Is there a BD139 equivalent which has a faster slew rate? ... - diyAudio
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Old 7th July 2014, 10:15 AM   #1
andyr is offline andyr  Australia
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Default Is there a BD139 equivalent which has a faster slew rate? ...

A mate of mine who designs amplifiers for a living, designed an emitter-follower regulated PS 12 years ago, which I have been using to power my active XOs and my phono stage.

I was talking to someone recently (who may or may not know what he is talking about! ) who said the BD139 is an old design and I should instead use a more modern transistor which has a "faster slew rate".

So, can anyone advise me which transistor would meet this criterion (and be able to output a few hundred milliamps)?


Thanks,

Andy
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Old 7th July 2014, 10:38 AM   #2
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A transistor doesn't have a "slew rate" per se. It has a maximum frequency response but Fairchild, the last manufacturer does not show a value. As it is designed for switching, I would assume ir is quite fast. The MJE340 is a good substitute if you feel the need to change.
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Old 7th July 2014, 10:41 AM   #3
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Quote:
Originally Posted by andyr View Post
A mate of mine who designs amplifiers for a living, designed an emitter-follower regulated PS 12 years ago, which I have been using to power my active XOs and my phono stage.

I was talking to someone recently (who may or may not know what he is talking about! ) who said the BD139 is an old design and I should instead use a more modern transistor which has a "faster slew rate".

So, can anyone advise me which transistor would meet this criterion (and be able to output a few hundred milliamps)?


Thanks,

Andy
Hi Andy,

But why the slew rate is important for a power supply (a slow and rather stable system by its purpose)?

I often use mje340/mje350 transistors in cap multipliers and other power supply-related applications.

Cheers,
Valery
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Old 7th July 2014, 10:44 AM   #4
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First of all slew rate is not defined in case of transistors, it depends on the application environment (follower, amplifier, driver stage, etc.) One thing that influences "slew rate" of the transistor stage, is the drive impedance of the previous stage, and the base capacitance of the transistor. A more typical data for transistors is ft (transition frequency), which is -in the case of BD139- 190MHz (at Ic of 500ma). Not quite sure that you need a "faster" transistor for any kind of audio application...... There are other types in the 2SA-SC series with better "sonic" reputation among DIY-ers, but in my practice there is nothing points against the BD-s.
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Old 7th July 2014, 11:36 AM   #5
andyr is offline andyr  Australia
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Thank you guys,

As I said - I'm not sure whether the guy knows what he is talking about.

JonSnell, I will investigate the MJE340.

Valery, I have no idea why a 'fast' transistor is needed for a regulated power supply - I am merely quoting what this guy said. The background to what he said is that we were comparing a SMPS against the E-F (BD139-based) analogue PS (for a component) and the sound produced by the SMPS seemed to more dynamic than when the analogue PS was used.

I will investigate the MJE340.

dragonweed, thank you for your comment. As I said, I'm not sure whether the guy knows what he is talking about ... hence my query. 190MHz sounds plenty fast to me!


Regards,

Andy
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Old 7th July 2014, 12:26 PM   #6
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MJE340/50 are designed for High Voltage use.
Their other parameters are compromised to achieve that HV capability.

If you don't need the HV characteristic then you can almost certainly find better and sometimes much better by selecting a lower voltage device.

The bd135/6/7/8/9/40 are Low Voltage devices.
Many modern devices cannot match their performance even though they are decades newer. New does not always mean better. Another example is the Hitachi 2sb649/d669, now obsolete.
I don't know of another device that can match the fT/hFE/temp de-rated SOAR in a To126 package.
When will we see a replacement that exceeds that original mix of parameters.
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Old 7th July 2014, 02:20 PM   #7
wg_ski is online now wg_ski  United States
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Quote:
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MJE340/50 are designed for High Voltage use.
Their other parameters are compromised to achieve that HV capability..
To achieve that HV capability, usually you end up with lower beta (by an order of magnitude sometimes) and a lower output capacitance. Depending on how high a voltage you're working with, that may equate to a faster swicthing speed even though fT is lower. Mr. Miller is not your friend.
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Old 7th July 2014, 02:24 PM   #8
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Quote:
Originally Posted by andyr View Post
Thank you guys,

As I said - I'm not sure whether the guy knows what he is talking about.

Andy
Well now you know

Jan
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Old 7th July 2014, 03:41 PM   #9
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I have a question about the BD139. I have quite a few that I got from Onsemi about 8 years ago. They measure around hFE 90-100. I have new ones that I think are ST transistors and they measure around hFE 150 and still another brand that say NXT on the front and they measure hFE 250. I've always wondered what, if any, difference the different hFE measurements make and if there are instances where a lower or higher hFE is prefered. Any comments are welcome.

Thanks, Terry
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Old 7th July 2014, 05:00 PM   #10
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Except in a few cases -like high bandwidth designs with low phase margin (ie.:lots of feedback)- higher beta is better, since it demands less drive current from the previous stage. Most manufacturers produce different Hfe group transistors from the same batch of wafers depending on their location on the wafer, marking them with different methods (numbers, letters, or colours), like BC 327-15, BC 327-25, or BC 413 A, B, or C.
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