Hfe PNP / NPN Distortion, Oscillation? - diyAudio
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Old 11th January 2007, 01:15 PM   #1
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Default Hfe PNP / NPN Distortion, Oscillation?

If you have a large difference in HFE between NPN and PNP output devices. will this have any ill effects such as distortion or more likley to oscillate etc?

For example. In my latest venture i have an amplifier with 4 pairs of output devices. the NPN's have an Hfe in the 10's and the PNP's have a Hfe in the high 80's.

Pretty large range. Now these devices came from the factory supposedly matched. i am assuming that they have matched these to whatever specs they prefer. A quick check with my little transistor checker shows that they are all grouped similarly.

So, im curious, with this sort of assymetry of Hfe, would this output stage be more prone to Oscillation? or have more distortion products then say the exact same set up with the Hfe's of NPN and PNP that were much closer to each other?

Understandable it may not be possible to get devices that close, but lets assume that we could. or at least get them with in 20 of each other. NPN's in the 50's and PNP's in the 60's for example. and of course each matched to like within there class.

Would an output stage as such be less oscilaltion prone or have less distortion or other ill effects??


Zc
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Old 11th January 2007, 03:41 PM   #2
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Oscillations I don't know, could be. But they sure will increase distortion. With such a low Hfe the driver will be loaded heavier and increase distortion.
Are you SURE the NPN's have Hfe in the 10's?? Very unusual, even cheap power transistors (if they are genuine) should have > 50 Hfe.
Normally the PNP's have lower Hfe within a complementary pair.

What are the types?

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Old 11th January 2007, 05:20 PM   #3
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MJ150024 & 25's

These were the numbers from my little tester.

NPN - Hfe- 7, 8, 8, 12
PNP - Hfe- 89, 89, 90, 93


Now this was not at the current level they will be running at. I dont know how the MFG matches these devices. But the NPN's seem very low. so i was curious what ill effects they will have being so far off.

again this is what was supplied to me by the MFG's service dept.


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Old 11th January 2007, 07:09 PM   #4
roender is offline roender  Romania
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You can not test power BJT's with a simple hfe tester because they need at least 90-100mA colector current to be on factory specs.
If you have bigger hfe dispersion between power BJT's, H3,5,7 will be greater than H2,4,6 and the sound of amplifier will be dull.
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Old 12th January 2007, 10:31 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
you are testing at the wrong current.

Output devices, if matched, should be tested at a current near where they operate.
I would aim for operational quiescent current, but others may say typical output currents (per device) should be used.
A range of currents from half Iq to half Ipk (curve tracing) may be a good way but I fear impossible to achieve.

Distortion performance may improve.

Oscillation, unlikely.

You will need to test at low voltage to minimise temperature changes during testing, unless you have access to a low duty pulse measurement facility. Low voltage will affect the accuracy and matching but it's usually all us amateurs can achieve.
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Old 12th January 2007, 11:19 AM   #6
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Well, unless he tests at 20A+, Hfe of around 10 is definitely anomalous. These are eirther fakes, or the test is wrong.
I would re-check, double check the test. Most probably there was an error in the E-B-C connections.

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Old 12th January 2007, 11:39 AM   #7
Nordic is offline Nordic  South Africa
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Lol, I just exerienced my first wrong connection problem when stuffing one of my Nuuk class A buffer boards a few minutes ago... Ran out of bc547 so I tried sticking a bc639 into the socket without proper orientation. Just got clicking sound from outputs... hey I'm stil a transistor noob... only learned 2 minutes ago about TO92A B and C. All great now! Now to wire the volume and balance controls between the 4 buffers (2 per channel with attenuator between stages).

My very first fully fledged solidstate hardware
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Old 12th January 2007, 11:43 AM   #8
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Quote:
Originally posted by Nordic
Lol, I just exerienced my first wrong connection problem when stuffing one of my Nuuk class A buffer boards a few minutes ago... Ran out of bc547 so I tried sticking a bc639 into the socket without proper orientation. Just got clicking sound from outputs... hey I'm stil a transistor noob... only learned 2 minutes ago about TO92A B and C. All great now!
First thing I do when starting to layout a pcb or assemble something is to get copies of all the data sheets into the project binder and check each type before inserting/soldering. Fell on my face too many times. Also great when needing to replace something xx years later...

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Old 12th January 2007, 12:07 PM   #9
Nordic is offline Nordic  South Africa
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As a matter of fact I now marked all my transistor baggies with the correct pinout according to my multimeter... I get confused with dataheets, some show pinout from top, some bottom....
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Old 13th January 2007, 12:42 AM   #10
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You MUST test Hfe near the operating idle current. NPN's tend to sag at low currents.
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