Methods, systems, and apparatus provide an accurate time constant for electroporation. A model voltage function is created (analog, digital, or combination) to provide a model voltage having the desired time constant. A voltage is applied to the sample. A comparing circuit compares the output voltage, which may be attenuated, to the model voltage and provides an output control signal. This output signal is used to modify a resistance in parallel with the sample, thereby altering the output voltage to approximate the model voltage, which has the desired time constant. In one aspect, the control signal may be used to turn on and off a transistor that is in series with a resistor in order to modify the parallel resistance.