[SOLVED] How to calculate/simulate/find out the output impedance?

[palmeiras]

How to calculate/simulate/find out the output impedance? Basic question

Hello guys,

The usual method to measure the output impedance of a circuit modeled by a VTH and RTH (Thevenin equivalent), is to measure the output voltage under two conditions: (a) without load (open condition) and (b) with a known load (Rload2)

The output stage of my circuit is a CMOS source follower with a resistor load (Rload1). Then, if I connect a second load (Rload2) in order to measure the output voltage, I'm going to decrease the RTH because the bias current will be changed.

So, how can I measure the output impedance if RTH is a function of RLOAD?

thank you very much,

 

[chuckey]

Use an AC test voltage and isolate your load with a capacitor. This way the DC working conditions are not upset. Select the capacitor so that at the test frequency, its reactance is very much smaller the the load impedance.
Frank

 

[palmeiras]

Thanks Chuckey!