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Need help with determining input impedance

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demodb

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input impedance

hello,

how would you determine Rin of this part of the circuit. I thought we could approximate this by β/gm. This gm is approx. by 40*Ic, but these numbers do not match my simulations. What am i doing wrong.
 

Re: input impedance

figured it out myself. so thx for anyone who read it and was looking for the answer.
 

Re: input impedance

actually i simulated it with a loading resistor and this put Q37 in saturation. Which off course will make your simulation different.
 

Re: input impedance

Still don't understand, Did it make Q36 also in saturation? Otherwise if Q36 was in active then the input resistance should still be β/gm and if you calculated gm from the simulated current through Q36 it should still have added up.
 

Re: input impedance

you are correct, but isn't the current through Q36 determined by the current through Q37 which acts as a current source. If this Q37 is saturated it isn't able to provide the current i want it too, and so calculation and simulation don't add up.
 

Re: input impedance

Oh ok, I thought you measured Ic through simulation and then calculated β/gm in which case it should have given you the correct result.
 

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