Disclosed is a transresistance amplifier for a charged particle detector, comprising a variable input resistance, which may be a phototransistor (IC3), a voltage amplification stage (IC1) and control means (IC3), operable to vary the variable input resistance. The variable input resistance includes a first light-dependent resistance and the control means includes a first variable intensity light source that is optically coupled to the first light-dependent resistance. Also disclosed is a charged particle detector that includes such a transresistance amplifier and an electron microscope that includes such a charged particle detector.