A transistorized potentiostat with very fast rise time for impulse techniques.

Abstract

The need for rapidly charging the double layer, in some electrode process studies, calls for a fast rise-time potentiostat, which, in turn, means a high-gain wide-band amplifier. Parasitic oscillations can occur within the closed loop feedback system: potentiostat + electrodes, when more than one RC product inside the amplifier is of the same order as the time constant resulting from the cell capacity (typically some μs or more). It is thus necessary to bring the other transition frequencies up to the 10–100 Mc/s region, to keep the phase shift sufficiently low to avoid instability. These requirements have been fulfilled with a one-stage modified “bootstrap”) amplifier (gain ∼1500, frequency response linear up to several hundreds kc/s) in which the major time constant is located, followed by a unity-gain amplifier, driving high frequency power transistors (complementary symmetry emitter followers). The regulated d.c. power supply is current-limited to ensure protection against overload. To improve rise time with highly reactive loads, a corrective RC network is associated with the anode resistor of the first stage. Depending upon the load, rise time ranges from 0·25 μs (purely resistive) to 20–25 μs (C = 50 μF). The output voltage is 0 ± 20 V (± 1 to 2 A).