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Old 11th April 2013, 12:03 PM   #11
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Location: Cambridge, MA USA
Which part types exactly did you test?

I'd like to say, with regard to hFE matching, it's an easy measurement for manufacturers and users to make (but perhaps relatively hard for manufacturers to meet).

I'm always disappointed when I see an hFE matching spec but no Vbe-matching spec. That's because hFE matching isn't a very useful parameter. hFE is not predictable, and well-done designs work around that parameter and depend instead on Vbe and Vbe matching. Vbe is the parameter that counts for differential-amplifier stages, etc. One can hope that well-matched hFE will imply well-matched Vbe, but one cannot count on it.

I wonder if you tried Vbe matches for your 10 parts?
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Old 11th April 2013, 01:51 PM   #12
AndrewT is offline AndrewT  Scotland
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Yes, I agree.
Set up an LTP pair and apply a common Vbe to both devices in the pair.
Discover just how well matched the output currents are into the exactly equal Collector loads.
Once I have found potential pairs I always test them thermally coupled as LTP pair. That really does show up mismatching of supposed good pairs.

The 2sa1312 datasheet is impressive.
1dB noise figure for 50r @ 7mA & 1kHz
Jumps to 6dB ref same 50r @ 10Hz.
Maybe for lower frequencies, around 2 to 3mA may be a better bias.
Any thoughts?
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Last edited by AndrewT; 11th April 2013 at 02:00 PM.
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