= Charge-sensitive preamplifier
A \b[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,
$$
\Delta V_{\mathrm{out}}=-\frac{Q}{C_f},\qquad Q=\int i(t)\,dt,
$$
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.<Preamplifier Introduction by ORTEC>[Charge-Sensitive Preamplifiers, pp.3–4: integrated charge and feedback capacitance]{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 $R_fC_f=50\,\mathrm{\mu s}$. 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.<Preamplifier Introduction by ORTEC>[p.1 detector matching; pp.3–4 finite gain and detector capacitance]{ref}
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