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Automatic variable resistance

  1. Oct 28, 2006 #1
    Hi, I am new to electronics and haven't been successful in my search, nor my efforts to create a solution to the problem.

    I need a circuit or IC that downconverts voltage but keeps the same frequency and signal pattern as the input over a very wide range of frequencies. No matter what the input voltage is (will always be less than 24v), I need the output voltage to be a maximum of 5v and minimum of 0v. For example, say I had an two input signals with a triangle wave running between 0 and 14v and a triangle wave running between 0 and 24v, the output in both cases would be a triangle wave with the same frequency as input but scaled to 0 to 5v. It would be nice to be able to tell what the original input voltage was for reference as well, but I can create a digital ciruit to do this if need be. :) Also, I am working with a very high input signal impedence, 1Mohm.

    I tried using a PNP transistor with a voltage divider on Vin and then ran the divided voltage to the base, without much success. I have looked into Op-Amps for this task, but can't seem to quite wrap my head around how to do it correctly. I have also scoured for an IC that does this for me, but can't seem to find anything.

    I am sure it is just my lack of knowledge and experience, please tell me that there is such a device or circuit out there.

    Last edited by a moderator: Oct 28, 2006
  2. jcsd
  3. Oct 28, 2006 #2


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    Look into AGC circuits. (Automatic Gain Control) Is it always the same type of waveform on the input?
  4. Oct 28, 2006 #3


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    The voltage divider is generally the correct way to reduce the voltage.

    However, you have to take into account the load your transistor imposes on the divider.
    Practially speaking, the transistor shorts out lower resistor of the divider.
    So in many cases there is only one physical resistor in the divider, with the transistor forming the other resistor.
    One problem is biasing the transistor in the liner range.

    Unlike a transistor an op amp has a very high input inpeadance, so they are actually easier to use.

    If you need a 1v signal to look like a 5v signal as well as the 24v signal looking like a 5v signal then you will need to consider AGC as Averagesupernova suggested.
  5. Oct 28, 2006 #4
    Automatic Gain Control it is

    Thanks for the help. AGC's are indeed what I am looking for. I found a lot of information about what an AGC does, usually in relation to audio applications, but not a lot of circuit diagrams. Then I found a thesis paper talking about AGC's. Now I understand why. It appears that AGC's are very application specific and have to be tailored to the application.

    Looks like I am going to have brush up on my math (what I really mean is re-learn my math), especially since what I want is a non-linear system. :) Any help with links, circuit diagrams or additional information on this would be greatly appreciated.

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