Charge-sensitive preamplifier

A charge-sensitive preamplifier converts a detector's input charge pulse into an output electric potential difference. A high-gain electronic amplifier with capacitive negative feedback directs the collected charge onto a feedback capacitor. In the ideal inverting approximation,where $C_f$ is the feedback capacitance and $i(t)$ the input electric current. The sign depends on the chosen current direction and circuit convention; the amplitude measures charge rather than just peak current.^[ref]

For an illustrative pulse with $Q=2\mathrm{fC}$ and $C_f=0.5\mathrm{pF}$, the ideal output step has magnitude4mV. A feedback resistor can discharge the capacitor and restore the baseline. With $R_f=100\mathrm{M\Omega }$, the simple decay time is $R_f{C}_f=50\mu \mathrm{s}$. Charge arriving on a comparable time scale will leak away during collection, so the ideal step formula requires a sufficiently short input pulse.

The circuit is useful with a germanium detector, whose radiation-induced electron-hole pairs produce a small charge signal. Finite amplifier gain, detector capacitance, input noise and subsequent pulse shaping affect the measured result. The preamplifier's role is to preserve information in that charge signal; increasing gain cannot by itself remove the uncertainty introduced before amplification.^[ref]