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Voltage controlled current source

Wasiu Popoola
Prodigy 90 points
Other Parts Discussed in Thread: OPA2673, LM6171

Hi,

I am looking to design a voltage controlled current source, operating at up to 500mA and 20MHz to drive an LED.

I managed a design but it appears to 'peak' at about 7MHz. That is the received signal amplitude is much higher at about 7MHz than at any other frequency.

Can anyone help please?

I will appreciate alternative circuit suggestions as well.

 

 

  • 0
    TI__Genius 17905 points

    Hi Wasiu,

    I"m curious as to what your current circuit looks like - could you post a rough schematic or description?

    There are different ways to implement current source circuits. Some current source circuits have an op amp drive a transistor base/gate in a buffer configuration so that the output current is supplied through the transistor to the load. Other circuits rely on the op amp's current drive directly.

    What is the input signal to the circuit (voltage/current, amplitude, DC level, bandwidth, etc) and what is the desired output current transfer function (bandwidth and what input corresponds to what output current)? I'm assuming unipolar output current since it's an LED.

    Must the LED load be referenced to ground or can it be floating? What LED is it? What supplies are available? Any other constraints?

    The OPA2673 is a dual op amp that can source over 500mA at each output and has a very wide bandwidth (G=1, 600MHz).

     

  • 0 in reply to Kris F
    Prodigy 90 points

    Hi Kristoffer,

    My circuit is the first one you described.  use LM6171 op amp to drive 2SC5707 transistor (Used bcos of its high power capability). The input signal is pure sinusoid of 1Vp-p, and up to 20MHz. I used another difference amplifier before this stage to add a DC value of about 2V to the signal and the sense resistor(At the emiiter of the transistor) is adjusted to get the required current of between 200-300mA.

     

    I am basically design a driver circuit for an LED and i want to drive this LED using a sinusoidal signal of up 30MHz. It is a white LED. I dont think i care a wheather it floats or not once i can modulate the light intensity.

    Any more suggestions please?

     

    many thanks