Viewing posts for the category SED

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.

Transistor Tester for laser noise canceller

As discussed in medium-gory detail in this paper, laser noise cancellers can let you do shot-noise limited measurements at baseband with lasers that are as much as 70 dB noisier than that.

They're limited by two main effects: beta nonlinearity (1/hFE-1/hfe) and log nonconformance (d ln(IC)/dVBE - kT/e).

This tester measures both of these quantities directly. It's a one-off, of course, so it's done with discrete logic and instrumentation amp parts. It has certain points of interest, for instance the use of a unity gain instrumentation amplifier as a precision +1/+2 gain amplifier. One loose end: U1 is a LT1043 switched-capacitor building block—a glorified MUX that has very low charge injection and very good common-mode rejection.

Schematic and data for the transistor tester