# Square signal's rise time and frequency

simulate this circuit – Schematic created using CircuitLab

I am analyzing work of amplitude limiter built on transistor, and I can see an incoherence as far as rising times for different frequencies and input voltages are concerned. The problem is: the bigger voltage on input for 1kHz is used, the longer rise time is, but the bigger voltage on input for 30kHz is used, the shorter rise time is. Are my measurements wrong?

I know that theoretically rising time for square wave equals 0, but what is the relation between rise time, frequency and input voltage?

Here are the oscilloscope output photos:

1: 1kHz 2V
2: 1kHz 5V
3: 30kHz 2V
4: 30kHz 5V

• I'm sorry but graph 1 and 2 look about the same to me - note the time base differences - maybe that's what's confused you. Also, precisely what is your input signal and where does it come from and what dc level might it have on it? May 14, 2014 at 19:06

There are a couple of unclear things in your schematic, which should be taken care of:

1. I assume that U0 is the output voltage which you want to measure with a high-ohmic oscilloscope. The way you depicted it, U0 is a 0 V voltage source which will defeat any functionality of your circuit.
2. The depicted circuit cannot work as a limiter. U0 is a full-swing output. I think you're confusing the terms limiter and limiting amplifier. This circuit is a limiting amplifier (at least the way you use it), to be more specific it's a common emitter stage.
3. To finally answer your question about the rise time: The main problem with this circuit is that it is not properly biased. You need to define the operating point of this common emitter stage so that the output voltage is in the middle between the rails for a 0V input signal.

I hope it helps! :)

what is the relation between rise time, frequency and input voltage?

That's actually a decent question, although only as a starting point. For a 2N3906, the better question is, what is the relationship between rise/fall time and current (frequency has no particular effect.)? For an answer, see http://www.onsemi.com/pub_link/Collateral/2N3906-D.PDF particularly figures 5 and 6. Over a fairly broad range of collector current, the harder you drive a BJT, the faster it goes.

Although, I'm a little dubious about your scope traces, since the rise and fall times seem rather long, but this may simply reflect the use of a solderless breadboard, with its' horrible stray capacitances.