Stability of a line receiver (0–T) preamplifier, connected to a capacitive detector via a transmission line

We analyze the stability of a line receiver (0–T) preamplifier proposed to readout the ionization signal of the ATLAS LAr electromagnetic calorimeter. The transport of detector signal to the preamplifier input via a cable is analyzed on the basis of classical theory of wave propagation in transmission lines. It is shown that the analysis of stability can be reduced to the examination of zeros of the expression Zin+ZDC , where Zin is the input impedance of the 0–T and ZDC is the impedance of the detector plus cable, as seen by the preamplifier. An experimental method to measure the preamplifier input impedance in a range 1–200 MHz is described. The stability function Zin+ZDC was built by using the combination of experimental data (Zin) and calculations (ZDC) and was used to analyze the stability of two versions of 0–T preamplifier at different experimental conditions. It is illustrated that if the vector end of this semiempirical stability function makes a clockwise encirclement of zero, then the preamplifier is definitely unstable.