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.