First Image: Detector from Perkin Elmer 337 IR spectrophotometer. The detector appears to be the one disclosed in US Patent 2,526,491 and US Patent 2,526,492 .

Second Image: PE 337 detector (held in my left hand) and a Pye Unicam detector on the right from a Pye Unicam IR spectrophotometer.

Third Image: Pye Unicam detector open showing the circuit board which includes one JFET, one bipolar transistor and other components. The three terminals on the disc above the circuit board are ground, V+ and signal output.

The Pye Unicam detector preamp is shown in the image below. The detector includes a first stage BFW12 N-channel JFET and a directly coupled BCY71 PNP bipolar transistor second stage. Other examples of N-channel JFET and PNP transistor preamps can be found by Googling "N-channel JFET PNP". British patent GB1506101A discloses a similar circuit for a photocell.