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Old 20th December 2009, 03:09 PM   #121
wahab is offline wahab  Algeria
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Quote:
Originally Posted by Bob Cordell View Post

Compare the actual datasheet information for the MJL3281 and the 2SC3281 for those parameters most important for an output stage.


Peak ft for the MJL is about 60 MHz, while that of the 2SC is 30 MHz.

Now for the biggie: ft droop at high current and only 5V;
MJL is 30 MHz at 4A; 2SC is only 8 MHz at 4A.
ft of the 2SC3281 really crashes at high current.


Bob
how those Ft curves are misleading..at first, i thought " how tricky is the
semi industry" , but in fact, it s me who was dumb, as figures speak for themselves..
the Ft curves display the gain AS A FUNCTION OF THE CURRENT, this, at a frequency of 1 mhz, thus showing no more than the gain dispersion as a
function of the current...
a perfect device would have a straight line...

no mention that their datasheet says 30mhz typ. at 1A ,the curves show 50MHZ+ at this bias point...mystery of the us industry..but let s take it as granted an discuss using the higher figure...

what you see as qualities for the MJL3281 are in fact serious drawbacks..
if we look at the curves, we see that from 0.1 to 2A, there s a gain
dispersion of 150%....
the 2SC3281 has a 50% dispersion in the same range, which tell that
it s three time more linear....at 0.01 A , his 0.1A gain is reduced by a factor of more than 4...
of course, onsemi did stop their curve at 0.1A, but extending it show clearly that at 0.01 A, their device has a gain close to 1....in fact, they display only
the part of the curve that is marketable...a key figure for amps designer is thus missing....
indeed, these curves tell absolutly nothing about the switching speed
or the bandwith of the product, despite their (misleading) name...
remember that 2SC is a more than twenty years old device, and the MJL
has increased current capacity and TDP compared to the original..
the question was to compare with the 2SC5200, the actual general
purpose bjts fo amps, but even then, the old toshiba device is better than
the onsemi one in an amp designer point of view, not adding that they are actually faster than the "modern one"....

the onsemi, as proved by the curves are not linear at all, not suited to audio, and are good at switching, as they have low gain at low current and gain increasing dramatically with the current...lumba ogir is right on this point...

attached are the gain curves in fuction of frequency with a 4A current, in common emitter mode for a 5V Vce...impedance source is 2R, so the
Cob influence is reduced to little value...


regards,

wahab
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Old 20th December 2009, 03:19 PM   #122
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Wahad, how did you make those curves you have attached?

jd
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Old 20th December 2009, 03:39 PM   #123
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There are good known manufacturer for vintages power devices (2N series in TO-3 e. g.). One example is "MOSPEC" about
:: Welcome :: MOSPEC SEMICONDUCTOR
Who knows more about the quality standart ??
In this case I have also started this thread:
Threshold 800A and other old models: Mospec 2N5876/2N5878 or ON MJ21193/MJ21194 ?

Independend of this, here (pdf attachement) one of the Case Studies of U.S.-Japan Technology Linkages in Semiconductors as read sample about Toshiba-Motorola - from the book
<< U.S.-Japan Strategic Alliances in the Semiconductor Industry: Technology Transfer, Competition, and Public Policy >>
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Old 20th December 2009, 04:37 PM   #124
wahab is offline wahab  Algeria
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Quote:
Originally Posted by janneman View Post
Wahad, how did you make those curves you have attached?

jd
using an ac source in sweep mode and running an AC analysis...
as the beta are different, i run three circuits that have optimised
base currents, so all the devices run exactly at 4.000A.....
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Old 20th December 2009, 04:48 PM   #125
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Quote:
Originally Posted by wahab View Post
using an ac source in sweep mode and running an AC analysis...
as the beta are different, i run three circuits that have optimised
base currents, so all the devices run exactly at 4.000A.....
OK thanks, so it was a simulation. Subject to accuracy (or lack thereof) of the models, of course. It looks too good to be true

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Old 20th December 2009, 04:59 PM   #126
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Quote:
Originally Posted by wahab View Post
the Ft curves display the gain AS A FUNCTION OF THE CURRENT, this, at a frequency of 1 mhz, thus showing no more than the gain dispersion as a
function of the current...
a perfect device would have a straight line...
No.

The fT variation of a BJT with current is described in equation (16) on this page. Note the dependency of fT on gm (listed as gmf, the forward gm, to distinguish it from the reverse gm). gm is given by Ic/VT, where VT=kT/q~26mV. The only way a BJT could have an fT that's independent of current would be if gm were independent of current, and that contradicts the physics of the situation. When gm becomes large, fT depends only on the forward transit time TF. But then TF starts to increase at high currents, causing fT do decrease above a certain point. If TF is made large (a slow device), then the variation of fT with current is decreased. If TF were zero, fT would be (almost) directly proportional to collector current.

Last edited by andy_c; 20th December 2009 at 05:07 PM.
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Old 20th December 2009, 05:11 PM   #127
PMA is offline PMA  Europe
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Based on experience, the MJL devices withstand much more than 2SA1302/2SC3281. Also, japanese datasheets usually show SOA for 25C - pretty useless parameter. Everything indicates that Toshiba chips are smaller than those of Motorola.
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Old 20th December 2009, 05:17 PM   #128
wahab is offline wahab  Algeria
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Quote:
Originally Posted by andy_c View Post
No.

The fT variation of a BJT with current is described in equation (16) on this page. Note the dependency of fT on gm (listed as gmf, the forward gm, to distinguish it from the reverse gm). gm is given by Ic/VT, where VT=kT/q~26mV. The only way a BJT could have an fT that's independent of current would be if gm were independent of current, and that contradicts the physics of the situation. When gm becomes large, fT depends only on the forward transit time TF. But then TF starts to increase at high currents, causing fT do decrease above a certain point. If TF is made large (a slow device), then the variation of fT with current is decreased. If TF were zero, fT would be directly proportional to collector current.
what i said doesn t contradict your stances...
the question was : is the MJL3281 a good device for audio?...
i was responding to say that it s pointless to use the Ft as
used by onsemi as valuable norm to define wich transistor is good
and wich is not..
this doesn t tell us wich one has a high bandwith, since the test is
simply a measure of the gain at a fixed frequencyof 1MHZ..
this is as measure the gm(I) at this frequency...
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Old 20th December 2009, 05:21 PM   #129
wahab is offline wahab  Algeria
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Quote:
Originally Posted by PMA View Post
Based on experience, the MJL devices withstand much more than 2SA1302/2SC3281. Also, japanese datasheets usually show SOA for 25C - pretty useless parameter. Everything indicates that Toshiba chips are smaller than those of Motorola.
no doubt, the toshiba 2SC is rated 20A Pk/150W and the MJL is 25A Pk/200W
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Old 20th December 2009, 05:27 PM   #130
wahab is offline wahab  Algeria
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btw, andy , can you explain me why the two devices have different
relative values of current increases part of the curves, since
the gm is linearly dependant of the current, at least in a part of the curves
, according to well established physic laws...
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