For measurement of currents less than 1 pA, the transimpedance,
G, which was equal to the feedback resistor, Rf, at low frequency,
must be in the range of 1012 X. Resistors with high
resistance value and at the same time with small temperature
resistance coefficient and voltage resistance coefficient are difficult
to obtain and very expensive. For this reason the single feedback
resistor, Rf, was replaced by a T-resistor configuration which could
achieve a current gain as high as 0.22 V/pA with 100 MX resistors.