Amplifier DC Current Limiting for Output Power Capping
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Solution Overview
Problem
Existing amplifier systems face challenges in limiting maximum output power without degrading linearity, particularly in the non-linear region, as existing techniques either require complex control systems, affect linear region linearity, or result in instability and reduced DC power availability.
Innovation Solution
A DC current limiting circuit is introduced between the DC power supply and the amplifier, allowing the amplifier to draw increasing current up to a predetermined level and then limiting it, maintaining linearity in the linear region while capping output power in the non-linear region, using a Nonlinear Mesa Resistor or Field Effect Transistor with Connected Gate and Source Electrodes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If feedback control is used to limit output power, then maximum output power can be limited, but the system requires extensive control theory analysis for stability and has finite response time
Solution Approach 1:
The patent extracts the power limiting function from the complex feedback control system and implements it through a simple DC current limiting circuit that directly limits the drain current of the amplifier transistor. This eliminates the need for detectors, switches, and control theory analysis while achieving the same power limiting effect.
Solution Approach 2:
The patent introduces a DC current limiting circuit as an intermediary between the power supply and the amplifier transistor. This intermediary component (implemented with a nonlinear resistor or diode) automatically limits the current without requiring active control elements, thereby simplifying the system while maintaining power limiting capability.
2Power
If a switch is placed in the signal path for power limiting, then output power can be limited, but the switch's insertion loss adversely affects linearity in the linear region
Solution Approach 1:
The patent segments the amplifier operation into linear region and compression region, applying different current limiting mechanisms to each. In the linear region, the current limiting circuit has minimal effect on linearity, while in the compression region, it effectively limits the output power through DC current control.
Solution Approach 2:
The DC current limiting circuit acts as an intermediary that controls the amplifier's operating point without introducing insertion loss into the AC signal path. By limiting the DC bias current rather than directly controlling the AC signal, the circuit maintains linearity in the linear region while achieving power limitation in the compression region.
3Power
If a nonlinear component is placed in the signal path for power limiting, then output power can be limited above threshold, but the component reflects AC signal causing circuit instability and channel temperature increase
Solution Approach 1:
The patent places the nonlinear limiting component in the DC current path rather than the AC signal path. The nonlinear resistor or diode limits the DC bias current to prevent the amplifier from entering excessive compression, thereby avoiding AC signal reflection and thermal instability while still achieving power limitation.
Solution Approach 2:
The patent replaces the mechanical/AC signal path limiting approach with an electrical/DC current path limiting approach. By controlling the DC bias current through a nonlinear element, the system achieves power limitation without the harmful effects of AC signal reflection and thermal runaway that occur with AC path limiting.
4Power
If DC bias is reduced to limit output power, then maximum output power is limited, but DC power availability is reduced affecting overall amplifier performance
Solution Approach 1:
The patent implements dynamic DC current limiting where the current limiting circuit automatically adjusts its resistance based on the operating conditions. The nonlinear resistor or diode provides low resistance during normal operation to maximize power availability, and high resistance during compression to limit output power, thereby dynamically optimizing both power availability and output power limitation.
Data Source
Figure 1
Figure 2~3
Figure 4
AI summary
An amplifier system (10) having: an amplifier (FET) having a linear operating region where an output signal produced by the amplifier (FET) at the output terminal (18) has a power level increasing proportionally with the increasing input signal power level up to a compression region of the amplifier (FET) where the output power is inhibited from increasing with increasing input signal power; and a DC current limiting circuit (14), coupled between a DC power supply and the amplifier (FET), to: supply DC current from the DC power supply that is equal to quiescent current to the amplifier (FET) from the DC power supply when the amplifier (FET) operates in the linear region.