Initial Design and Implementation of a Cryogenic Preamplifier for FTICRMS.

Improvement in signal to noise ratio (SNR) in a Cryogenic FTICRMS leads to better sensitivity1, which will eventually enable the FTICRMS to detect single charge. In FTICRMS there is a potential to dramatically improve SNR by cooling the preamplifier to 4K. However, to achieve this goal of single charge detection sensitivity, we came across several issues which are discussed below. Temperature: It is critical to design the amplifier specifically to function at 4 K, because current FET-input preamplifier designs lose charge carriers <~25 K. With some simple design modifications and swapping the input Si-FETS with the suitable active devices (GaAs FETs or HEMTs), a preamplifier can function at liquid helium temperatures with good noise and gain. Power Dissipation: The power dissipated in the amplifier is crucial as it will determine the heat generated in the circuit, and therefore, the additional heat load placed on the cryogenic system. From literature, we find that this limit is about 10mW2. Magnetic Field: Si-FETs which are usually used in designing preamplifiers often don’t work in high magnetic fields. If the amplifier is placed in a cryobath, then it sits in very high magnetic field (7T in our FTICRMS), which may cause deflection in the conduction path of the charge carriers. In order to address these challenges, we designed a 3-stage transimpedance amplifier based on the PNNL design3 using an input cascode, an NPN current amplifier, and a differential amplifier. We have evaluated the gain, noise and power consumption characteristics of this amplifier and compared it with two other preamplifier designs at room temperature, namely, (i) a commercial in-vacuum preamplifier, and (ii) a simple instrumentation amplifier. The prominent features of these designs are listed in the table below