Cascode Enhancement pHEMT Photodiode Preamp

This is pretty small, because it has to be--those are microwave transistors, and will oscillate at the slightest provocation. The axial resistors and TO-92 parts are all for biasing---the actual amplifier is the part between the output coupling cap (the small orange thing in the middle) and the photodiode, which is the white square with the black middle at the right.

The 0.1-inch pitch holes round the outside and 25-mil pad pitch set the scale.

This amp works fine below 6 mA of bias current, but above there it wants to oscillate at 6 GHz. Interestingly this is just what SPICE predicts if the capacitance across the cascode device's base isolation bead is about 0,2 pF, which is a plausible number.