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Old 19th December 2010, 08:48 PM   #1
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Default Need a circuit to noise test BJTs

Anyone have a simple test circuit?
I have a bunch of BC556B (pnp) and want to select them for an LTP stage.
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Old 19th December 2010, 09:57 PM   #2
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Google turned up THIS.
Hope that helps.
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Old 19th December 2010, 10:36 PM   #3
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I don't bother with matching, the difference is so little its in the region of 0.0001% THD difference or less in matched or unmatched.
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Old 19th December 2010, 10:42 PM   #4
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Tekko,

You need to match the transistors to minimize the offset voltage. Yes, unmatched device will produce very little distortion in a closed loop amp, but that's not the reason for matching.

Rick
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Old 19th December 2010, 11:02 PM   #5
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That is matching for Hfe, he asked for a circuit to measure noise. I assume he wants the 'quietest' specimens for his LTP.
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Old 19th December 2010, 11:06 PM   #6
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Even offset difference will be so small that matching is a waste of time, atleast in a hobby project.

However if it was a commercial product like a high precision instrumentation amplifier, then matching is highest priority.
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Old 19th December 2010, 11:52 PM   #7
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Quote:
Originally Posted by Tekko View Post
Even offset difference will be so small that matching is a waste of time, atleast in a hobby project.
I tend to agree. The 'correct' way is to match them though. I haven't and didn't have any significant output offset.
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Old 20th December 2010, 04:26 AM   #8
RJM1 is online now RJM1  United States
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Something like this?
Attached Images
File Type: jpg Vnoise.JPG (40.3 KB, 94 views)

Last edited by RJM1; 20th December 2010 at 04:37 AM.
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Old 20th December 2010, 08:39 AM   #9
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RJM1, how do i work out the component values for a +/-12v supply and 1ma thro Q1?
what R values for the opamp?
use a scope on the output?
thanks.
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Old 20th December 2010, 08:49 AM   #10
effebi is offline effebi  Italy
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I really appeciate the idea. In the short time that I am readig this forum, I have seen that much concern is taken for THD, then less for offset, but very few for noise. If I remember well the noise should be indeed of concern specially with FET transistors that should (correct me if I am wrong) be affected of 1/f noise. Also I never have seen explicitely (maybe it was done anyway) dimensioning resistors for noise avoiding. I mean trading a higher dissipation with a smaller R. This is done always when you design measurement instruments. Sometimes I had also the impression ( i should research on that) that sophisticated schematics (i.e. cascoded and double CCSs, especially Zener ones) besides of the linearity (read THD) advantages may bring in additional noise. Very interesting post
effebi
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