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Old 26th March 2006, 09:34 PM   #1
Tedyp is offline Tedyp  Israel
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Join Date: Mar 2006
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Question HELP: White or Pink noise problem

Hello all,
It's my first post here, hoping that maybe someone may help me with a white/pink analog problem: I have several schematics using a NPN transistor as the source noise. the Base is at ground, the Emitter via a 470k resistor to VCC and Collector NC.
It works perfect from 10V, but as I need it portable, I wish to make it work from 6-9V (beginning with a 9V battery, that in an hour or two become 6V due the using an AF -lm386- amp.
The problem that any transistor I try (ANY!!!) - no matter what I'm doing, stops working under 8.5 - 8.6V.
I have to finish this project ASAP - but need some ideas - what can I do to make it working without redesigning my PCB?. I'm using this noice source, connected by a 0.47uF to a transistor (amp), an rc network for the "pink", another transisor as second stage amp and the 386. Nothing new, but I cannot find a decent noise source
I'll appreciate any help.
TIA,
Tedy
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Old 26th March 2006, 10:03 PM   #2
Onra is offline Onra  Germany
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Join Date: Jul 2005
Location: near Frankfurt/M
Hi,
you may use a voltage doubler driven by a CMOS-Timer-IC 555.
An other solution is searching for a Transistor with an Emitter-Base Breakdown Voltage of less than 4 - 5V.
Usable parts are BC368/369, BC868/869 and for example BFT92 as RF-Transistor with an Emitter-Base-Breakdown Voltage of 2Volts.
Many RF-transistors have very low VBE-breakdown.

Onra
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Old 26th March 2006, 11:42 PM   #3
Tedyp is offline Tedyp  Israel
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Join Date: Mar 2006
Location: World
Thanx a lot for responding, Onra,
I've tried BC868 - I had only one - worked well for about 3 minutes and then - R.I.P. At 8V !!!) there is no way for me to find BFT92 - it's a very hard to get one (I'm working more on the RF field, surveillance, transmitters etc,) - but I have several RF transistors that work OK - I'm not exactly sure what are those transistors - they are about half of TO92 case, round and marked as 668CB. The same thing happen to them as well: they begin to work, work for some time - but when I begin to "play" with them, and I mean turning the device on and off several time - they drop dead as well
I've seen my design at endless sites, I've checked it endless times - there must be something that make those transistors to "burn"....and I cannot find any device that may be used instead. I've tried Zenners - from 2.4V to 12V - no noise. Leds, - nothing... I'll find the design in a short time and up it here, as I'm not sure I have it on this PC...
Regards,
Tedy
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