This third volume of the book series Lessons In Electric Circuits makes a departure from the former two in that the transition between electric circuits and electronic circuits is formally crossed.
Electric circuits are connections of conductive wires and other devices whereby the uniform ﬂow of electrons occurs. Electronic circuits add a new dimension to electric circuits in that some means of control is exerted over the ﬂow of electrons by another electrical signal, either a voltage or a current.
In and of itself, the control of electron ﬂow is nothing new to the student of electric circuits. Switches control the ﬂow of electrons, as do potentiometers, especially when connected as variable resistors (rheostats). Neither the switch nor the potentiometer should be new to your experience by this point in your study.
The threshold marking the transition from electric to electronic, then, is deﬁned by how the ﬂow of electrons is controlled rather than whether or not any form of control exists in a circuit. Switches and rheostats control the ﬂow of electrons according to the positioning of a mechanical device, which is actuated by some physical force external to the circuit.
In electronics, however, we are dealing with special devices able to control the ﬂow of electrons according to another ﬂow of electrons, or by the application of a static voltage. In other words, in an electronic circuit, electricity is able to control electricity.
The historic precursor to the modern electronics era was invented by Thomas Edison in 1880 while developing the electric incandescent lamp.
Edison found that a small current passed from the heated lamp ﬁlament to a metal plate mounted inside the vacuum envelop. (Figure 1.1 (a)) Today this is known as the “Edison effect”.
Note that the battery is only necessary to heat the ﬁlament. Electrons would still ﬂow if a non-electrical heat source was used.
Figure 1.1: (a) Edison effect, (b) Fleming valve or vacuum diode, (c) DeForest audion triode vacuum tube ampliﬁer.
By 1904 Marconi Wireless Company adviser John Flemming found that an externally applied current (plate battery) only passed in one direction from ﬁlament to plate (Figure 1.1 (b)), but not the reverse direction (not shown).
This invention was the vacuum diode, used to convert alternating currents to DC. The addition of a third electrode by Lee DeForest (Figure 1.1 (c)) allowed a small signal to control the larger electron ﬂow from ﬁlament to plate.
Historically, the era of electronics began with the invention of the Audion tube, a device controlling the ﬂow of an electron stream through a vacuum by the application of a small voltage between two metal structures within the tube.
A more detailed summary of so-called electron tube or vacuum tube technology is available in the last chapter of this volume for those who are interested.
Electronics technology experienced a revolution in 1948 with the invention of the transistor. This tiny device achieved approximately the same effect as the Audion tube, but in a vastly smaller amount of space and with less material.
Transistors control the ﬂow of electrons through solid semiconductor substances rather than through a vacuum, and so transistor technology is often referred to as solid-state electronics.