Phantom Design Problem

Discussion in 'The Projects Forum' started by Analog_, Mar 20, 2014.

  1. Analog_

    Thread Starter New Member

    Oct 23, 2012
    17
    1
    Hi there,
    Im working on a project which is basically a transductor, connected to a LNA (with an easy Impedance Matching Network between them), and then the load resistor who's reciving the amplification.
    The fact is that i've introduced a Phantom stage at the end, between the LNA and the load resistor, but it's not working properly.
    As you may know, the Phantom is useful for using the same pair of cords for power supply and signal comming out.
    The LNA works fine, and it should put out an 657mV on the load resistor (it works fine without the Phantoms Stages).
    But, when I connect these stages, the voltage on the load increases until 79 mV, and the power supply for the LNA its not the correct value (it should be 12 v)
    I've attached a screen capture of the DC simulation in PSpice.
    Sorry for the arrows, it's easier and faster for me to draw and text in MSPaint, but seems like if i were 6 :rolleyes:
    I hope someone could tell me what's wrong in the circuit, I really don't know why is it not working.
    Please ask me if you dont understand anything on the circuit.
    Thank you in advance,
    Regards.:)
    D.

    [​IMG]
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,142
    1,790
    You have a DC current path from:
    +12V through R18 through R16 through R10 through R13 to the collector supply on Q1 which sits at 4.895 volts.

    The bias network for Q1, R1 in series with R2 completes the path to power ground.

    Now tell me again, using Ohms law, how you expect the junction of R1 and R3 to be at any other voltage. Certainly 12 Volts is completely out of the question given that there are at least four resistors between 12 V and the junction of R1 and R3.
     
  3. Analog_

    Thread Starter New Member

    Oct 23, 2012
    17
    1
    Yep, it seems its an bad design failure.
    But, I just put the Phantom stages as they are described in theorical circuits.
    Any idea of how to connect it to the circuit to make it work?
    Ty,
    Regards-.
     
  4. Papabravo

    Expert

    Feb 24, 2006
    10,142
    1,790
    Would there be anything dramatically wrong with connecting the junction of R1 and R3 directly to +12V?
     
  5. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    Is Q1 the "Low Noise Amplifier"?
    My problem is that I can't see how the 12 volts gets to V7 with C8 in the way.
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,535
    1,251
    Since you have a single-ended source, with its return tied to system ground (battery negitive terminal), I don't see the need for a balanced signal connection. This looks more like a cable TV problem, getting power to an amplified splitter. As long as the DC source impedance *at the frequency of interest* is much greater than the characteristic impedance of the interconnect, a simple current injector should work. Usually it's just an inductor.

    On the power source side, delete R16, 17, 18, C14 C15. Replace C14 with a short. Connect Bat+ to the C10-C13 node with inductor L99.

    On the power destination side, delete R8, 10, 13. Connect C10-C13 node to C12 with inductor L98. L98-C12 is the new power node; connect directly to R1-R3. Add 0.1 uF and 0.001 uF ceramic caps in parallel with C12 for way better decoupling.

    Or something like that.

    ak
     
  7. Analog_

    Thread Starter New Member

    Oct 23, 2012
    17
    1
    Not dramatically, but Phantom stage is used to get the feature that is, using a single pair of wires, power supply (DC) comes from to the amplifier, and the signal output (AC) goes out to the load resistor R14.

    I need to use this stage on the project, otherwise I wont have the issue i'm having :rolleyes:
     
  8. Analog_

    Thread Starter New Member

    Oct 23, 2012
    17
    1
    It's going to be used with a AC signal (you can see the symbol on V7) and with signals whichs freq are going to be between 35 and 45kHz, so C8 will work as a wire on that situation.

    But the issue is on the final stage, with the phantom and the power to supply the amplifier stage.
     
  9. Analog_

    Thread Starter New Member

    Oct 23, 2012
    17
    1
    I cannot do that, since I need to keep the feature of having DC supply on one way (to the amplifier from the output) and AC signal to the other way (from the amplifier to the output) just using 2 wires.
    Sorry, I forgot to tell you guys that the final conection is designed for using with a XLR connector, where 2 wires for signal-DC supply and the third one for GND.
    Thats how Phantom works. Here is the diagram:
    [​IMG]
     
  10. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    So you're using a 2N3904 as a LNA? I was thinking satellite dish frequencies and that a 2N3904 probably won't work in that range.

    My bad.
     
  11. Analog_

    Thread Starter New Member

    Oct 23, 2012
    17
    1
    For sure will work. I've also made other projects using signals hanging around 300kHz and it worked fine.
     
    #12 likes this.
  12. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    I was thinking about 300 MHz (and up).
     
  13. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,535
    1,251
    So was I. It's still not clear what the freq range is and what the application is.

    Also, Analog, I agree with your drawing about phantom power in a balanced audio application, but nothing in your original schamatic looks like that. The 30 mV source is unbalanced and tied to the same ground potential as the amplifier. If the power is going only one way and the signal is going only one way, it doesn't matter which way either is going. You can do this with three XLR pins and no phantom:

    pin 1 - gnd
    pin 2 - signal back
    pin 3 - power out (the hottest is the highest)

    ak
     
  14. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,535
    1,251
    Analog, I just reread your last post. You said you have to do the task in just two wires, but your original schematic clearly shows three. ...?

    ak
     
  15. Analog_

    Thread Starter New Member

    Oct 23, 2012
    17
    1
    The 30 mV is just put there because of simulation reasons. It's simulating the signal that comes out from a microphone (actually, it's an hydrophone); the resistor and capacitor after that is simulating the output impedance. Forget about that zone, it works fine. In fact, if I put the phantom stage away, the whole circuit works properly, with a correct gain/amplification of those 30mV for a 40kHz work frequency.

    I mean, with two wires for the DC/AC both ways stage. The third one is of course for GND, that's why I didn't mention. Sorry
     
  16. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,535
    1,251
    I still don't see the need for a phantom power system. Attached is a quickie sch with close-enough parts in my libraries. 3-wire XLR connector interface, volts going out, signal coming back. If you are trying to bring back the signal pseudo-balanced for noise immunity, that's another thing. But what you have actually described isn't that. Is it?

    If you do want to bring your signal back balanced, there's a better way. Can your preamp expand to two transistors (or a dual opamp), or are you stuck with only one?

    ak
     
  17. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,535
    1,251
    Here's the sch.
     
  18. Analog_

    Thread Starter New Member

    Oct 23, 2012
    17
    1
    Thank you AnalogKid for your sch, but I need to have the signal that comes out from the amplifier in two wires (balanced).
    The reason of using a phantom interface is to reduce the number of wires. With an usual circuit, 2 pairs of wires are needed (2 for DC supply, 2 for signal comming out). But using phantom you can reduce it to 3 (2 common and 1 for GND), placing properly the phantom filter, which will allow DC comming to the LNA and AC going out from LNA.
    The problem I' having is that I cannot set the connection between this:
    [​IMG]

    and the improvement of connecting a typically Phantom circuit, such as:

    [​IMG]

    without losing Power Supply voltage or getting a poor amplification at the load resistor

    Tha's basically my problem.
     
  19. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Is L4 part of the hydrophone equivalent circuit? Or what is it for?
    The way I see it you have a hydrophone to the left of R1, R2, and an amplifier is the rest.
    Now add a capacitor between L4 and junction R1,R2, and add a resistor from +12V to L4 to power the microphone. Is that what you are after? (actually that is how you connect electret microphones which also need phantom power)
     
  20. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,535
    1,251
    OK, now that you've said that you need a balanced output, what about this? This will give much better noise immunity than the pseudo-balanced circuit in your first schamatic. OBTW, a patent application for this was filed in 1942.

    ak
     
    Analog_ likes this.
Loading...