Differential MOSFET Peak Detection for Weak RF Interferers
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Solution Overview
Problem
Conventional peak detectors (PDETs) struggle to detect weak interferer RF signals relative to desired RF signals, leading to saturation of low-noise amplifiers (LNAs) and degradation of signal-to-noise ratio (SNR) in wireless communication systems, while also attenuating desired RF signals, which affects detection sensitivity.
Innovation Solution
A peak detector arranged in a common-source topology using a differential pair of transistors, which detects interferer RF signals by combining second-order non-linear drain currents at the drain terminals, providing a higher small-signal gain without degrading LNA performance and improving detection sensitivity for weak interferer signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional peak detectors are used to detect RF signal power levels, then the circuit can detect desired RF signals, but weak interferer RF signals cannot be detected and the LNA saturates
Solution Approach 1:
The peak detector is segmented into a differential pair configuration with separate transistors (first and second transistors) that process different signal components. This segmentation allows the circuit to separate desired signal detection from interferer signal detection, enabling weak interferer detection without LNA saturation by processing signals through distinct transistor paths that can be differentially combined.
2Power
If conventional peak detectors are used, then the circuit operates with standard gain, but small-signal gain is insufficient to detect weak interferer signals
Solution Approach 1:
The invention transitions from a single-ended peak detector to a differential pair configuration, adding a dimensional aspect to the signal processing. This differential architecture provides signal path separation and enables higher small-signal gain through differential amplification while maintaining power efficiency by utilizing the inherent differential structure that rejects common-mode noise and interferers.
Solution Approach 2:
The transistor operating point and bias conditions are optimized to achieve higher small-signal gain in the differential pair configuration. By adjusting the bias currents and voltages of the individual transistors, the circuit achieves approximately 13 dB higher small-signal gain compared to conventional peak detectors, enabling detection of weaker interferer signals without proportionally increasing power dissipation.
3Measurement precision
If conventional peak detectors are used, then the circuit structure is simple, but detection sensitivity for weak interferer signals is degraded
Solution Approach 1:
The differential pair peak detector serves multiple functions simultaneously: it detects desired RF signals, detects weak interferer RF signals, provides automatic gain control feedback, and rejects common-mode noise. This multi-functionality is achieved through the differential transistor pair configuration that inherently provides both signal detection and interferer rejection capabilities without requiring separate dedicated circuits for each function.
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 enables the detection of weaker interferer RF signals with a higher small-signal gain, around 13 dB higher than conventional PDETs, without increasing power dissipation, thereby improving the sensitivity and maintaining the performance of LNAs and mixers in receiver chains.
Implementation Method 1
Transistors have a non-linear voltage and current relationship. A small-signal voltage is an alternating current (AC) voltage signal where the amplitude of the small-signal is small relative to direct current (DC) bias voltages that are applied to bias the transistors in the PDET.
Implementation Method 2
A non-linear voltage and current relationship during small-signal operation can be represented with first-order and higher-order drain currents as a function of the small-signal gate-to-source voltage. For instance, a small-signal drain current is modeled as a sum of a linear current source gmVgs, and non-linear current sources, for example, gm2Vgs2, gm3Vgs3, and higher-order non-linear current sources.
Implementation Method 3
The value gm is a first-order linear transconductance coefficient, gm2 is a second-order non-linear transconductance coefficient, gm3 is a third-order non-linear transconductance coefficient, etc. Using the transistor non-linearity, a PDET can detect the power of a received alternating current (AC) signal as direct current (DC) voltage from the non-linear currents.
Data Source
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Figure 2A~2B
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AI summary
An apparatus comprises a transistor pair including a first metal oxide semiconductor field effect transistor (MOSFET) coupled to a second MOSFET. The first MOSFET includes a first gate terminal and a first drain terminal. The second MOSFET comprises a second gate terminal and a second drain terminal. The first gate terminal is configured to receive a first signal. The second gate terminal is configured to receive a second signal that is phase shifted with respect to the first signal. An output node is coupled to the first drain terminal and the second drain terminal and configured to output a third signal that is proportional to a power of the first signal and the second signal.