Need a circuit to noise test BJTs

Henry8 · 19th December 2010, 08:48 PM

Anyone have a simple test circuit?
I have a bunch of BC556B (pnp) and want to select them for an LTP stage.

MJL21193 · 19th December 2010, 09:57 PM

Google turned up THIS.
Hope that helps.

Tekko · 19th December 2010, 10:36 PM

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.

sawreyrw · 19th December 2010, 10:42 PM

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

MJL21193 · 19th December 2010, 11:02 PM

That is matching for Hfe, he asked for a circuit to measure noise. I assume he wants the 'quietest' specimens for his LTP.

Tekko · 19th December 2010, 11:06 PM

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.

MJL21193 · 19th December 2010, 11:52 PM

Quote:

Originally Posted by Tekko

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.

RJM1 · 04:37 AM

Something like this?

Henry8 · 20th December 2010, 08:39 AM

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.

effebi · 20th December 2010, 08:49 AM

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

19th December 2010, 08:48 PM	#1
Henry8 diyAudio Member Join Date: Jul 2010	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.

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
PGA 2311 Test Circuit	HarryK	Chip Amps	13	15th March 2011 09:42 AM
Tube aging, in-circuit test?	Skorpio	Tubes / Valves	5	27th September 2010 09:11 PM
How to test noise level of PSU?	Sunsun22	Power Supplies	2	10th August 2006 01:48 AM
Tube tester and noise test	Jeffy74	Tubes / Valves	1	7th March 2004 06:49 PM
Toshiba JFETs BJTs Sanken BJTs	Praise!	Swap Meet	5	14th February 2004 06:45 AM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

19th December 2010, 09:57 PM	#2
MJL21193 Account disabled at member's request Join Date: Mar 2007	Google turned up THIS. Hope that helps.

19th December 2010, 10:36 PM	#3
Tekko diyAudio Member Join Date: Jan 2005	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.

19th December 2010, 10:42 PM	#4
sawreyrw diyAudio Member Join Date: Apr 2006 Location: Minnesota	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

19th December 2010, 11:02 PM	#5
MJL21193 Account disabled at member's request Join Date: Mar 2007	That is matching for Hfe, he asked for a circuit to measure noise. I assume he wants the 'quietest' specimens for his LTP.

19th December 2010, 11:06 PM	#6
Tekko diyAudio Member Join Date: Jan 2005	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.

20th December 2010, 08:39 AM	#9
Henry8 diyAudio Member Join Date: Jul 2010	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.

20th December 2010, 08:49 AM	#10
effebi diyAudio Member Join Date: Nov 2010 Location: Milan	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

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)