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.
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:
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).
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.
Here is its 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:
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.
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