Multisim and Ultiboard

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Confused about the "input impedance" in Transfer Function Analysis

This is a simple circuit:



I get the input impedance through the "Transfer Function Analysis":



But in my textbook, it says that the input impedance should be rbe, which approximately equal to 1kΩ. The input impedance is 1TΩ as the simulation result shows, it is so large and conflict with my textbook.


Does I improper use the multisim "Transfer Function Analysis"?


P.S. The settings of Transfer Function Analysis:



Can anyone help me?

Best regards.

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Message 1 of 3

Hi trifarmer,



Input impedance is a network property not a transfer function. I don't have Multisim so I don't have detailed knowledge about the functionality of Transfer Function Analysis.


In this "simple circuit" an input impedance of 1 TΩ is too high. On the other hand, your textbook's estimate of 1 kΩ might be erroneous. Am I correct that the BJT is operating at (luke)warm temperature? R1 will cause the impedance seen from the base node with capacitor C1 open to be less than r(be) although R1 which is 250 kΩ can still be neglected. Nevertheless, the input impedance of the circuit is


1 / (s•C1) + 1 / (1/R1 + 1/r(be))


Generally the input coupling capacitor is treated as shorted for ac signals. Unfortunately the frequency of your input signal is only 10 Hz, this makes the capacitive reactance of C1 roughly 16 kΩ.


So please verify this, the input impedance of your circuit is not simply equal to r(be) (not just about 1 kΩ).



Best regards,

G. Goodwin


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Hi, @Goodwin, thank you very much. Those days I find that in the help document of multisim it says that the software use " DC small-signal" when calculating input impedance, so...the software make VCC=0V. I find that there is a method to calculate the impedance: the AC Sweep. Put a "Voltage and Current" probe in the node 2 as the picture above shows, set the output of "AC Sweep" is V(PR1)/I(PR1), leave other settings as its default. The input impedance can be getted. It is a curve with frequency. Best regards.
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Message 3 of 3