Bob Cordell Interview: BJT vs. MOSFET - Page 153 - diyAudio
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Old 21st May 2007, 04:16 PM   #1521
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Quote:
Originally posted by Workhorse


If we ignore the price tag, then TO-3 has a "STEEL" casing while TO-247 is just a mix of copper n plastic...BOB, TO-3 is a LUXURY item in Mosfet reign and there are only few amps on this planet who afford this piece of Steel. Their Heat Dissipation capability is much greater and they are very easier to mount on heat sink provided no mica insulators used.

The Steel Mosfet is a Mosfet with Muscle !
The "Shining Chrome Thing" claiming its superiority over dull black plastic.

Lots of caveats there, especially the last one. I just don't think they are worth the money and hassle. I don't think the steel hermetic package buys us anything in a consumer product, and in fact there are those out there who would probably complain about the presence of seel :-).
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Old 21st May 2007, 10:55 PM   #1522
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Quote:
Originally posted by G.Kleinschmidt



Well have I been out of line by attempting some kind of discussion on the performance potential of high fT BJTís over low fT BJTís?

I mean, there has been an obvious lack of interest, so perhaps Iím just a dummy talking from ignorance about something that everybody knows about already, but an appreciation of BJT performance does pertain to a comparison between BJT's & MOSFETís, doesnít it?.

The input capactiance of MOSFET's has be discussed here extensively. What about BJT's?
That we do not post does not mean 'no interest'.
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Old 23rd May 2007, 05:16 AM   #1523
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Quote:
Originally posted by myhrrhleine


That we do not post does not mean 'no interest'.

OK
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Old 23rd May 2007, 08:40 AM   #1524
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Sorry for not responding earlier. I have been away for a week.

Quote:
Originally posted by G.Kleinschmidt




The "source" being that delivering the drive current to the base of the BJT. What else could it have meant in relation to a BJT?

But we talking about the effective input capacitance, as seen by the "source". That depends on how the transistor is connected.

Yes, but I am mostly talking about the contribution of Cbe to the effective base to ground capacitance.
In an emitter follower it is much less than when the emitter is tied to a supply rail, as with a common emitter connected output (not driver) transistor, as used in a CFP.

The person first mentioning bootstrapping explicitly wrote that the bootstrapping reduced Cbe, not the input capacitance. I asked how bootstrapping could possibly alter the Cbe, to which you seemed to respond. Obviously you were not answering the question I asked, but took the liberty of answering a different question without telling us, which explains all the confusion you mention above. Cbe and input capacitance are very different things, even though they sometimes match approximately in value.

People make mistakes and write things they don't meant. That is life. That is excusable. However, all too often such mistakes go unnoticed by the people doing the mistakes and cause endless pointless discussions based on other peoples assumption that people mean what they write and not something else. I can't read peoples mind and I don't think anybody else on the fourm can either.

Anyway, I think Andy had already helped to clear up the confusion.
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Old 23rd May 2007, 08:59 AM   #1525
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Quote:
Originally posted by Christer
Sorry for not responding earlier. I have been away for a week.
The person first mentioning bootstrapping explicitly wrote that the bootstrapping reduced Cbe, not the input capacitance. I asked how bootstrapping could possibly alter the Cbe, to which you seemed to respond. Obviously you were not answering the question I asked, but took the liberty of answering a different question without telling us, which explains all the confusion you mention above. Cbe and input capacitance are very different things, even though they sometimes match approximately in value.

People make mistakes and write things they don't meant. That is life. That is excusable. However, all too often such mistakes go unnoticed by the people doing the mistakes and cause endless pointless discussions based on other peoples assumption that people mean what they write and not something else. I can't read peoples mind and I don't think anybody else on the fourm can either.

Anyway, I think Andy had already helped to clear up the confusion.

I was replying to this post of yours......

http://www.diyaudio.com/forums/showt...67#post1209267

....which wasn't explicitly address to anyone, but seemed to be a reply to my post preceeding it, here:

http://www.diyaudio.com/forums/showt...14#post1209214

I answered your bootstrapping question here:

http://www.diyaudio.com/forums/showt...10#post1209310

When you replied here:

http://www.diyaudio.com/forums/showt...16#post1209316

....you did not tell me that you were talking to somebody else.
I canít read minds either.

Thanks.
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Old 23rd May 2007, 09:10 AM   #1526
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Glen,

severel of your own posts that you link too confirm that you said that Cbe is bootstrapped, not the input capacitance, just as John said. How could I possibly know you meant something else? As you can see from my posts, I eventually started to suspect you might mean the input capacitance, not Cbe, which Andy confirmed to probably be the case.

I am not trying to accuse anybody of anything. I am just trying to explaing what the confucion was and where it came from. We already know now what the misunderstanding was, and that we seem not to disagree on the technical details, so I suggest we leave it at that. There is no point in throwing accusations at each other.
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Old 23rd May 2007, 09:22 AM   #1527
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Quote:
Originally posted by Christer
Glen,

severel of your own posts that you link too confirm that you said that Cbe is bootstrapped, not the input capacitance, just as John said. How could I possibly know you meant something else? As you can see from my posts, I eventually started to suspect you might mean the input capacitance, not Cbe, which Andy confirmed to probably be the case.

I am not trying to accuse anybody of anything. I am just trying to explaing what the confucion was and where it came from. We already know now what the misunderstanding was, and that we seem not to disagree on the technical details, so I suggest we leave it at that. There is no point in throwing accusations at each other.

OK, but one last niggle - I think that Cbe (same as Cib) IS bootstrapped. In the emitter follower configuration, the emitter 'follows' the base. Cib is the capacitance between the emitter and the base, so this capacitance is bootstrapped. Because of this, the effective input capacitance is equal to:


Cin = Cob + (1-Av * Cib)


Cheers,
Glen
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Old 23rd May 2007, 11:37 AM   #1528
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Quote:
Originally posted by G.Kleinschmidt



OK, but one last niggle - I think that Cbe (same as Cib) IS bootstrapped. In the emitter follower configuration, the emitter 'follows' the base. Cib is the capacitance between the emitter and the base, so this capacitance is bootstrapped.
Yes, I think I can see your point of view too here, although I am not so sure it is appropriate or useful to call it bootstrapped. However, that is just a matter of terminology and the important point is that regardless of whether we think of Cbe (or Cib) as bootstrapped or not, its value is unaffected by the actual topology. But we seem to agree on that.
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Old 23rd May 2007, 06:30 PM   #1529
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Quote:
Originally posted by Christer


Yes, I think I can see your point of view too here, although I am not so sure it is appropriate or useful to call it bootstrapped. However, that is just a matter of terminology and the important point is that regardless of whether we think of Cbe (or Cib) as bootstrapped or not, its value is unaffected by the actual topology. But we seem to agree on that.

Hi Guys,

Yes, it is so easy for us to get tangled up in semantics. I tend to agree with Glen in that I would say that the Cbe is bootstrapped, thus reducing its effective contribution to Cin.

So, in very rough terms, if Cbe was 1 uF (a 4 mHz bipolar operating at about 600 mA), and the gain of the follower stage was 0.99 (gm = 25 siemens against a 4 ohm load), then the Cbe contribution to effective input capacitance would be about 1 uF/100 = 10,000 pF. Then if Ccb were another 500 pF, the net effective input capacitance would be on the order of 10,500 pF.

The key thing to keep in mind is the importance of the follower stage gm against the load impedance, which determines the amount of bootstrapping. With a bipolar, since both Cbe and gm tend to be proportional to Ic, some of this goes out in the wash with current variations.

Bob
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Old 23rd May 2007, 11:53 PM   #1530
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Quote:
Originally posted by Bob Cordell
The key thing to keep in mind is the importance of the follower stage gm against the load impedance, which determines the amount of bootstrapping. With a bipolar, since both Cbe and gm tend to be proportional to Ic, some of this goes out in the wash with current variations.

Bob


G'day Bob.

I think this strengthens the argument for using more BJT pairs than that necessary for the given output power in high performance designs. Beta droop and sky-rocketing input capacitance at high currents are best avoided. Doing such not only improves the linearity of the output stage, but lightens the load on the driver stage and VAS as well. A typical 16A BJT starts going down the crapper proper at Ic > ~2A.

Cheers,
Glen
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