hello every one,
can any one please explain why all foundaries will give ft only, they don't mention fmax? because fmax is more informative than ft.

Thanks.

raja.cedt wrote on Jul 31^st, 2011, 7:42am:


I can imagine (guess) what you mean - however, don't you think it would be appropriate to give us some more details?
What is your definition of fmax? What do you mean with ft? (Probably transit frequency, but nobody knows).
You even did not mention if you speak about an amplifier or a passive part.

raja.cedt wrote on Aug 1^st, 2011, 12:20am:


Where are your "definitions" from? Which textbook?
I must confess that I never have heard about a "unity gain short circuit  current" that defines a frequency like ft.
And what is "unity gain power gain"?

What I know is the following:
* for transistors, the transit frequency is defined for current gain beta=1
* for opamps, open-loop gain Aol=1.

* for opamps, fmax=large signal bandwidth (caused by the slew rate).
_____________
Does this help?

Hi Sheldon,

thank you for your message.
However, I think it is sufficient and correct to speak about the "unity-gain frequency" of a BJT since the signal current gain always is specified for a constant bias point (i.e. Vce=const and Ic=const, which is equivalent to a signal voltage short circuit).
Thus, at the same time the EARLY effect is without any influence.

Therefore, I think it is not necessary - and, more than that it is confusing - to state that "ft is unity-gain short circuit current".
(Is ft a current?). That was the background of my answer.
However, I could imagine that the question was related to the transistor transit frequency. On the other hand, I must confess that I try to teach people to ask clear and logical questions.
Example: Some time ago I read the question: What is the meaning of B?
And what about "unity gain power gain"? Is it silly to ask for clarification?
Regards
Buddy

Hi Aaron,

by the way, what is your answer to the original question:

raja.cedt wrote on Jul 31^st, 2011, 7:42am:

Regards

aaron_do wrote on Aug 1^st, 2011, 7:51am:


Hi Aaron,
the extent of this discussion regarding the meaning of two simple parameters (ft and fmax) - for my opinion - confirms that I was not completely wrong by asking for questions that are more clear.
As you will agree, most of us are rather busy but, nevertheless, take the time and try to answer some questions that are placed here in the forum - in many cases basic problems from beginners.
And because of this I think it is not appropriate that the reader who is willing to answer is forced to assess and to guess what the questioner may have meant.
In the present case, two symbols have been mentioned (ft and fmax) without any additional information resp. explanation. No mention if it is a matter of amplifiers, parts, ADC's or what else.
Therefore, my unpatient response (hopefully not unpolite).
(By the way: Did you really know from the beginning that fmax was related to power transfer only? For example, for my understanding fmax could also be the large signal bandwidth of an opamp caused by the slew rate).
There are so many misunderstandings round the world - should we as engineers not try to express ourself as clear as possible? And should we not try to teach this "art" to other forum members ?    
Kind regards
Buddy

aaron_do wrote on Aug 2^nd, 2011, 1:00am:


No, in no way. - don`t worry.
Greetings
Buddy