Wide-Band Low-Noise Charge Sensitive Preamplifier

This paper presents the design considerations and experimental characteristics of a transistorized charge-sensitive preamplifier that has a noise line-width significantly lower than that of the best vacuum-tube preamplifiers, and a high gain-bandwidth product which allows stabilization of the charge gain against detector capacitance fluctuations over a wide range of pulse-shaping time-constants. The open-loop gain of both the input charge-sensitive section and the output cable driver section is sufficiently stabilized against temperature fluctuations by compensating elements to allow the feedback capacitor in the charge-sensitive section to control the gain drift of the entire preamplifier. The input device is a new type of n-channel field-effect transistor (2N3823) that has excellent gain characteristics and an unusually low noise output. The measured gain and noise characteristics of the device are presented in this paper as functions of frequency (500 Hz to 750 kHz), temperature (-200°C to + 25°C), and bias point. The maximum signal-to-noise ratio for the best devices occurred when the case temperature was about -110°C. The charge-sensitive input section of the preamplifier has a midband open-loop voltage gain of about 3700, a low-frequency 3-db point less than 100 Hz and a high-frequency 3-db point at about 500 kHz. This high gain-bandwidth product (1.85 GHz) was obtained by developing a new type of input cascode circuit. The new circuit employs a single-stage shunt fedback current amplifier connected between the first and second stages of the conventional cascode circuit.