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Old 6th August 2012, 12:15 AM   #1
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Join Date: May 2011
Location: Virginia
Default Vbe matcher?

I'm interested in matching the Vbe of power transistors in NPN/PNP pairs. I found this pdf document:
http://home.comcast.net/~ijfritz/pro...ransmat001.pdf
That shows a simple way to match a pair of NPN or PNP, but not a complementary pair.

Any ideas?
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Old 12th August 2012, 04:57 PM   #2
AndrewT is offline AndrewT  Scotland
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Build up a two transistor CCS that can run continuously at the test current you require. You need a second opposite polarity CCS for the PNP.
50mA to 150mA for a power device biased to ClassAB might be in the ballpark. ClassA devices will need much more.

Set up your NPN DUT with a collector load resistor of 0r1 to 1r0 (depending on what Ic you want and what PSU voltage you have available).

Attach a fixed resistor of about 1k to the base lead and then connect that 1k resistor to the +ve supply using a variable resistor, 20k to 50k should be about right.
Connect the load resistor to the +ve supply via your CCS.
Tie the CCS -ve lead to zero volts.
Now adjust the VR until the voltage across the CCS is ~ 2V or so. If this voltage is too high, the CCS will require an even larger heatsink and that will take longer to reach equilibrium.
You want to keep that CCS Vdrop about the same for each DUT, so that your don't need to repeatedly check the voltage across the collector load.

Now measure the Vbe @ your CCS controlled test current. You can also measure and record the hFE @ the same test current. That 1k0 Vdrop gives the denominator for hFE.

Repeat the whole thing with opposite polarities for your PNP DUTs.

The accuracy of your testing is affected by Tj of the DUT and the repeatability of the CCS test current. You need to develop a method that minimises these two errors.

Ian's method uses current checking resistors in the emitter leads. I don't like that method when doing LTP pair comparison. The emitter resistors act like degeneration resistors and hide the effects of errors in technique.
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Last edited by AndrewT; 12th August 2012 at 05:06 PM.
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