Page 159 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

Gyrators (aka Simulated or Active Inductors)

When generating FM was in its infancy, there were no varactor tuning diodes. The
FM transmitter back in the 1930s or 1940s used vacuum tubes for amplification, for
l
oscillation, and for modulation of the frequency of the oscillator. The choice circuit
to generate FM of that era was the reactance modulator. The reactance modulator
circuit consisted of a vacuum tube that acted as a variable inductor. With no audio
signal into this vacuum tube, an inductance of fixed value was formed in parallel
with the oscillator's main coil. If the vacuum tube changed in plate current due to

an audio signal, the inductance changed as well. So for a fixed plate current in the
vacuum tube, a fixed inductor was synthesized from the plate to the cathode of the
tube. Today, the reactance modulator can be designed with field-effect transistors,
bipolar transistors, and/or integrated circuits.
For this book, we will be working with simulated inductors or gyrators consisting of
solid-state devices, and the inductance will be fixed.

Figure 11-4 shows a very simple gyrator. The gyrator in the figure is a "simulated"
inductor with one lead that is grounded. So looking into the Cl and R2 input
terminal of the gyrator is equivalent to looking into a regular inductor as in the
lower portion of the figure with the other lead grounded.