The full equivalent circuit of the transimpedance inverting preamplifier formed by the operational amplifier with gain bandwidth f gbw and feedback network R f C f. The device under test is modeled with the resistor R s connected by the signal line with capacitance C L.
The circuit diagram of the composite current preamplifier with the current gain 107 V/A. The composite op-amp circuit is marked by the dashed box. In order to minimise the parasitic stray capacitance the preamplifier was assembled on a surface mount printed circuit board and enclosed in an aluminium box. All the components used were of the surface-mount type; the 1206 surface mount device (SMD) metal-film resistors (Philips Electronics), 0805 SMD ceramic and 1210 SMD tantalum capacitors (AVX), and the op-amps in small-outline integrated circuit (SOIC)-type package (analog devices).
The frequency response of the composite current preamplifier shown in Fig. 2. (a) The measured gain flatness at different values of the input capacitance C L presented by the normalized gain . (b) The signal phase frequency response at different values of the input capacitance C L. The value of C L increases for curves from top to bottom.
The noise characterisation of the composite current preamplifier shown in Fig. 2. (a) The preamplifier total noise density spectrum measured for the open input (“O.I.”) and different values of R s (12 kΩ, 24 kΩ, 51 kΩ, 246 kΩ). R s was immersed in liquid 4He (T = 4.2 K) and connected by the coaxial cable with C L = 438 pF. The red dash dotted line represents the thermal noise of the feedback resistor R f (i f = 41 fA/). (b) The total current noise of the open-input preamplifier measured for different input capacitances C L. The dashed line shows the current noise expected from coupling of the voltage noise e n = 2.9 nV/ through the capacitor C L = 1 nF. The dash dotted line represents the thermal noise of the feedback resistor the same as in (a). (c) and (d) The extracted op-amp voltage (e n) and current (i n) noise density spectra, correspondingly.
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