1. Analysis of a MOSFET operating as an RF power detector
Centro Tecnológico
Departamento de Engenharia Elétrica
Laboratório de Circuitos Integrados
Analysis of a MOSFET operating as an RF power detector
Authors: M.Sc. Daniel Silva Piovani –
Ph.D. Fernando Rangel de Sousa -
Abstract
This paper presents the analysis of a MOSFET biased in the triode region operating as a power detector. Two modes of operation are evaluated, current mode and voltage mode. For each mode, we develop expressions relating a DC signal and the squared amplitude of a RF signal. A MOSFET compact model, valid for any inversion level, is used, permitting to predict the detector behavior with a reasonable accuracy. Optimization of the CMOS power detector is discussed remarking the trade-off between sensor’s sensitivity and input impedance. The approach is validated through simulations and measurements results, which present good agreement with the theory.
Simulations and Results
MOSFET as an RF Power-to-DC current transducer
There are two techniques to measure the DC current, dependent on the DC impedance of the DC amplifier (Fig. 1):
Current mode technique (low DC impedance).
Voltage mode technique (high DC impedance).
Fig. 2- Output DC current (IDC) according to (2), ADS HB simulation and measurements as a function of if ( f = 2.4 GHz – CMOS 0.18 ).
Fig. 3 - Output DC voltage (VDC ) according to (3), ADS HB simulation and measurements as a function of if ( f = 2.4 GHz – CMOS 0.18 ).
Fig. 1 – Behavioral model of the power transducer.
MOSFET drain current using a charge based model (ACM).
DC current generated by the RF input signal in current mode.
DC voltage generated by the RF signal in voltage mode.
Fig. 4 – Simulated and theoretical power detector input impedance as a function of the forward inversion level ( f = 2.4 GHz – CMOS 0.18 )
DC current and voltage for RF weak signals.
Conclusions
Expressions for the sensitivity of a CMOS power detector from low to high input signal power are obtained and compared with simulated and measured values. The behavior of the input impedance with the aspect ratio and inversion level values is studied, emphasizing the tradeoff between sensitivity and input impedance, showing the versatility of the design variables which allows power detector designs for different applications.
Input impedance seen by the power detector input.
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