OT: Does anyone have a good, simple random noise generator?

there is no real danger in reverse biasing the base emitter junction of

a standard general purpose transistor for this purpose. at 1ma and 7v

the dissipation will be 7mw, practically nothing. you can experiment

with the amount of current that produces the best noise.

however, as i mentioned, the transistor should not be used for anything

else afterward as it will be damaged in regard to speed and noise

specifications. a zener diode is designed to be reverse biased and will

take orders of magnitude more power - unfortunately, it will not produce

as much or as good quality of noise. the noise voltage will vary from

transistor type to type and within each type from one sample to another,

but i doubt it will be significant unless you need a calibrated accurate

level. the zener noise will also vary from unit to unit.

and yes, you must use a resistor to limit the current. if you run it

from 9v and the junction drops 7v, your resistor should be 2v/1ma or

2kohm. the voltage will be developed between the emitter and base (which

  you will most likely consider ground) so yes, take the output from the

resistor emitter connection.

try a gain of 100 as a first guess, though you may need more like 1000.

it depends upon what final amplitude and frequency range you need. the

whole problem with these simple noise generators is getting enough

output, that's why the transistor is better. the more gain you try to

get from the op amp, the less bandwith you will get. for 25khz, to get a

gain of 1000 you'd need an op amp with 25Mhz unity gain bandwidth.

that's not likely a part you have in your junk box. a better bet is to

use two stages of lower opamp gain if you need it.

it sounds like you are just learning about analog electronics - this is

a great circuit to teach yourself the fundamentals of analog

electronics. simple but full of options, like all analog circuits. do

you think you should use the inverting or noninverting op amp circuits?

where should you put the gain, is it better to have more noise to start

with or more amplification? how about power supply rejection, should you

make a differential connection to the noise source? have fun, try a

zener diode, try an LED, try different junctions and see for yourself

what effects they have!

there is no real danger in reverse biasing the base emitter junction of

a standard general purpose transistor for this purpose. at 1ma and 7v

the dissipation will be 7mw, practically nothing. you can experiment

with the amount of current that produces the best noise.

however, as i mentioned, the transistor should not be used for anything

else afterward as it will be damaged in regard to speed and noise

specifications. a zener diode is designed to be reverse biased and will

take orders of magnitude more power - unfortunately, it will not produce

as much or as good quality of noise. the noise voltage will vary from

transistor type to type and within each type from one sample to another,

but i doubt it will be significant unless you need a calibrated accurate

level. the zener noise will also vary from unit to unit.

and yes, you must use a resistor to limit the current. if you run it

from 9v and the junction drops 7v, your resistor should be 2v/1ma or

2kohm. the voltage will be developed between the emitter and base (which

  you will most likely consider ground) so yes, take the output from the

resistor emitter connection.

try a gain of 100 as a first guess, though you may need more like 1000.

it depends upon what final amplitude and frequency range you need. the

whole problem with these simple noise generators is getting enough

output, that's why the transistor is better. the more gain you try to

get from the op amp, the less bandwith you will get. for 25khz, to get a

gain of 1000 you'd need an op amp with 25Mhz unity gain bandwidth.

that's not likely a part you have in your junk box. a better bet is to

use two stages of lower opamp gain if you need it.

it sounds like you are just learning about analog electronics - this is

a great circuit to teach yourself the fundamentals of analog

electronics. simple but full of options, like all analog circuits. do

you think you should use the inverting or noninverting op amp circuits?

where should you put the gain, is it better to have more noise to start

with or more amplification? how about power supply rejection, should you

make a differential connection to the noise source? have fun, try a

zener diode, try an LED, try different junctions and see for yourself

what effects they have!