Noise generators are used for measuring the self-noise of amplifiers and receivers and for some acoustic measurements. The noise of traditional low-frequency noise generators is based on the stochastic properties of an ion current resulting from a gas discharge. A simple noise generator can, however. be designed without a special gas discharge tubes the reverse-biased base-emitter junction of a bipolar transistor is a compact and inexpensive alternative.
In the circuit diagram. the noise voltage is taken from the emitter of Tl. The base-emitter junction of this p-n-p transistor begins to behave like a break-down diode at a reverse bias of about 9 V, but it really starts to generate noise at 10 V.
To ensure that the circuit works satisfactorily with a battery supply of 9 V, a step-up generator, based on 1Ci, is used. This stage provides a rectangular output voltage at a frequency of around 2750 Hz. The diode pump, consisting of C1, C3, D1, and D2, doubles the battery voltage, which results in a stable direct voltage of 10 V across D3.
Low-pass filter R17—C6 prevents frequency components of the rectangular-waveform generator appearing in the noise spectrum. Each of the opamps in IC2 raises the noise voltage in the ‘frequency range from 10 Hz to 300 kHz tenfold. The amplitude of the output voltage can be preset with Pi. The noise signal can be tested with the a.c. buzzer by closing S2.
Powered by a fresh 9-V battery, the circuit draws a current of 5-6 mA.