Help function generator...

Discussion in 'The Projects Forum' started by JonnyP, Mar 7, 2013.

  1. JonnyP

    Thread Starter New Member

    Mar 6, 2013
    I need some function generator help in AC bridge circuit.

    First of all sorry for any English mistake, as it is not my native language.

    First post here and I expect not to be the last. Just found this forum in google.

    Just some background… I work as a researcher in a laboratory at my University. My main focus application in eddy current testing (

    So let´s go. The main focus of eddy current testing is using coils interacting with conductive materials to measure a variation of characteristics of those materials. I am using an AC bridge circuit to measure difference of two identical coils as can be seen below:


    Everything is going well except that when I connect the AC generator to the bridge circuit (the coils have ~50ohm and I used another 50ohm resistor and a trimpot to balance the bridge) I see that the voltage amplitude of the sine wave drops. So I am supposing that the circuit is draining more current than the AC generator can supply, which can damage the equipment (I´ll post all my assumption, to show all my steps, please correct If I am wrong at any part!).

    So I made a non-inverting buffer using a LM324, which works well while disconnect from the bridge, but when I use the output of the buffer to feed the bridge the sine wave become all fuzzy. And now I am stuck at this point… I need a circuit that feeds the bridge without damaging the AC generator. Can you help me?

    Just some more data:
    - Frequency of the sine wave: 5-30 kHz
    - Amplitude of the sine wave: 4 volts peak-to-peak.

    Thanks in advance!

    João Rocha – UFRJ - Brazil
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
    What is the inductance?
  3. JonnyP

    Thread Starter New Member

    Mar 6, 2013
    It is about 6mH each coil.
  4. MrChips


    Oct 2, 2009
    You need a power amplifier, something like an LM675.
  5. JonnyP

    Thread Starter New Member

    Mar 6, 2013
    Thank you! With this chip the only modification that I must do is not to use as buffer, but decrease the amplitude of the input signal and have a gain of 10 or more in the op amp, right?