Passive Active Network

Discussion in 'General Electronics Chat' started by Narwash, Jan 3, 2013.

  1. Narwash

    Thread Starter Member

    Jun 27, 2012
    Hi All,

    So I'm trying to build a passive active network. It's so my antenna can operate in the passive mode once the batteries for my amplifier run out of juice.

    I'm using the MAX8211 voltage monitor to look at my battery and output a HIGH when the battery is a-ok and a LOW once the voltage drops below a certain threshold voltage, as set by the bias resistors on the MAX8211 chip.

    So the idea is to pass my AC signal from my antenna straight to the output when the voltage monitor goes LOW and pass it to the amplifier when it's HIGH.

    I thought I may be able to do this with a transistor switching network. However, I'm not too good with transistor networks, so I figured I would ask for some help.

    My idea is to use a combination of NPN and PNP switches. I figure I would have a NPN that would be operating in the saturation region (Switch ON, Vbe > .7V, etc.) when the output of the voltage monitor is HIGH and operating in the cutoff region when the output is LOW. This logic would be inverted for the PNP.

    In summary:
    IF output of MAX8211 Voltage Regulator HIGH:
    NPN acts as closed switch
    PNP acts as open switch

    IF output of MAX8211 Voltage Regulator LOW:
    NPN acts as open switch
    PNP acts as closed switch

    However, I think I'm biasing the transistors wrong. I attempt to run a simulation in LTSPICE, with the output of the MAX8211 simulated as just a simple DC supply (5V for HIGH, 1V for LOW) and while I'm able to get the active track to be fully on at 5V with some minimal attenuation and fully off at 1V, the passive track doesn't seem to vary in either the 5V or 1V configuration.

    I've attached a picture of the LTspice schematic I'm using. The PNP transistor I'm using is:

    And the NPN is:

    My input signal frequency varies from 50 MHz - 500 MHz and is on the order of microvolts.

    I thought that I might need a different bias network (R1/R2) for the NPN and the PNP, but when I tried that my active network stopped working so I went back to the single bias network.

    I can provide the LTspice .asc and .lib files if necessary.

    Sorry for the long post and thank you in advance for your help!
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    OK. I cannot still get you what you are trying to do.

    But a suggestion, why not use a relay to couple the antenna feed. Unless you are trying to amplifier the antenna input ( since you are saying about tr biasing ).

    Need a better info or details on ur project

    PS...Passive Active network is rather confusing.
  3. Narwash

    Thread Starter Member

    Jun 27, 2012
    Sorry about the lack of clarity! Let me try to explain further. I should of put Passive/Active...not just passive active.

    I have an antenna I'm using to pick up signals in the 50-500 MHz band. I also have an amplifier as the signals can be very quiet (-50 dBm). I would like to be able to pick up signals in both the passive (no amplifier, only antenna) and active (amplifier with antenna) modes.

    To this goal, I am attempting to design a transistor network that will switch between the two modes.

    In addition to being able to switch between these two modes at will, I would like for the network to automatically switch from the active to the passive mode when the battery has dropped below a certain voltage.

    To this end, I am hoping to use a voltage monitor from Maxim Integrated Products called the MAX8211. The datasheet is here:

    The idea is that when the output of the MAX8211 is LOW, the network will switch from active to passive modes, if the switch currently in active mode. When the output of the MAX8211 is HIGH, the network will be driving the active track, unless the switch is thrown to set it to passive.

    By "the switch", I mean that the user would be able to set whether if it is in the passive or active mode. If the user has set it to active and the output of the voltage monitor is low, then the network should automatically switch to passive.

    Perhaps a truth table would be more illustrative but I hope the logic is somewhat clear.

    I was considering relays but I was concerned about their current consumption as well as their physical size. Since this is a battery powered application, I thought a clever transistor network might be better suited. I could very well be wrong, however. :)

    I thought this would be easier, but I may have bitten off more than I can chew. I thought it would be fun to attempt it, though.

    Let me know if it remains unclear! I did my best to try and explain it.
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    OK. what I do not understand is how are you going to switch transistors or anything without power.

    You can use a small relay ( like 3V ones) to cut off when the battery drains.

    I dunno how you can keep a transistor switched with draining battery. The switch too needs power to work u know.

    By the way, do you anything about high frequency switching
  5. Narwash

    Thread Starter Member

    Jun 27, 2012
    Good point. I suppose it would be easier to have a normally closed relay which would then switch to the passive track when the battery starts to die.

    I don't know much about high frequency switching. I'm not really sure what considerations are needed for the switch in the frequency range I'm using. Can you point me to some application notes or other documentation regarding high frequency switching?

    Thanks for your help!
  6. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    did you try google ?
  7. kubeek


    Sep 20, 2005
    You could use a bistable relay, that draws power only when you change the state.
    The problem will be if the relay can pass the antenna signal without attenuating it too much.
  8. Narwash

    Thread Starter Member

    Jun 27, 2012
    Hi all,

    Thanks for all your help! With your suggestions, I believe I've drawn up a viable solution using low voltage relays. Attached is a circuit "diagram", but it's kind of more just a sketch of the general idea.

    I'm looking to use the 3V version of this relay

    I think a bistable relay would be better as I wouldn't drain the battery at all in the passive state, but I couldn't find any cheap 3V ones.

    I think I over-complicated the logic a lot previously. It's simple: If the switch is thrown to active and the battery is good, drive the active track. Else, drive the passive track.

    I think this is going to work. I wouldn't think the relay would attenuate the antenna signal too much as a relay is just a moving conductor. Are there relays specifically designed for RF signals?

    Another question: Is the relatively high frequency of my signal going to affect the relay operation or anything? I wasn't sure if certain parasitics are going to introduced that will shoot me in the foot or something.

    Also, I'm going to put a flyback diode across the relay but I didn't put it in the sketch.

    Thanks again for all your help!
  9. crutschow


    Mar 14, 2008
    For RF it would be best to use a coaxial relay if you are concerned about impedance matching and SWR.