[R-390] OT: Pass Transistor Question

[ews265]

Don,

Wow.  Collector current vs. Base Emitter voltage.  That would be 
difficult to measure and more so to characterize.  Crown of course 
doesn't specify what should be matched.  The expectation is that you buy 
THEIR kit of matched parts.  My supposition was that for an "equal" 
drive top and bottom, the output circuit should supply equal currents to 
the load, top and bottom so that at max output, the output stage both 
top and bottom would go into current limit at the same time.

Given all that, I put the curve tracer in pulse mode, cranked it up as 
high as I dare and matched for Hfe.  BTW, the unit is a Crown D150A.

I'd say this post is off-topic at this point.  should we go off list?

Regards,
Jon


2002tii wrote:
> Jon wrote:
>
>> I purchased 10 new transistors and after sorting ended up with a 
>> measured Beta (current gain) RANGE of about 2:1.  I now have two well 
>> matched sets installed and two lesser well matched sets as spares.  
>> The two unused outliers  have the 2:1 Beta ratio
>
> Beta does not really matter much for emitter follower applications 
> like the NPN pass transistors on a positive voltage regulator or the 
> "top" output transistors in the Crown amp.  It has some limited 
> relevance to the "bottom" transistors in the old Crown 
> quasi-complementary output stage.  By far the greatest worry as far as 
> current-sharing is concerned is the threshold voltage of the parallel 
> transistors (for power transistors, this is usually measured as the 
> base-emitter voltage required to produce a specified small collector 
> current, generally in the 1-10 mA range).  [Note that this parameter 
> is rarely specified on data sheets.]  The problem is that the increase 
> in base-emitter voltage required to increase the collector current 
> from 1 mA to 1 A (a factor of 1000:1) is very small -- on the order of 
> the normal variability of the threshold voltage from one transistor to 
> the next.  So, one transistor tends to hog all the collector current 
> before the other one even turns on.  Then, as I mentioned, even if 
> they are reasonably well matched and the other one carries some 
> current, the one that is carrying more current gets warmer, which 
> lowers its threshold voltage, which makes it carry an even greater 
> portion of the current, which makes it even warmer, etc., etc.
>
> Best regards,
>
> Don
>
>
>
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