the white noise generator attached circuit is from circuit cellar magazine, i don't understand the part with red outline. i know that TR1 is inverted to generate white noise but i don't know its analysis for example how could VR1 control the noise level? what is the importance of c1?why don't use only one capacitor instead of c5 and c6?
C1 filters the power supply to the first stage. This prevents oscillations from power supply coupling.
The noise of the input resistor is amplified by the transistor stages. The noise voltage is proportional to the square root of the resistance.
The two parallel capacitors bypass over a wider frequency range. The larger one has more series inductance which prevents it from bypassing higher frequencies.
what about TR1? isn't it used for noise generation not amplification. what is the use of R2? in the simulation i have measured the voltage at the node between R1 and R2 and it was 12v, this means that the current in R2 branch is so small.
TR1 has the base emitter junction reverse biased. This produces noise also. The noise is proportional to the current which is controlled by the variable resistor.
I was not observant at examining the circuit the first time and did not notice that the first stage was not a PNP type.
The noise spectrum of this reverse biased junction in breakdown is not exactly flat. A noise generator using just the resistor would be more flat.
The R2 and the C1 form a voltage divider at AC and decouples the signal path from the power supply rail.
Think of R2 and C1 as being a low pass filter. Although the first transistor generates random noise, it is sensitive to changes in the supply voltage. R2/C1 are there to remove most of the supply borne signals.
As Keith noticed, there was a spam added to the thread and it was deleted.
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