We have already
published designs that use a transistor junction operating in Zener
breakdown as a noise source. Anyone who has experimented with a
reverse-biased transistor knows that the amplitude of the noise voltage
generated in this manner is strongly dependent on the supply voltage.
The variation between individual transistors is also rather large. An
obvious solution is to use an adjustable supply voltage for the noise
generator stage. A BC547B starts to break down at around 8V.
Using P1 and R1, you can adjust the voltage across T1 and R2 between
8 and 12V. C3 decouples the reduced supply voltage. An impedance buffer
in the form of T2 and R3 is added to the circuit, to prevent the
connected load from affecting the noise source. This buffer is powered
directly from the 12-V supply. To adjust this circuit, connect the
output to an oscilloscope. Then adjust P1 to obtain the highest signal
amplitude, combined with the best ‘shape’ of the noise signal. The
output voltage is approximately 300mVpp, and the current consumption is