The 277 is used as a pre-amplifier for a lock-in amplifier in ac systems employing current source detectors such as photomultipliers and photodiodes (eg. Si, Ge, InGaAs, InSb) which, at low frequencies, give their best performance when connected to a virtual ground.

The input, which is dc coupled, effectively has zero impedance, consequently no voltage is generated across the detector as a result of the photocurrent it produces.

This short circuit operation enhances linearity of detectors and reduces the effect of cable capacitance.

The programmable six decade sensitivity range, in conjunction with the 225 lock-in amplifier, allows the user to choose a combination of current and voltage gain which optimises the trade-off of noise performance versus dc current sinking.

The unit has two inputs which can be selected via the IEEE/488 interface which can be used with detectors of the following types: Si, Ge, InGaAs, InSb, and photomultipliers.