Will Lt Spice show me impedence?

[ItZaLLgOOd]

I've been messing around with LT Spice for a little while now.  I would like to know if it is possible to calculate input and output impedence at different points in the circuit?

[teemuk]

Yes.

The waveform viewer allows waveform arithmetics so you can derive impedance from voltage and current. Naturally, you need make the simulation perform an AC analysis for this, otherwise you'll get strange results. When you're done just convert the scale of the vertical axis to linear and it will show the values in ohms instead of decibels.

[ItZaLLgOOd]

When I switch to linear I get volts not ohms.

[brett]

Hi
V=IR
so plot V/I.
cheers

[changes]

sorry but i dont really get it
can someone help me how to do it in microcap

for example i want to calculate input-output impedance of a circuit

[brett]

Hi
RE: calculate input-output impedance of a circuit

Input Z = divide signal voltage by signal current  (Inside LTspice it's a cntrl click or shift click or something to get to the calculator box where you can combine parameters.  Check the LTspice guide or search the web for details.  You could also use the .param Z = X/Y function and then output Z.  This is good for dynamic characteristics.)

For output impedance I often do a cheat:  use a very large load (10 Meg).  Note the output voltage.  Then use the .step parameter function to step through smaller output loads until you find one that reduces the output voltage to half the voltage recorded with 10 Meg.  That load equals the output impedance (because the voltage drop is spread equally between the equal internal and external resistances.  Hint: for BJT common emitter stages, try values roughly equal to the collector resistor (JFETs = drain resistor plus a bit.)
You can also do this trick for input impedance.  Start with a nil source resistance, and increase until the input voltage halves.  That's your input impedance.

Maybe that was clear as mud, but for LTspice info you are probably better off searching the web for quality stuff from universities, and by finding stuff out by trial and error.
cheers

[changes]

that was really helpful thanks Brett