[sdiy] Help with (lack of) noise circuit

[Oren Leavitt]

On 12/23/2013 4:19 PM, cheater00 . wrote:
>
> On 23 Dec 2013 22:47, "Oren Leavitt" <obl64 at ix.netcom.com
> <mailto:obl64 at ix.netcom.com>> wrote:
>  >
>  > On 12/21/2013 5:52 PM, Tim Ressel wrote:
>  >>
>  >> Funny how the simplest circuits can kick your butt. I'm trying to do
> the classic transistor noise generator, with no luck. I started with a
> 2N3904 with the emitter to +12 with a 100K resistor and the base to
> ground. I've tried resistors from 500 ohms to 1 meg, voltages up to 24
> volts, and different transistors (PN2222, PN2484). No joy. What am I
> doing wrong?
>  >>
>  >> Thanks!
>  >>
>  >
>  > Tim, Having build a number of these myself, I've found about 470k to
> 1M is a good range for reverse bias current thru the transistor on a 12
> to 15 volt supply.
>  > Be sure to AC couple the noise transistor via a capacitor to the
> input a high AC gain (say 500 to 1000x) amplifier. Use an AC gain
> configuration so that you don't amplify DC offsets.
>  >
>  > If you can find one, try a 2N2712.
>
> Why that transistor in specific?
>
> Cheers,
> D.
>

2N2712s have a higher noise output in reverse bias. I've found I don't 
need as much amplification on these guys.
"Old process" Zener diodes from the 1960s/70s (10V or higher) can be 
very noisy, too.

- O