Reverse Bias NPN Transistor for White Noise - Page 1
Forum section: electronics / beginners

author: kbarnette
author_rank: Contributor
timestamp: August 20, 2016, 06:00:45 pm
content: Hi all,New here, and I have a question. I'm trying to build a white noise (maybe evolve into a pink noise with some filtering after I can reliably get white noise) generator for my daughter's room. I can do it with a simple arduino sketch but I was looking for something a little more...analog (simpler, in its way). I've been attempting to get a circuit running with a reverse biased NPN transistor and an opamp. I've been using an S8050 transistor with 9v on the emmiter and the base grounded. I was trying to view the noise coming off the transistor on a scope at my local makerspace, as I'm still saving up for a DS1054z. Unfortunately I get a strong 60hz signal in the lab from surrounding power/equipment at the makerspace and couldn't make out if I was actually getting noise from the transistor. I was hooking the positive lead to the emitter leg and the negative lead to ground (i.e. the base of the transistor). I wanted to know what sort of amplitude the noise was off of the transistor so I could calculate the gain I needed on a negative feedback op amp config. I have a few questions:Is there a general expectation in terms of peak to peak voltage I'd be getting from the transistor in this configuration?Is 9v sufficient to drive the zener effect on this kind of transistor?Am I probing in the correct location?Is there any other way (don't have an electro-magnetic quiet location at the moment) for me to take a look at the noise coming off of the transistor?I'm basing my design off of the following:http://electro-music.com/forum/phpbb-files/whitenoise_969.jpgI also have a wide range of zener diodes I could try this on if it's suggested that might be easier (at least I'd know the break down voltage). I'm not sure if I'd need to exceed that voltage or ride the breakdown voltage line to get clean white noise off of it though.Thanks in advance!

author: Zero999
author_rank: Super Contributor
timestamp: August 20, 2016, 08:18:25 pm
content: Selecting the current for maximum noise output can be tricky. As far as I'm aware, the best way to achieve this is to put the reverse biased BJT in the feedback loop of a common emitter amplifier.https://www.circuitlab.com/circuit/8wzr22/white-noise-generator/

author: Andreas
author_rank: Super Contributor
timestamp: August 20, 2016, 09:18:51 pm
content: Quote from: kbarnette on August 20, 2016, 06:00:45 pmHello,the zener voltage of the transistor will be typically in the range of 7-9V (depending on sample).So a single 9V supply will not be sufficient for all devices as you will need some headroom for the pull-up resistor.I expect a noise amplitude of around 1 mV (0.1-10mV) at the transistor.If probing directly you will short the zener with the capacitive + input impedance of the scope.So without amplifier you will hardly see something on a normal scope.Usually you will need a (metal) paint can or cookies box + battery supply to shield the 60 Hz hum.With best regardsAndreas

author: Zero999
author_rank: Super Contributor
timestamp: August 20, 2016, 09:36:33 pm
content: Sorry, I missed the part about 9V.If lower voltage operation is required then why not simply use a zener diode? It won't be as noisy as a BJT but should do. Higher voltage zeners tend to be more noisy than lower voltages but it needs to be comfortably lower than 9V so it still works, as the battery discharges. 5V is probably a sensible place to start.Another option is to use a voltage doubler (555 or better still the ICL7660) to power the noise part of the circuit but it will need to be filtered to prevent the switching frequency form interfering with the noise output.

author: kbarnette
author_rank: Contributor
timestamp: August 21, 2016, 06:30:34 am
content: Thanks for the replies! I'm not limited to 9v so I'll take a look at bumping it up and seeing what happens. I didn't expect to not be able to measure the noise at the transistor itself so that's enlightening. I'll use the 0.1 - 10mv range to calculate the gain I'll need for my 8ohm speaker. I'll post again when I've done some more experimenting!

author: kbarnette
author_rank: Contributor
timestamp: August 22, 2016, 10:29:09 pm
content: I tried the noise maker circuit that Hero referenced from circuitlab.com and still only really see a 200mVpp 60hz sine wave coming off the outputs. I'm not sure what sort of noise amplitude one would expect from that circuit but, assuming it's working, it's evidently WAY less than 200mV. Even if I decrease the time base by a factor of 10, I then see a 60khz 4mVpp triangle wave that's carried by the 60hz since wave. No evidence of white noise as it's all pretty stable on the scope. I believe all of this means I can't really tell if the noise circuit is working because of all of the interference here at the makerspace. I hope to have a 1054z soon and maybe I can find a quiet place to test this out. Thanks for all of the help! Hopefully I'm not misunderstanding what's going on here and can make progress when I have the right tools myself.

author: kbarnette
author_rank: Contributor
timestamp: August 22, 2016, 11:15:05 pm
content: Nevermind, I'm an idiot. I was using a new breadboard on which the power rails apparently don't extend the entire length of the board. I'm getting great white (from what I can tell, seems pretty even across the band) noise from it now and am proceeding with the amplifier (2Vpp noise is barely coming through on my 0.5W 8ohm speaker). Thanks all!

author: fubar.gr
author_rank: Unknown
timestamp: August 23, 2016, 08:22:59 am
content: Quote from: Hero999 on August 20, 2016, 08:18:25 pmCan somebody please explain why this circuit generates white noise?Could the transistor be substituted with a diode?

author: Andreas
author_rank: Super Contributor
timestamp: August 23, 2016, 06:11:13 pm
content: Q1 is switched as a zener diode (about 7-10V zener voltage).Q2 is a amplifier stage and also sets the bias voltage of the zener.So you could replace Q1 with a (around 7V) zener diode.With best regardsAndreas

author: Lee Leduc
author_rank: Regular Contributor
timestamp: August 23, 2016, 08:19:13 pm
content: Here's a site that describes the avalanche phenomenon of a PN junction. It's basically an introduction to the concept, but it's a good place to start.http://www.circuitstoday.com/pn-junction-breakdown-characteristics.

author: igeljo
author_rank: Newbie
timestamp: July 23, 2023, 02:54:29 pm
content: Quote from: Andreas on August 23, 2016, 06:11:13 pmI know its an old topic, but i hope youre still active: Can you please explain the reason for the C1 capacitance? When i simulate the circuit it seems like theres no difference, if i just exclude it so im wondering if i even need it?Kind Regards,igeljo

author: magic
author_rank: Super Contributor
timestamp: July 23, 2023, 04:10:56 pm
content: There should be a difference. This circuit can be regarded as an opamp, with IN+ at Q2 emitter and IN- at Q1 emitter and about 7V offset voltage.R2 and C1 are the feedback network. Noise gain of this configuration is unity at DC and starts increasing at the corner frequency of 47kΩ·1μF. Actual output will not increase infinitely, but level off once it reaches full open loop gain (not a lot) of this stage.

author: MrAl
author_rank: Super Contributor
timestamp: July 23, 2023, 06:04:06 pm
content: Hi,I prefer using a microcontroller with a short logic program.