A preamplifier circuit with a very low noise characteristic can be built by simply combining a FET transistor with a bipolar one.
The input impedance of the preamp circuit is almost the same as the gate impedance of the FET transistor (around 1MΩ) The output impedance at the other end is about 1KΩ.
The frequency of this preamplifier is linear (-3dB) between 10 Hz and 450 kHz and -1dB between 20 Hz and 200 kHz. The amplifying factor is aroung 100.
This preamp circuit can be powered from 12 to 30 volts without significant deterioration in its amplifying characteristics
Low noise preamplifier circuit diagram
Low noise preamp printed circuit board
- T1 = 2N3819, 2N3823
- T2 = 2SA1515, 2SB822, 2SB909, 2SB911