Measurement of an Active Circuit

We would now like to find the output impedance of the jFET source follower circuit shown below. The output is taken at the source terminal of the jFET, so the impedance we are interested in is between nodes Vo and 0 (ground). 

When you are finding output impedance you must set the input AC source to zero. Since the input is a voltage source, we 

Next we need to add an AC source at the output to measure the impedance. We could use either an AC voltage source or an AC current source. If you use a voltage source, wire the circuit shown below  

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AC voltage source (part name VAC) added here. 

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