Currentless RF switch

Discussion in 'General Electronics Chat' started by kampianakis, May 15, 2013.

  1. kampianakis

    Thread Starter Member

    Jul 10, 2010
    20
    0
    Hi all,

    I am currently building an RFID system in which i need a transistor for the communication front end. In principle the system achieves backscatter modulation by switching a single RF transistor, connected to the scattering antenna. The transistor’s gate is directly connected to a MCU output pin and its collector to a SMA antenna connector (Fig. 2). The transistor’s gate is either biased to a ‘low’ voltage level (0V) or a ‘high’ voltage level (3V). This way, the transistor acts as an open or closed switch, and the antenna-load reflection coefficient takes two distinct values Γi, i = 1, 2. In order to consume as less power as possible i need to utilize a MOSFET transistor as a switch. With a quick search I found numerous products capable for the job. However, most (if not all) are N-channel, depletion type MOSFETS, meaning that i, somehow have to produce a negative potential to the transistor gate. Taking under consideration that the reason why i use MOSFETS is to avoid consuming any power (apart from the leakage current which is on the order of <1uA), are you aware of either an enhancement mode RF transistor or a power-less trick in order to achieve Vgs < 0 without having a negative potential available in my circuitry?

    Thank you in advance
    Lefteris
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    I recently used IRLML2402 and it worked pretty well. What kind of project are you working on and what frequency band?
    I am doing my thesis on making an 13.56MHz tag emulator.
     
  3. kampianakis

    Thread Starter Member

    Jul 10, 2010
    20
    0
    I use 868Mhz frequency therefore I need something working at the UHF band. Have you measured S11 parameter on the IRLML2402 for on and off state respectively? Btw, with a first look IRLML2402 looks good ;)
     
    Last edited: May 16, 2013
  4. kampianakis

    Thread Starter Member

    Jul 10, 2010
    20
    0
    After some more research and experimentation I have found the products below that suit my needs (for anyone interested)
    NXP BF1118 : |Γ1-Γ2| ~= 1.68, switches @ 2V
    NXP BF1107 : |Γ1-Γ2| ~= 1.8, switches @ 3V

    Both are depletion type, but i thought that by connecting the source to the Vss i can get a Vgs = -Vss and therefore reach the transistor threshold
     
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