How to define s + 2, no denominator, in a transfer func block in simulink ??

s + 2 isn't a causal transfer function since there is no such thing as a pure derivative, i.e., the s term. You can either send a signal to both a derivative block and a gain of 2 in parallel with each other and then add the output of the two blocks, or you can add a denominator with a high frequency pole (i.e., add a low-pass filter to the derivative function) and normalize the gain, e.g., (as+2a)/(s+b), where b is a positive integer >> 2 and a is b/2. If you have need of the signal into the function to be a step signal, use the latter suggestion, or the derivative block may cause the output signal to blow up.

xnuke
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thank you xnuke