The paper describes a transistor chopper-type d.c. amplifier which is intended for use in the field of process measurement and control. It has a voltage drift below ±10 μV and a current drift below ±4×10−9amp.The forward gain in the steady state is about 10kV/μA and 15 volts/μV, and, in consequence, the full output swing of 0 to 5 volts is provided by an input signal little greater than the drift. The application of sufficient overall negative d.c. feedback therefore enables input signals of 0–10 mV or 0–4 μA to provide full output with an accuracy of ±0.1%.The low drift is obtained by operating the transistor chopper 200 c/s, stabilizing its temperature within ±2°C, and providing suitably stable waveforms to drive the transistor and compensate for its voltage offset.The a.c. gain prior to the demodulator is only 50 volts/μA, the remaining voltage gain, of about 200, being provided by a low-drift d.c. amplifier which is connected as a feedback integrator and used smooth the demodulated output. This arrangement enables the system to handle, with a minimum of saturation, the large error signals which occur during a rebalancing operation, and thus maintain a fast response to large changes of input signal.The system is first analysed in general terms, and expressions are derived which describe its performance and act as a guide to the design of this type of amplifier.