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Old 23rd February 2011, 08:45 PM   #1
BigE is offline BigE  Canada
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Default Vebo

I have a concern about a transistor replacement I've made. The transistor is MPSA06, replacing the TIS97. It is configured in a differential pair on the input of the phase linear 400.

The TIS97 has a VEBO of 6 volts, the MPSA06 only 4.

Is there any concern in this application?

Thanks.
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Old 23rd February 2011, 10:51 PM   #2
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Differential pairs are often matched transistors.
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Old 24th February 2011, 02:50 AM   #3
BigE is offline BigE  Canada
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My apologies,

the differential pair are *both* either TIS97 or MPSA06. I just don't know if the different Vebo of the mpsa06 poses a problem.
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Old 24th February 2011, 02:53 AM   #4
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Quote:
Originally Posted by BigE View Post
My apologies,

the differential pair are *both* either TIS97 or MPSA06. I just don't know if the different Vebo of the mpsa06 poses a problem.
I would say no, as the transistors are not driven as switches but as amplifiers.
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Old 24th February 2011, 03:03 AM   #5
BigE is offline BigE  Canada
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Thank you for corroboration!

I find that there is no detectable sonic difference between the two in the phase linear 400.

I'll be putting in another matched pair next week, to replace the existing mismatched differential pair of TIS97 and ECG123. Their HFE is off by 400. Which makes a DC offset of 65 mv. It's more important to reduce that.

Thanks again.
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Old 24th February 2011, 02:10 PM   #6
BigE is offline BigE  Canada
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According to fairchild, the TIS97 characteristics are the same as PN100.

The Vbe(on) voltage of the MPSA06 is identical. Seems like high hfe versions of the MPSA06 are a valid substitute. Given that it's in a differential pair, the surrounding resistance is supposed to dominate anyway.

So why bother with high hfe?
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Old 24th February 2011, 03:36 PM   #7
wg_ski is offline wg_ski  United States
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Quote:
Originally Posted by BigE View Post
So why bother with high hfe?
Differential balance. Higher input bias currents through the bias and feedback resistors will produce more offset. Anything you do to "correct" this will upset the vbe balance and and produce an increase in distortion. Other distortion mechanisms likely predominate in the PL amps, so it may not be a big issue.
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Old 24th February 2011, 03:41 PM   #8
wg_ski is offline wg_ski  United States
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As to the issue of Vebo specifically, a lower Vbeo will not be able to "stand off" as much input overload before breaking the input transistors down. This will not blow them (avalanching the EB junction will turn a transistor very hard OFF), but long term exposure to this will permanently degrade the Hfe, and possibly increase any mismatch between the pair.
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Old 24th February 2011, 04:22 PM   #9
BigE is offline BigE  Canada
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Thanks!
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