The use of a capacitor (22) to serve as the principal impedance in a negative feed-back loop in a voltage amplifier component (21) of a trans-impedance amplifier and actively controlling the amount of charge accumulated within the capacitor appropriately to improve the responsiveness and/or dynamic range of the amplifier. A switch (25) is electrically coupled to the inverting input terminal of the voltage amplifier and electrically isolated from the output terminal (23) of the voltage amplifier. The output voltage of the amplifier is proportional to the accumulation of charge, and the switch is operable to ‘reset’ the charge/voltage on the feedback capacitor, as desired. This arrangement decouples the structure of the switch from the output port of the voltage amplifier, and so avoids leakage currents and/or interfering voltage signals emanating from the switch structure and being felt at the output port of the voltage amplifier.