Building an amplifier to drive a noise diode

Discussion in 'Wireless & RF Design' started by ngmwave, Dec 4, 2012.

  1. ngmwave

    Thread Starter New Member

    Dec 11, 2011
    I am trying to use a Noise/Com NC302L noise diode to balance the output of a 12 GHz microwave radiometer receiver. The balancing is done by injecting the noise diode signal into the receiver input via a directional coupler. The noise diode generates a 12 GHz signal of increasing power as the DC voltage increases between 7 and 18 volts, with current increasing from 0 to 10 ma. To accomplish the balancing I sample the receiver output (0 to 10 volts) and input it into an op amp to energize the diode. I use a OPA445 power op amp with a 22 volt supply in a simple inverting configuration hoping to get an output of up to 18 volts at 10 ma. However, I find that the noise diode loads down the op amp, reducing its output voltage from 19 volts to a maximum of 14 volts at 6 ma independent of the op amp gain. Any suggestions would be welcomed.
  2. bertus


    Apr 5, 2008

    When reading the datasheet, I must conclude that the NC302L diode generates noise between 10 Hz and 3 GHz.
    The operating voltage would be 6 - 8 Volts.

  3. ngmwave

    Thread Starter New Member

    Dec 11, 2011
    You are correct that the specs show it being used up to 3 GHz. However, by
    using higher volatages and currents it easily generate large output at 12 GHz and beyond. My 12 GHz microwave radiometer is a highly sensitive receiver and detects a large signal strength from the diode even through a 10 dB coupler when using voltages between 7 and 18 volts.
  4. KL7AJ

    Senior Member

    Nov 4, 2008
    Very clever arrangement on your part! i don't think I've ever seen a noise diode dynamically driven like that!

    You can use a simple pass transistor in an emitter follower configuration to drive the diode, or even a darlington pair. (after the op amp).

    Good luck! let us know how it turns out!