White Noise Generator

[redhat]

what is the purpose of R2 in this noise generator?

regards

 

[_3iMaJ]

Thats an interesting question... How do you control the variance of the noise?

I needed one of them awhile back and I have no clue how to do it with analog components so I simply did it digitally.

 

[Nigel Goodwin]

R2 (and C1) are supply decouplers to the noise generator transistor. Without them there's a good possibility the unit would be unstable, as supply variations would be fed back from the output to the input.

 

[mstechca]

R2 can increase the chance of circuit stability.

and why is the collector of the BC547 connected to ground?
Why isn't it rearranged like the other transistors?

 

[JimB]

and why is the collector of the BC547 connected to ground?
Why isn't it rearranged like the other transistors?

The point of the circuit is that the BC547 is reverse biased and the b-e junction is in zener breakdown which generates noise.
The transistor could be replaced by a zener diode which would probably be just as effective.

JimB

 

[Nigel Goodwin]

The point of the circuit is that the BC547 is reverse biased and the b-e junction is in zener breakdown which generates noise.
The transistor could be replaced by a zener diode which would probably be just as effective.

It would be interesting to compare the noise levels from a transistor version and a zener version, I wonder if transistor 'zeners' are perhaps more noisy? - it seems common to use them in place of a zener in noise generators, but it might just be for convenience?.

 

[JimB]

It would be interesting to compare the noise levels from a transistor version and a zener version, I wonder if transistor 'zeners' are perhaps more noisy? - it seems common to use them in place of a zener in noise generators, but it might just be for convenience?.

I just did a quick and dirty test on my noise generator "thingy" (just a BNC connector, a zener and a resistor)

I replaced the very old zener (marked ITT ZF 9.1 P) with a 2N3904 (just connected base and emitter) and looked at the output on a spectrum analyser.

The zener gave more noise output at low frequencies (up to about 10 Mhz) then tailed off to zero at about 250 Mhz.

The 2N3904 gave less at low frequencies than the zener but then the output was fairly flat output up to more than 1250Mhz (the limit of the analyser).

JimB

 

[Nigel Goodwin]

That's interesting Jim!, so I imagine a transistor is usually chosen just for convenience?, as noise sources are 'generally' used for audio?.

 

[vitale green]

hi, in this arrangement how to compute the gain of 2N3904 and BC549B
I have a problem with the transistor TR3 (which is identical with TR2), R5 linked core and collector at the same time, I can not find the equivalent diagram for calculating the gain, please help me