I'm trying to understand the operation of a single coil magnetometer (marketed as "fluxgate", but with only a single coil). I've backed out the schematic as shown. The coil is driven by a 17kHz oscillator (not shown other than as "oscillator").
I'm stumped on the part I've called U4. It appears to be a SOT-143 device with the marking "E9" on the top. There is no continuity between any of its 4 pins to the others. Sadly the diagram doesn't lend many hints either - I was initially leaning to an NPN RF transistor, but the pinouts/functionality don't make sense. Any thoughts would be appreciated. Photo of the part included with U4 circled.