Concerning the preamplifier to measure the ultra low current in the Quadrupole mass spectrometer

low noise and wide bandwidth is the key to this system as the current is from 10-11A to 10-6A and varies quickly. So we design the Non-inverting input composite preamplifier (NIICP)

and analyze the open-loop gain (ac) to optimize the frequency response of signal and noise. In the lower magnification

the circuit optimizes ac to reduce intrinsic noise. In the higher magnification

the circuit optimizes ac to recover high frequency signal which is damped down by the low pass transfer function of the large feedback resistance and parasitic capacitance. Compared with inverting input composite preamplifier (IICP) in the commercial Quadrupole mass spectrometer

experimental results indicate that NIICP reduces 5-30% in the 106-107V/A magnification

expands signal bandwidth 1.8 times and increases 10-15% MS peak signal amplitude in the 108 V/A magnification

improves the SNR of MS peaks detection.