[R-390] OT: Pass Transistor Question

[ews265]

Hi all,

Here are some thoughts that fit in with Don's.

I would probably still go with emitter degeneration resistors as it's 
considered "good engineering practice".  Also noteworthy, I just 
replaced the output transistors on an old Crown Audio stereo amplifier.  
Parts list calls for a matched set of two transistors for each channel.  
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

As a result, if you happened to install the two outliers of my group as 
parallel pass transistors, you would most likely want some emitter 
degeneration resistance to minimize the effect of the Beta variation.  
Another issue beyond possible Beta variation is in regard to what Andy 
Moorer pointed out regarding Vbe (Voltage, base to emitter) and it's 
effect on collector current.  Regardless of the transistor heatsinking 
arrangement used, it would be difficult to guarantee that the 
transistors will see exactly the same  temperature over the power 
supply's entire operating range.  Turns out that Vbe is quite sensitive 
to temperature so in regard to Andy's remarks, collector currents over 
the power supply's operating range could differ solely to the 
transistors' temperature difference.  Emitter degeneration also tends to 
help out  these effects.

Regards,
Jon WA3MVM



rbethman wrote:
> Barry,
>
> The equalizing resistor use is the result of broader mfr tolerance in 
> days of yore.  You pointed out a reference to a 1978 handbook.  
> Remember in that time when we built ridiculous bridge rectifiers for 
> HB Amplifiers?  Same philosophy!
>
> If you are using more modern components, don't need the equalizing!  
> If using "OLD" saved parts, equalize!
>
> I built a power supply about 1980 - 1981.  used 5 2N3055s on one BIG 
> heatsink.   Go for it!
>
> Bob - N0DGN
>
> Barry wrote:
>>> A few weeks ago, I mentioned I wanted to build a regulated, 
>>> low-voltage power supply that uses an LM317 with some pass 
>>> transistors to enable it to handle a larger current than the LM317 
>>> can handle.  The circuit uses a 2N2905 to drive a 2N3055 in a "pass 
>>> transistor" configuration around the LM317 and the circuit 
>>> description states it's capable of delivering up to 3 amps in this 
>>> configuration.
>>>
>>> I have a transformer with a 6-amp capacity and I would like the 
>>> power supply to deliver up to 5 (maybe all 6 amps) if I want.  Is it 
>>> possible to put another 2N3055 in parallel with the one in the 
>>> circuit allowing thus allowing the pass transistors to deliver the 
>>> extra amperage.  Will this work?
>>>
>>> I ran across something like it here:
>>>
>>> http://ourworld.compuserve.com/homepages/Bill_Bowden/page12.htm
>>>
>>> (Scroll down to the middle of the page).  The two 2N3055s are 
>>> paralled and have 0.1-ohm resistors in the emitter lines.  Not sure 
>>> what these equalizing resistors do, but perhaps that's all I need to 
>>> add?
>>>
>>> By the way, the original circuit is on page 127 of the 1978 ARRL 
>>> handbook.
>>>
>>> Thanks guys for the OT help.
>>>
>>> Barry
>>
>> Hmmm.  I backed up a page or two (p. 124) in the ARRL Handbook and it 
>> discusses parallel pass transistors, specifically 2N3055's.  I'm 
>> still wondering what the equalizing resistors do.  Anyone care to 
>> explain?
>>
>> Thanks again,
>> Barry - N4BUQ
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