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I've tried to find a way to calculate the input resistance of a darlington pair (TIP120), using the values given in the datasheets.

https://www.onsemi.com/pub/Collateral/TIP120-D.PDF

After Searching for a while, i was only able to find solutions for discrete darlingtons,(these required knowing parameters for each transistor (like the hfe,hie that are missing in tip120 datasheet).

I need this parameter to perform the calculations concerning the AC Analysis of this circuit (T1 is the darlington):

enter image description here

How do I find Hfe and Hfi for this circuit?

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After Searching for a while, i was only able to find solutions for discrete darlingtons,(these required knowing parameters for each transistor (like the hfe,hie that are missing in tip120 datasheet).

The data sheet is particularly good for this device. Here are a few snippets: -

enter image description here

Given that hIE is the input impedance and hFE is both stated and graphed I'm not sure you are looking deeply enough at the data sheet.

I need this parameter to perform the calculations concerning the AC Analysis of this circuit

As with virtually any analysis I would urge you to use a simulator to double check any hand calculations you may make. Sims are free and good (usually) and far more accurate than a hand-calculation.

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