20 Mhz Led Driver

Discussion in 'The Projects Forum' started by sonaht, Mar 18, 2011.

  1. sonaht

    Thread Starter New Member

    Mar 18, 2011

    This is my first post.
    Also not very experienced as electronics student as well.

    I need to make a circuit to drive some IR Leds for a project.
    IR Leds needed to be able to pulse at least @ 10Mhz.

    So far, I have made this design:

    Op-amp= AD8056 (Voltage feedback, 300 Mhz, high slew rate, etc.)

    Vin=3.2Vp-p square wave (from a signal generator)
    R3= 402Ω
    R2= 402Ω (recommended resistor values for a GAIN=2)
    R1= 10Ω

    C1=C3=C5= 0.1μF
    C2=C4=C6= 1μF (added for stability at high freq.)

    IR Leds= OPE5685

    My problems are:
    > @ 5Mhz I am getting an optical signal (from Leds) square wave (good) but with a little delay (bad),
    > @ 10Mhz the optical signal is not a square wave any more (bad)

    > Also, when I connect the input signal (from signal generator) to my circuit, the input signal (as seen on the oscilloscope) become noisy.

    Vin and Optical signal @ 10Mhz:
    Yellow= Vin
    Purple= Optical signal from IR Leds

    I think the problem is not due to Leds.
    The output of the ap-amp @ 10 Mhz is as shown below:
    Blue= Output of the op-amp (transistor base).

    I have tried:
    > several gains,
    > a faster op-amp,
    > the circuit is on a breadboard but I have tried also on a stripboard (the final circuit will be on a PCB, hopefully),
    > to reduce Vin to a lower voltage,
    > ++

    but the result is almost the same,

    To conclude:
    > Up to 2 Mhz working fine,
    > 2~5Mhz square wave but with delay,
    > 5+ not any more square wave, delay, noise, etc.

    Any help is much appreciated :)
  2. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
    Why are you trying to modulate it at such a high frequency?
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
    What kind of signal generator do you have?
    What is your transistor's part number?
    Are you using coax between the generator and the op amp? If so, how long is it and what kind is it?
    What is the value of R4?
    How much LED current do you want?

    These questions are all relevant to your problem.
    You probably will need to have a higher supply voltage for your LEDs, or put them in two parallel strings, or drive them with two transistors.
    Last edited: Mar 19, 2011
  4. hgmjr


    Jan 28, 2005
    How about posting the schematic of the receiver circuit you are using. Maybe it is not optimized to respond fast enough to faithfully reproduce the signal from your IR emitter.