diode detectors, amplifier, stabilze

Discussion in 'The Projects Forum' started by ninjaman, Oct 9, 2014.

  1. ninjaman

    Thread Starter Member

    May 18, 2013
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    hello

    I have been reading about diode detectors and two states, square law region and when it has the forward voltage. below the forward voltage the diode doesn't work well and something is required to make it more linear from one state to another. I think this is where op amp compensation comes in. I have read about compensation a little. most of what I find is a lot of math that does my head in. I know that an op amp can have internal (capacitor inside) compensation and external (capacitor outside) compensation. I think the 741 is internal. I don't know if this is what makes the diode detector linear I cant find anything about it. I think its a case of knowing what to look for.

    any suggestions would be great!!!

    upload_2014-10-9_21-46-50.png

    I have cut this from a circuit im trying to research. I think the compensation is provided by C6 (10nF). I think D1 and C2, D2 and C4 provide full wave rectification. I think R6, R7, R10 and R11 provide a feedback system making the op amp stable. I don't know what R4, C3, R5, R8, C5 and R9 do?
    I think maybe R8 and R4 provide some sort of loading and C5, C3 some sort of smoothing. actually looking at it R8 and R9 look like a voltage divider?
    anyone know if this is right?
    the first person to respond gets a free ferrarrarrarri in the post!!!

    Christmas soon, WOOOOO!!!!!

    thanks
    simon
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Last edited: Oct 9, 2014
  3. alfacliff

    Well-Known Member

    Dec 13, 2013
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    that looks like the power monitor and swt protect circuit of a transmitter. the top one is the swr protect detector, and the d2 diode is the power monitor to feed back the power output of the transmitter.
     
  4. DickCappels

    Moderator

    Aug 21, 2008
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    C6 is for power supply decoupling, not compensation.

    D2 and C4 form a straight-foward diode peak detector.

    Apparently this circuit does not have any compensation for the square law region; if the signal is large enough, the nonlinearity resulting from square law operation can be ignored.

    There does not appear to be anything special in the circuit to linearize the detectors. If you need to handle small signals (tens or hundreds of millivolts) you can improve linearity by applying a small forward bias to the diodes.


    [​IMG]
     
  5. ninjaman

    Thread Starter Member

    May 18, 2013
    306
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    its an antenna analyzer project using arduino and ad9850 module for the signal. the output of the dds is 2vpk-pk
    there is a wheat stone bridge at the top and what looks like two rectifiers/peak detectors so you get full wave ( I think its full wave)
    I don't know what the voltage going through the diode is I haven't yet built the circuit. I cant find the diodes cheap anywhere so I was thinking about a 741 op amp and germanium schotky diode. I think the diode has to have the lowest forward voltage and biggest bandwidth.
    the op amp, from what I have read, is a buffer so the gain ( from what I understand) is one. meaning what goes in is what comes out and the opamp provides some stabilization???

    anyone got any thoughts on this, im not sure if this is correct. I cant find a manual that explains it that well. im getting bits from different places.

    thanks
    simon
     
  6. alfacliff

    Well-Known Member

    Dec 13, 2013
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    good luck findinding a germanium shotkiey diode, last I saw they were silicon. the max voltage across the diodes is determined by the power appplied.
     
  7. DickCappels

    Moderator

    Aug 21, 2008
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    Both opamp circuits have a closed loop gain of 9.
     
    Last edited: Oct 11, 2014
  8. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
    hello

    DickCappels, did you find the gain by Av = R2/R1 + 1 = 5000/648 + 1 = 8.7?
    upload_2014-10-12_12-8-50.png
    this is the circuit im trying to learn about. I understand that its a non inverting amp, is this a "super-diode"?
    so the buffer is the detector part of the circuit and provides more accurate detection than a regular diode (series) and capacitor (parallel)
    im guessing that the diodes in the reflectometer provide a one way path and rectification?
    would that buffer require any further components? I have a schematic for this part of the circuit but its very difficult to read
    upload_2014-10-12_12-20-30.png
    it looks very similar to the other circuit I posted.
    does anyone know if this circuit has a name or fall under a title. seeing as the components seem to be in similar places I thought it might have a name. that way I could search for it.
    I know the op amps provide a more linear result from the diode. though I thought they were buffer amps or voltage followers meaning that there wouldn't be any amplification. what goes in is what comes out? but if they provide a gain of 9 then what ever goes in is multiplied by 9 before it comes out. so this isn't a buffer?
    any help with search options on google, what should I put in to research this circuit and the function of individual components.
    thanks

    simon
     
  9. DickCappels

    Moderator

    Aug 21, 2008
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    Not quite. I took R1/(R1+R2) and got 8.7.
    Edit: Wrong: its really (R6+R7)/R7, it is equivalent to what you wrote.

    I would not call that circuit a "super diode". I suspect that the diode in the feedback is to compensate for the diode drop in the detector.

    You wrote:
    "im guessing that the diodes in the reflectometer provide a one way path and rectification?"

    Yes good. That's how rectifiers (diode detectors) work.

    You also wrote:
    "...but if they provide a gain of 9 then what ever goes in is multiplied by 9 before it comes out. so this isn't a buffer?"

    It is a buffer with a gain of 9.

    Before suggesting internet search terms, how about telling us exactly what you are trying to do?
     
    Last edited: Oct 18, 2014
  10. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
    hello
    thanks for the help there DickCappels!

    I am trying to make an antenna analyzer using an arduino, DDS ad9850 module, rotary encoder, wheat stone bridge circuit with peak detector.
    I would like to add TDR to this as well as I think I can create a fast signal but I don't think arduino is fast enough to record the results.
    the measuring part of the circuit is the wheat stone bridge and diode detector and I want to learn how they work so I put one together without copying someone elses design. a lot of analyzers look like they have a very similar way of measuring impedance (bridge and diodes).

    im guessing that the diode in the feedback position is providing the difference that the amplifier has to add, the initial diode drop that you mentioned. I know that there will be a drop of 0.2v as I would use germanium and that the op amp linearizes the diode but im not sure how the feedback diode fits in exactly. I watched a video on youtube about op amp precision rectifiers. I think I will have another look at that.

    thanks again

    simon
     
  11. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
    any one know any information about the circuits in the schematics. im trying to do searches to find information about them but cant seem to find anything.
    any hints would be good

    thanks
    simon
     
  12. DickCappels

    Moderator

    Aug 21, 2008
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    The diode in the feedback path of the opamp compensates for the temperature dependent forward drop of the detector diode. For example, D1B compensates for the offset of D1A.

    Most likely your SWR meter will look a lot like an existing design because there many SWR measurement circuits have been designed and there are only a few efficient topologies around.

    The SWR meter discussed in the pdf file at the link below is a more modern looking implementation, but the basic circuit blocks are similar to those of other meters.
    http://www.arrl.org/files/file/Product Notes/2012 Handbook/KAUNE.pdf
     
  13. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
    hello DickCappels

    thanks for that, im still looking into the diode and op amp part of the circuit and I should have been more specific about what part of the circuit I was talking about. I wanted to understand more of the op amp and diodes operation. you mentioned the buffer had a gain of 9. I read today about buffers with current gain on Wikipedia and I will try to read more about that circuit.

    thanks again

    all the best
    simon
     
  14. DickCappels

    Moderator

    Aug 21, 2008
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    There are probably other definitions, but generally speaking (writing) a buffer is an amplifier that does not affect the source signal, such as by loading it down, and provides a relatively low impedance output to the next circuit stage. The buffer is a function, and does not have to have any particular gain.
     
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