Current-collector head-amplifier

To amplify small current signals such as from ah electron-collector inside a vacuum chamber, it is convenient for reasons of noise and bandwidth to have a `head-amplifier` attached to the chamber. The op amp N1 is a precision bipolar device with extremely low bias current and offset voltage (1) as well as low noise, which allows the 100:1 feedback attenuator R4:R5. The resistance of R3 can be varied from above 10M to below 1R, and so the nominal 0 to 1 V-peak output signal corresponds to input current ranges of 1 nA to 10 µ; this current i enters via the protective resistor Rl. Light from the bulb B1 shines on R3, and the filament current I is controlled by the op amp N2.
Current-collector head-amplifier - schematic

The reference voltage VR is "shaped" by the resistors R9R10 so as to tailor the bulb and LDR characteristics to the desired current ranges. Thus, rotation of the calibrated knob gives the appropriate resistance to R3 for the peak-current scale shown.

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