A low-noise pre-amplifier was developed for use with a high-impedance germanium bolometer operating at LHe temperatures. The input stage is a selected low-noise silicon N-channeI junction FET. A large value of input resistance (Ri = 100 MΩ, metal-film) is used at room temperature to make the Johnson noise contribution of Ri unimportant, because it is in shunt with the cooled bolometer system; the large value also essentially eliminates its attenuation of the bolometer signal. Additional transistor stages connected in a unity feedback arrangement provides exceptional gain stability and linearity, and although the voltage gain is only unity, the low output impedance of 1.0 Ω results in a high power gain of 106 from a 1 MΩ source. This high power gain in conjuction with a suitable coupling transformer make negligible the noise contributions from all stages beyond the pre-amplifier. Since the noise of this pre-amplifier is low, a reliable measurement of its noise characteristic was made possible by cooling an equivalent source resistance R8 of 1.5 MΩ to 2°K, so that the noise contribution of R8 in conjunction with that of the pre-amplifier input resistance, became less than the preamplifier equivalent noise voltage, Enp, appreciably so at low frequencies. The value of Enp for R8 = 1.5 MΩ shunted by 40 pF varies from 63-25 nV/√Hz m the frequency range 25-1000 Hz, respectively, and follows an f-0.29 dependence from 25-400 Hz. Curves are shown for Enp and for the composite Johnson noise contributions of Ri and R8 at both 290°K and 2°K, for comparison. The noise figure relative to the Johnson noise voltage contribution of R8 at 290°K varies from 0.71-0.19 dB in the range 25-1000 Hz, respectively. On the other hand, the noise figure relative to the noise voltage contribution of the bolometer system at 1.65°K varies from 0-39-3-8 dB in the same frequency range, respectively. The opposite sense of frequency dependence is due entirely to the excess noise characteristic of the bolometer. © 1968.
