Hearing the noise of BJT junction

Thread Starter

Willen

Joined Nov 13, 2015
285
Hi,
I have now more than 10 types of TO-92 transistors. Few of them are manufactured by world's wellknown manufacturer like FAIRCHILD, National etc. And some of them are maybe from Chinese source. I am not sure these Chinese BJT have exact chip of exact part number. So I guess these BJTs may not perform as datasheet's spec.

Now a days I am trying to play with pretty low noise audio. I am just a poor hobbyist (in knowledge & wealth) so I don't have advanced tool to measure junction noise of BJT. So is there any way to amplify just the junction noise so I can hear them? If there is any circuit then I would place every unknown source of bjt one by one and would be able to compare.
 

DickCappels

Joined Aug 21, 2008
5,760
Just because they were made in China, that doesn't mean they won't meet the specifications. Parts bought from Chinese manufacturers through a legitimate authorized distributor are probably fine. I have used plenty of Chinese parts from my favorite distributor and never had a problem with the parts. Parts bought on the Gray Market (eBay, for example) you never really know.

You will often (usually?) find a noise figure test circuit in the datasheets for low noise transistors. The next problem is you need to filter the noise so you know its bandwidth and then some sort of RMS to DC converter so you can get a number.

See the schematic for a useful RMS to DC converter.
http://sound.whsites.net/project140.htm
 

dl324

Joined Mar 30, 2015
8,769
Just because they were made in China, that doesn't mean they won't meet the specifications.
I agree. Legitimate companies (like Intel) run manufacturing facilities in China; they just don't use bleeding edge process technology because that's restricted IP. Companies don't want their Chinese partners stealing IP; neither does the US Government.

The problem is caused by people who acquire rejects or relabel to "up spec" parts and dump them on the market. Or those who salvage components from electronic waste and sell them as something other than salvaged parts.
 

ArakelTheDragon

Joined Nov 18, 2016
1,262
I agree. Legitimate companies (like Intel) run manufacturing facilities in China; they just don't use bleeding edge process technology because that's restricted IP. Companies don't want their Chinese partners stealing IP; neither does the US Government.

The problem is caused by people who acquire rejects or relabel to "up spec" parts and dump them on the market. Or those who salvage components from electronic waste and sell them as something other than salvaged parts.
There is nothing new to steal in the past 30 years. The desire is to ruin quality, not improve it.

Why would it matter if there is some switching noise from the BJTs? Unless you are making precise medical electronics, you do not really care.
 

Bordodynov

Joined May 20, 2015
2,356
Willen,
You can simply assemble a simple amplifier circuit, and simulate a signal source with a resistor. Turn on your simplest amplifier and send a signal to the power amplifier from it. You can listen to the result or measure the variable signal at the output of the power amplifier. Of course, you can use your own, self-made amplifier on a low-noise operational amplifier instead of a ready-made audio amplifier. You can make the gain equal to 1000.
And so you can choose the best transistor.
 

DickCappels

Joined Aug 21, 2008
5,760
That circuit uses avalanche current in the transistor to generate white noise. It will not tell you anything about the transistor's noise performance because in amplifier applications you don't run transistors with the emitter-base in avalanche mode.
 
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