1-18
You should see from this graphic analysis that when a linear impedance is combined with a
nonlinear impedance, the resulting characteristic curve is nonlinear. Some examples of nonlinear
impedances are crystal diodes, transistors, iron-core transformers, and electron tubes.
AC APPLIED TO LINEAR AND NONLINEAR IMPEDANCES
Figure 1-10 illustrates an ac sine-wave generator applied to a circuit containing several linear
impedances. A sine-wave voltage applied to linear impedances will cause a sine wave of current through
them. The wave shape across each linear impedance will be identical to the applied waveform.
Figure 1-10.—Sine wave generator applied to several impedances.
The amplitude, on the other hand, may differ from the amplitude of the applied voltage. Furthermore,
the phase of the voltage developed by any of the impedances may not be identical to the phase of the
voltage across any of the other impedances or the phase of the applied voltage. If an impedance is a
reactive component (coil or capacitor), voltage or current may lead or lag, but the wave shape will remain
the same. In a linear circuit, the output of the generator is not distorted. The frequency remains the same
throughout the entire circuit and no new frequencies are generated.
View (A) of figure 1-11 illustrates a circuit that contains a combination of linear and nonlinear
impedances with a sine wave of voltage applied. Impedances Z2, Z3, and Z4 are linear; and Z1 is
nonlinear. The result of a linear and nonlinear combination of impedances is a nonlinear waveform. The
curve Z, shown in view (B), is the nonlinear curve for the circuit of view (A). Because of the nonlinear
impedance, current can flow in the circuit only during the positive alternation of the sine-wave generator.
If an oscilloscope is connected, as shown in view (A), the waveform across Z3 will not be a sine wave.
Figure 1-12, view (A), illustrates the sine wave from the generator and view (B) shows the waveform
across the linear impedance Z3. Notice that the nonlinear impedance Z1 has eliminated the negative half
cycles.
