OnSemi says they're already matched (MJ15022G/23G)... Do I believe them? - diyAudio
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Old 27th August 2008, 07:56 PM   #1
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Join Date: Sep 2004
Location: North Alabama
Default OnSemi says they're already matched (MJ15022G/23G)... Do I believe them?

Preface: I'm an electrical engineer, but my specialty is digital electronics, not analog... (I never paid enough attention on small signal circuit design, etc.) So I'm hoping you guys can enlighten me a bit here... thanks in advance!

I have received some samples of OnSemi MJ15022G/23G power transistors, that I plan to use to refurb an old Audire Forte power amplifier ... and I have a few questions...

1) I asked the On Semi FAE about wanting to match these transistors (all the 22's have the same date code, and all the 23's have the same date code, but the 22's and 23's date codes are off by a few weeks), and he passed my question along and the factory came back with the following answer:

Quote:
The easy solution is to go for “Audio Transistors” to start with and MJ1500 falls into that category…
The tolerance on all our Audio Transistors are so close that there is no need to match Date Codes.
- Is this true?

2) If I did want to setup a test circuit to parameterize these guys to verify how well/poorly they matched one another... can anybody point me to some sample circuits to perform such parameterization (PNP and NPN circuits please) ?

3) The original Forte power amp uses the Pb (lead) versions of these parts (sans the "G" suffix), but the parts I received are all the "G" (Pb-free) versions. When searching the forum I noticed someone indicated their testing showed the Pb-free parts had lower hfe ranges than the Pb versions. (It was a different part, but still within the MJ15xxx family IIRC.) So -- IF in fact the Pb-free parts have lower hfe values, is there a chance I need to consider adjusting some other part of the input stage to compensate for a lower hfe value? Or does it simply mean the overall sensitivity will be reduced, requiring a (slightly) stronger input signal to achieve the same output power?

More on hfe values

3a) ... how much of a difference is "important?"
3b) ... is this the parameter that is needing to be "matched?"


Sorry to be such a "noob" ... I can deal with crazy complicated state machine logic and 0's and 1's all day long and twice on tuesdays, but when it comes to analog, I'm still trying to find money in the budget (1 income - [3 kids + dog] == NO MONEY LEFT ) to go back to school...


Thanks everybody...
..dane
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Old 29th August 2008, 06:10 PM   #2
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Join Date: Sep 2004
Location: North Alabama
Bummer... thought I'd get more replies than this ...
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Old 29th August 2008, 06:19 PM   #3
AndrewT is online now AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
I don't believe ONsemi.
I cannot accept that they can manufacture all their devices to such a tight tolerance that in effect they are pre-matched.

Set up the transistor to pass operational bias current and measure the Vbe @ that current.
Look up AKSA's matching description. He goes on to describe how to match Vbe accurately and then select a close match for hFE by measuring the base current (via voltage across the base resistor).
But, be careful that you measure Vbe and not include the voltage drop across the base resistor.

Parallel sets need to be matched preferably better than 5mVbe. If you can get <2mVbe then great.

BTW,
some builders insist that the more important parameter is hFE, but I don't agree.
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regards Andrew T.
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