bipolar (bjt) transistor families for audio power output stages
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andy_c
Banned

Join Date: Apr 2003
Quote:
 Originally Posted by wahab 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...
The easiest way to see that mathematically is on this page, equations (11) and (12). Equation (11) makes it look like fT is proportional to gm, but a closer look in equation (12) shows that Cbe in the denominator of (11) consists of the sum of two terms:

Cbe = gm * TF + Cje(Vbe)

So there's really an implied gm term in the denominator of equation (11). Let's neglect Ccb for the moment, as it will be much less than Cbe.

If the first term for Cbe above (= gm * TF) dominates as it does at high currents, then Cje(Vbe) can be neglected, and the gm terms in the numerator and denominator of the fT expression in equation (11) cancel (neglecting Ccb). In that case, fT is independent of current, having value 1/(2*pi*TF). So the slope really depends on the relative size of the two terms that make up Cbe above. If the second term dominates, fT will be nearly proportional to gm (and to Ic). I say "nearly proportional", because when Ic increases, Vbe increases also, which makes Cje(Vbe) increase somewhat.

Of course, real transistors don't approach a constant fT at high currents. Rather fT falls off once it hits the peak. SPICE models this by making TF itself vary with current.

Last edited by andy_c; 20th December 2009 at 07:59 PM.

 20th December 2009, 07:49 PM #132 PMA   diyAudio Member     Join Date: Apr 2002 Location: Prague Andy - is this just a coincidence? Analysis of the subtractive algorithm for greatest common divisors Regards, __________________ Pavel Macura http://pmacura.cz/audiopage.html https://www.diyaudio.com/forums/ever...cord-test.html
 20th December 2009, 07:52 PM #133 andy_c   Banned   Join Date: Apr 2003 Here's one other thought. Surely, the fact that fT varies with current indicates nonlinear behavior, right? But who's the culprit here? Let's imagine that Cbe were completely constant, independent of Ic and Vbe. Surely that's a requirement for linearity too. But equation (11) shows that for a constant Cbe and Ccb, fT is proportional to current! Indeed, the way to make fT independent of current is to arrange for Cbe to be proportional to current just as gm is. So the culprit of this variation of fT with collector current is the fact that gm is proportional to collector current.
andy_c
Banned

Join Date: Apr 2003
Quote:
 Originally Posted by PMA Andy - is this just a coincidence? Analysis of the subtractive algorithm for greatest common divisors
I don't know. I'd have to take some time to read the paper and find out what it's all about.

Bob Cordell
diyAudio Member

Join Date: Sep 2006
Quote:
 Originally Posted by homemodder I agree no simulation is going to tell you anything about these parts but one thing is for sure Onsemi datasheets have errors, Bob you have much better equipment than me, compare the capacitances as stated by onsemi, actually the the p cob is much higher as it should be, given the structure, little datasheet errors, please also keep in mind you compairing 30 year old japanese component to a recent onsemi one, and overall one shouldnt forget that the onsemi is a 1302 with a little bit bigger die, so will obviously have better SOA but worse cob. I can think of more modern bigger die japanese transistors which are better if you want to compare apples with apples, here youll see better SOA than ONsemi with half the cob. Also youll notice that japanese datasheets are very very accurate unlike the western datasheets which are very optimistic sometimes, use curvetracer and do some comparisons and youll get interesting results. Onsemi devices are not bad, they are copies of the japanese with much better quality control than like comparing chinese manufactured copies. Through the channels I get the japanese devices the onsemi would cost me more than double the price, the US goverment still have heafty own industry protection taxation on goods coming in from japan, through the channels I use they are on equal footing and this way the japanese parts are much cheaper so it all depends on where you live and if your goverment is trying to protect your own industry, on level footing its a very different ball game. Theres some interesting things written about ,,buy local practices,, still being used in the US. Through tiefbassuebertrlinks you can get some details about the toshiba motorola onsemi marriage.

I was just comparing the devices that were mentioned here as being the same device, and where the japanese part was alleged to be superior. What are the more modern japanese transistors that you are referring to? I always like to do apples-apples comparisons.

BTW, I don't have the perception that Japanese data sheets are more complete or more accurate in their curves. Just look at the 2SC3264 LAPT datasheet; just one page.

Crappy or not, I have not been able to get SPICE models for many Japanese transistors.

My bottom line is that it is still usually dangerous to make generalizations, even though all of us are occasionally guilty of it.

Don't get me wrong. The Japanese do great things very well. I drive a Lexus and will probably never buy an American car again, although that is probably unfair on my part.

Cheers,
Bob

bobodioulasso
diyAudio Member

Join Date: May 2008
Quote:
 Just look at the 2SC3264 LAPT datasheet; just one page.
Could you be more precise, please?
...I'am just finishing an amp using them.

PMA
diyAudio Member

Join Date: Apr 2002
Location: Prague
Quote:
 Originally Posted by andy_c I don't know. I'd have to take some time to read the paper and find out what it's all about.
Sorry for making you confused, I thought if you were an author.

Bob Cordell
diyAudio Member

Join Date: Sep 2006
Quote:
 Originally Posted by andy_c 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.
Exactly! Indeed, the reduction of ft at low current is a reflection of the somewhat fixed portion of junction capacitance as ratio'd against gm.

BTW, in a power amplifier output stage, beta droop and ft droop at high current is a far bigger problem than at low current.

Cheers,
Bob

Bob Cordell
diyAudio Member

Join Date: Sep 2006
Quote:
 Originally Posted by bobodioulasso Could you be more precise, please? ...I'am just finishing an amp using them.
First and foremost, my complaining about the datasheet does not mean the parts are bad. These are very good parts. Maybe I'm just cranky, but to me any one-page datasheet for anything other than a resistor is lacking.

Cheers,
Bob

andy_c
Banned

Join Date: Apr 2003
Quote:
 Originally Posted by andy_c 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.
Oops. That statement is an error. If Cbe were directly proportional to collector current as it is in the mid-current region, and Ccb were negligible compared to Cbe, fT could be made constant with variations in collector current. This amounts to two nonlinearities canceling each other out. I probably should have said, "...independent of current in the low-current region...".

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