Electronics > Beginners
Transistor rise fall time & frequency response
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T3sl4co1l:

--- Quote from: Benta on July 21, 2018, 03:27:41 pm ---If the transistor is operated as a saturated switch, it's a completely different story, and  fT will not tell you anything at all.

--- End quote ---

Exactly.  Rise times are not measured in a linear circuit.  A switching circuit is only partially representative of the true performance of the part, anyway, because of voltage swings.  The fT parameter is more fundamental, but only true for small signals at the given operating point, so it's not exactly relevant to a large signal circuit.

Tim
fonograph:

--- Quote from: Benta on July 21, 2018, 03:27:41 pm ---If the transistor is operated as a saturated switch, it's a completely different story, and  fT will not tell you anything at all.

--- End quote ---

Why?
fonograph:

--- Quote from: T3sl4co1l on July 21, 2018, 02:39:23 pm ---The point is, your choice of criteria shows ignorance of the underlying principles.

--- End quote ---

How does my choice of criteria show ignorance?


--- Quote from: T3sl4co1l on July 21, 2018, 02:39:23 pm ---Indeed, 10-90% is both more common, and simpler, as you might've noticed from all the edge cases presented in reply.

--- End quote ---

Ofcourse it is more common,I never claimed anything else.I forgot to explicitly mention this before so I do it here,its simpler to ME,its question by me for me,I dont care what is simpler to you or anybody else.


--- Quote from: T3sl4co1l on July 21, 2018, 02:39:23 pm ---Likewise, you aren't learning anything by getting answers to this overly specific case.
--- End quote ---

Thats ridiculous statement,completly wrong.Its perfectly valid question that I can learn alot from,you dont know jack sh!it what I am or what I am not learning from my question.Answers? What answers? I got 0 of my questions answered,all I got was "Oh,but what about overshoot though" and "Oh,but what about 100% though".

Overly specific? What is that even to supposed to mean? Real problem are vague,unclear questions that arent specific enough ,the more I specify various variables,the clearer the question become.Asking very specific questions is normal and effective way to learn.Being highly specific is good and desirable,not faulty.


--- Quote from: T3sl4co1l on July 21, 2018, 02:39:23 pm ---What you actually need to know are the underlying principles, and then you can answer any case yourself, whatever the percentage and whatever the frequency response.

Do you understand? :)
--- End quote ---

I agree but I dont understand how dismissing my question as being  useless while not answering single thing I asked for helps me to understand the principles?





rhb:
I have recently observed 3%, 7% and 10% overshoots on a 40 pS rise time square wave on some $20K 1 GHz scopes.

The rigorous answer to your question can be obtained with Octave.

Create one cycle of a square wave:

a=[ones(1,1000000),zeros(1,100000)];
A=fft(a);
plot(abs(A));

Then zoom in on the spikes. That's the amplitude of an infinitely short rise time.  If you take the period as 1 second, then the spikes are at 1, 3, 5, 7, 9... Hz.  And the resolution bandwidth of the plot is 0.000001 Hz.

Now substitute a ramp for the leading and trailing edge.  That will get fiddly to avoid introducing even harmonics.  The FFT is defined on the semi-closed interval from -pi to pi or -1 to 1.  The maximum frequency in the plot is the center.  LHS is the positive frequencies, RHS is the negative frequencies.

QED

Edit:  The 0 to 100% ramp is a convolution with a short rectangular pulse.  So the spectrum of the square wave gets multiplied in frequency by a sinc(f) function.  The wider the pulse, the narrower the sinc(f) function.   I'll let you look up the relationship between the period T and the zeros of sinc(f).  I *should* be able to remember it, but after 30 years I still feel obliged to look it up if I'm actually doing something.  But I do the same with the quadratic equation.
tggzzz:

--- Quote from: fonograph on July 21, 2018, 11:14:22 pm ---
--- Quote from: T3sl4co1l on July 21, 2018, 02:39:23 pm ---The point is, your choice of criteria shows ignorance of the underlying principles.

--- End quote ---
How does my choice of criteria show ignorance?

--- End quote ---

See the points in my previous message.

I suggest you run a spice simulation of an abstraction of your question. I suggest a voltage step with a 1ns risetime driving the series combination of:

* a 10nH inductor (inductance of a 1cm wire)
* a 10pF capacitor in parallel with a 1Mohm resistor (a typical load)and observe the voltages and currents. Extra points for replacing the inductor with a 10ns transmission line.

Understanding how and why those waveforms occur will be enlightening.
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