Spectrum Software
Industrial Strength Simulation




Spectrum Software has released Micro-Cap 11, the eleventh generation of our SPICE circuit simulator.

For users of previous Micro-Cap versions, check out the new features available in the latest version. For those of you who are new to Micro-Cap, take our features tour to see what Micro-Cap has to offer.




Plotting AC Impedance


To plot AC impedance for two-terminal devices like diodes, resistors, etc. you need to measure the current into and the voltage across the two terminals. Consider the simple circuit below.
Inductor whose impedance is to be analyzed
This inductor uses a model statement in which a 1pF parallel capacitance and a .01 ohm series resistance are specified. To measure its AC impedance we must plot V(L1) / I(L1). When we do we get the plot below:

Inductor AC Impedance plot.

Because this inductor has a parasitic capacitor in parallel with it, there is a resonance in the impedance which approaches infinity at about F0 = 5.032 MHz. Before this point the impedance is approximately the ideal inductor impedance of j*2*PI*F*1m. After F0 the impedance is approximately that of the 1p parallel capacitance, 1/(j*2*PI*F*1p).

For diodes, resistors, capacitors, and inductors there is an easier way to plot AC impedances. Simply plot Z(D1), Z(R1), Z(C1) or Z(L1). Here is the same plot with both V(L1) / I(L1) and Z(L1).

Inductor AC Impedance plot.

As you can see the plots overlap because Z(L1) gets internally computed as V(L1) / I(L1).

For another illustration of plotting two-terminal impedances here is a plot of the impedance of a zener diode, biased near breakdown. Here is the circuit.
Zener AC Impedance Test Circuit.
Here is its AC impedance plot.

Zener AC Impedance plot

In this case, the low frequency impedance saturates at about 38 ohms and the impedance at high frequency saturates at just under 1 ohm.

AC impedance is a complex quantity, having both real and imaginary parts. Z(D1) plots the magnitude of Z(D1). You can also plot PHASE(Z(D1)), RE(Z(D1)), and IM(Z(D1)).

You can also plot the inverse of AC impedance, AC conductance. The syntax is the same as impedance except that you use G instead of Z. For example G(D1) would plot the complex conductance of D1.

Could you use the same Z(X) or G(X) syntax for a general two terminal subcircuit? No, because there is no intrinsic way for Micro-Cap to measure the current through the two terminals, whereas for standard two-terminal devices there is a readily available way to measure the current.

For a two-terminal subcircuit you can always use the general method; plot V(VIN)/I(VIN), where VIN is a voltage source placed across the two terminal device. The source can be either a current or a voltage source. The main concern is that its DC conditions produce or at least not alter the desired operating point. In the zener circuit just used, the source has a small DC value to bias the zener near breakdown.

The same principle can be employed for more complex circuits like amplifiers or filters. You measure the current into the circuit and the voltage across two terminals that represent its input. Consider the simple circuit below:
General Network

Since the source VIN can be used to measure both the input voltage and the input current, all we need do is plot Z(VIN) (which internally translates to V(VIN)/I(VIN)). This plot, which shows both quantities, demonstrates their equivalence.

General Network AC Impedance plot

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