A LOW NOISE LEVEL PRE-AMPLIFIER

A low noise pre-amplifier built for use with ionization chambers and with proportional counters is described. The first stage of the amplifier is a cascade'' using two grounded cathodes in parallel and a parallel grounded grid double triode. The noise level of the pre-amplifier is determined mainly by the anode current fluctuations in the input stage and is 2 mu v for a pass band 50 kc/s to 1.1 Mc/s, input capacity of about 11 pF and V/sub a/ of 75 v. The preamplifier gain is 140 with the pass band 5 Mc/s to 600 cps. The further two stages of the pre-amplifier consist of a pentode and a double triodes a heavy current feed-back is applied. The anode supply is stabilized, and d-c is used for the heater chain; the total galn of the last three stages is 170 with feed- back and about 4000 without feed-back. The whole arrangement is very stable, has a very good linearity, and has been giving very good service in high precision physical instrumentation for the last three year's. The circuit diagram of the pre-amplifier is given. (TCO)