
Solution Approach 1:
The patent segments the gain control function into two independent parts: (1) bias current control for setting the operating point, and (2) gate voltage control for adjusting the channel resistance. This segmentation allows the circuit to achieve gain control through resistance modulation rather than bias current changes, thereby reducing distortion while maintaining adaptability.
Solution Approach 2:
The patent introduces gate voltage as an intermediary control mechanism. Instead of directly adjusting the bias current (which causes distortion), the gate voltage serves as an intermediary that modulates the channel resistance, thereby controlling the gain indirectly through resistance changes rather than current changes, reducing distortion in the process.
3Reliability
If external PIN diode is used for attenuation, then attenuation function is achieved, but device complexity and cost increase
Solution Approach 1:
The patent combines the attenuation function with the existing LNA transistor, eliminating the need for external PIN diode components. The transistor's channel resistance naturally provides the attenuation function when properly biased, reducing device complexity by integrating multiple functions into a single component rather than requiring separate external parts.
Solution Approach 2:
The LNA transistor is designed to perform multiple functions: signal amplification, signal attenuation, and gain control. By making the transistor multi-functional, the patent eliminates the need for dedicated external components for each function, thereby reducing overall device complexity while maintaining reliable attenuation performance.
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 approach effectively reduces distortion and increases the range of attenuation, integrating the attenuator within the receiver circuitry to reduce cost and power consumption, maintaining signal integrity across the desired frequency range.
Implementation Method 1
a first MOSFET transistor (M1) having a drain, a source, a gate and a channel between the drain and the source. The gate is coupled to a first control voltage (Vctl) that regulates a channel resistance (Rch) of the channel between the drain and the source.
Implementation Method 2
A distortion compensation circuit (200) reduces second-order distortion in a signal that has been attenuated by an attenuator (100). The distortion compensation circuit (200) includes a second MOSFET transistor (M2) that is identical to the first MOSFET transistor (M1)... the distortion compensation circuit generates a distorted version of an input signal and then subtracts the distorted version from the input signal to reduce second-order distortion.
Implementation Method 3
The attenuator (100) includes a first capacitor (C1) coupled between the drain and the gate of the first MOSFET transistor (M1).
Implementation Method 4
The attenuator (100) includes a second capacitor (C2) coupled in parallel with the second MOSFET transistor (M2)... a third capacitor (C3) coupled in parallel with the third MOSFET transistor (M3)
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.