We describe a charge detector that utilizes a high-gain, differential solid-state amplifier with active reset and a novel technique to measure the gain using a custom optoelectronic system. Implemented in a 180-nm complementary metal–oxide–semiconductor (CMOS) process, the amplifier realizes a high gain of $8.9~\mu \text{V}/\text{e}-$ and single-pass rms noise of 475e− for 10-ms time interval with a 300-kHz low-pass filter corner. Operating at a sampling frequency of 10 kHz, the active reset extends the dynamic range of the detector. The amplifier is the first instance of a charge detector that combines a differential topology, active reset, and small feedback capacitors for low-noise, high-gain, high-dynamic-range, and robust operations. The proposed optoelectronic test system injects an adjustable input photocurrent as low as 33 pA to precisely quantify the detector’s gain without the need for a known calibration capacitance. The amplifier is directly wire-bonded to a PCB that has a built-in 1-cm2 Faraday cup. The gain and noise performance have been characterized for a range of input capacitance, and the results show good agreement with simulations. The test system additionally demonstrates the ability to inject a time-varying input at the picoamphere levels to characterize the dynamic response of the detector. An electrospray system confirms that the detector responds to free-space charge and that it can process an input as large as 56 000e− without saturation. The compact and high sensitivity design qualify the device for use in charge detection applications, such as mass spectrometry and space instruments.