A differential-input, differential-output preamplifier topology for the design of ultra-low noise voltage amplifiers

We propose a simple topology for the realization of ultra low noise differential-input, differential-output preamplifiers. The input devices can be either low noise JFET pairs (for high impedance sources) or low noise BJT pairs (for minimum low frequency voltage noise). Besides the input devices, the circuit requires only two matched low noise BJT transistor pairs (either PNP or NPN, depending on the input device type) and a few excess noise free resistors for consistently obtaining differential gains of 100 (40 dB) with a bandwidth in excess of 100 kHz and an equivalent input noise floor comparable to (or better than) those reported in the literature for similar devices. The preamplifier is to be regarded as an off-the-shelf component that can be employed as a building block for a number of applications in the field of low frequency noise measurements.