LNA Current Bleeding Branch for Wide Gain and Output Headroom
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Solution Overview
Problem
Low-noise amplifiers (LNAs) in radio receivers face challenges in accommodating a wide range of input signal levels without compromising noise figure and linearity, particularly in mobile environments where signal strength varies, leading to issues of under-amplification or saturation, and increased power consumption.
Innovation Solution
The implementation of a current bleeding branch in the LNA circuit, which allows a portion of the current from the current source to flow through parallel-connected load resistors, controlled by a control transistor, enabling variable gain without increasing load voltage and reducing noise, thereby maintaining output headroom and improving linearity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the gain of the LNA is increased by increasing the transconductance gm of the input device or the load resistance, then the amplification capability is improved, but the voltage across the load resistor increases requiring higher operating voltage
Solution Approach 1:
The current from the current source is segmented into two paths: one through the load resistor and another through the current bleeding branch. This segmentation allows the load resistor to operate at lower voltage while still achieving the required amplification through the controlled current division.
Solution Approach 2:
The current bleeding branch acts as an intermediary element that controls the current distribution between the load resistor and ground. By introducing this intermediate current path, the system achieves variable gain without increasing the voltage across the load resistor.
2Adaptability or versatility
If multiple input devices with various sizes are used to implement various gains, then the variable gain capability is improved, but the device complexity increases
Solution Approach 1:
Instead of using multiple static input devices of different sizes, the invention uses a dynamic current control mechanism where a single input device's effective gain is varied by controlling the current through the bleeding branch. This dynamic approach achieves variable gain with a single device configuration.
Solution Approach 2:
The gain is varied by changing the current parameter through the bleeding branch controlled by the control transistor, rather than changing the physical size or configuration of multiple input devices. This parameter-based control simplifies the device structure while maintaining adaptability.
3Reliability
If the bias current of the input device is increased to improve noise figure and linearity, then the signal quality is improved, but the power dissipation increases
Solution Approach 1:
The noise figure and linearity are improved by optimizing the bias current parameter of the input device at lower power levels, rather than continuously increasing power consumption. The bleeding branch current is adjusted to achieve the desired signal quality at minimum power dissipation.
Solution Approach 2:
The current bleeding branch provides self-regulation of the current distribution, allowing the LNA to maintain optimal bias conditions for noise and linearity performance without requiring excessive external power input. The system automatically balances current to achieve quality at efficiency.
Data Source
AI summary
An LNA circuit for providing a wide range of gain while maintaining the output headroom. In a radio frequency (RF) receiver, the signal received by the receiver may be extremely small. For a transmitter in a short distance, the received signal may be relatively strong. A low power amplifier usually is used to amplify the input signal. The LNA has to be designed to accommodate a wide range of gain. A convention LNA circuit supporting a wide range of gain often suffers from reduced output headroom due to increased current through the load resistor. The present invention discloses the use of current bleeding branch to allow a portion of current to flow through the current bleeding branch and consequently reduces the current that would have flown through the load resistor. Consequently, the voltage across the load resistor may be maintained low to allow adequate output headroom.