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Transistor rise fall time & frequency response

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fonograph:
If we have transistor with 1 micro second 0% - 100% rise time and 1 micro second 100% - 0% fall time,at what frequency it starts rolling off? At what frequency  there would be the "knee",I mean the point where it begins struggling to keep up and as result start acting like low pass filter,the point where harmonics start dropping off more rapidly than otherwise perfect waveform would becose transistor isnt fast enough.

Lets pick square wave for example,perfect square wave harmonics decrease by amplitude by 1f,that is harmonic that is 90 times higher than fundamental is 90 times weaker than fundamental.Do they start rolling off by 1f^2 or 1f^3 or maybe its not linear but gaussian curve?

1. At what frequency does 1 μs rise & fall time transistor start rolling off?

2. Does transistor FALL time matter when it comes to high frequency roll off point?

3. How rapidly does the frequency response start decreasing after the roll off point?
 

tggzzz:
Have a look at the references in https://entertaininghacks.wordpress.com/2015/08/11/measuring-digital-signal-edge-rates-without-an-oscilloscope/#more-652

fonograph:

--- Quote from: tggzzz on July 20, 2018, 07:54:50 pm ---Have a look at the references in https://entertaininghacks.wordpress.com/2015/08/11/measuring-digital-signal-edge-rates-without-an-oscilloscope/#more-652

--- End quote ---

Thank you,I like that link but it doesnt  answer any of my questions.I guess looking at the graph the roll off have gaussian shape?

T3sl4co1l:
It's never 0 and 100%.  Or actually it is, but the exact time depends on the amount of overshoot and noise.

Why not 10-90%, or 63% (one time constant), or...?

The least arbitrary case is a time constant, but strictly speaking, that's only applicable to a single order case (e.g. RC time constant).

Also, what rises and falls are you measuring?  If input current to output current, for small changes around a mean (bias) value, then it will be close to the fT figure.  If not -- if there is any voltage swing, say -- then expect the times to be much slower.

Tim

fonograph:
0 - 100% becose its simple,I know 10 - 90% is more commonly used but for the purpose of this theoretical educational question,we will go with the former.

Overshoot... Correct me if I am wrong,but when you switch to 100% it cant overshoot becose there is no more headroom,there is no additional voltage that can be provided for the overshoot by the source,any overshoot will be stopped instantly and completly by hard clipping.

Lets ignore noise for sake of simplicity,in practice we must take it into account but this is just theoretical question,no need to complicate it.Imagine its gods personal magic noiseless mosfet or whatever.

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