Feedback stability of charge amplifiers with continuous reset through forward-biased diode junctions
A low noise charge sensitive amplifier that does not require a feedback resistor nor an additional reset device has recently been proposed and successfully tested in several spectroscopy systems. The discharge of the leakage and signal current from the detector is performed continuously through the forward-biased gate to channel junction of the input JFET or, in general, through a diode shunting the preamplifier input. Up to now this forward-biased FET charge amplifier (FBFA) has been used only with detectors having currents limited to a few tens of pA. Recent applications of the FBPA for larger area detectors operating at room temperature with leakage current up to few nA reveals that the original feedback configuration of the FBFA is no longer stable and can show low frequency oscillations. In this work the double feedback loop of the FBFA and its stability is analyzed in detail, and a solution to the mentioned problem is found. The acceptable limit of the input current value in the original configuration of the amplifier is analytically determined, and a new configuration of the singularities of the loop gain, which guarantees stability of the FBFA response at higher input current, is proposed. Experimental tests are presented that show the critical condition of the original FBFA configuration and verify the feasibly of the new proposed solution.