The model 181 is a current-to-voltage preamplifier of low noise and low input impedance designed to amplify the extremely low low currents encountered in such areas as photometry and semiconductor research. In photometric applications the low input noise allows the use of photodetectors with dark currents as low as 10-14 A/root Hz, while the wide frequency range permits high modulation frequencies to avoid 1/F noise and power-line pick-up.
The unit has a high dynamic range, allowing small AC currents to be amplified without overload in the presence of quiescent (DC) detector currents up to ten times the current to voltage converter setting. In semiconductor applications its low input impedance permits the actual bias voltage applied to the device under test to be measured without having to correct for the effects of back bias.
Six switch-selectable gain settings from 10-4 A/V to 10-9 A/V are available and the instrument has a useable frequency range from DC to 200 kHz. A signal monitor connector is provided on the rear panel and there is an overload indicator light on the front panel.
A bias control (accessible through an opening in the bottom of the unit) allows the application of a detector bias voltage at the input connector in the range 0 V to -5 V, with a nominal source impedance of 10-5/S, where S is the selected sensitivity. For example, if the sensitivity is set to 10-7 A/V then the source impedance will be 10-5/10-7, or 100 W. In some cases it may prove convenient to use this bias control to cancel the effect of DC bias accompanying the input signal.
A second control, also accessible through an opening in the bottom of the unit, allows the internal electronics to be DC zeroed.
The unit can be powered from an external low voltage, a lock-in amplifier via a suitable power cable, or the models PS0055 or PS0056 remote line power supply modules