output impedance simulation

[khorramrouz]

Hi all
I designed an op-amp for an improved Howland current source and want to plot it,s output impedance.I applied an ac voltage source with Vac=1V at output and measured output current and then plot V/I , also with applying I=1A, the output impedance will be V.(with grounding input)
1- But this 2 procedures in my simulation results different Zout. why?
2-If i apply a Sin. signal at input and measure output voltage and current with two amount of load resistors(say RL1 and RL2), can i evaluate output impedance by (V2-V1)/(I2-I1)?
3-If i do this method at various frequencies and plot this points,can i state that this graph is the same that came from ac analysis .
Best regards

 

[LvW]

Answer to 2) and 3): Yes - under the assumption/requirement that the condition of small-signal operation is fulfilled!
Answer to 1): Did you perform an AC (small-signal) simulation? In this case, the results should match.

 

[khorramrouz]

thanks for your favor
i grounded input signal and applied an ac source at ouput node and did an AC simulation in cadence (frequency analysis)

 

[FvM]

I would go for the current source.

An AC voltage source probably shorts the amplifier output during operation point calculation and causes a wrong DC bias point for the succeeding small signal analysis. Checking the bias point is useful anyway.

 

[khorramrouz]

Dear FvM i put a capacitor series with ac voltage source and the results became the same.
As i explained in 2, i tried to calculate the output impedance by simulation and by transient analysis.why the results differ? i want to know if ac analysis in this case is usable?which analysis can result exact answer.
best regards

 

[LvW]

Because the output resistance is a small-signal parameter you are obliged to apply an ac analysis because this is a small-signal analysis too.
And - as FvM has mentioned - the best method is to use an ac current source.

 

[khorramrouz]

thanks for reply
If i apply an input signal which fulfill small signal assumption,why the output impedance derived from (V2-V1)/(I2-I1)differ from one extracted from ac analysis? I have done a test with ideal current source parallel with a resistor and capacitor (as output impedance) .the output impedance from ac analysis and transient was the same.