We present a method for simulating the performance of millimetre-wave and submillimetre-wave STJ direct detectors when combined with commercially available op-amp based readout circuits. We employ full
nonlinear modelling, together with frequency-domain analysis, to determine the responsivity, and then we use this responsivity, in conjunction with a detailed noise model, to calculate the NEP. By modelling the saturation of these devices, we are also able to calculate the dynamic range. Our method is capable of simulating a wide range of devices and takes into account the RF matching circuits. Using this approach, we have explored the effect of cooling STJs to different temperatures, and the effect of changing the frequency of operation. To achieve the best noise performance, the energy gap should be tailored to suit the operating frequency, and the device should be biased at a low voltage. We have performed detailed simulations to show that by using TaAl devices, and suitably chosen op-amp feedback components, an NEP of 6.0x10-18 W/√Hz and dynamic range of 80 dB should be possible
at 150 GHz: these conclusions draw on results already known from optical photon-counting experiments.