AMPLIFIER GAINS

AV = Voutput / Vinput

AV = 30 V / 2 V

AV = 15

Correspondingly, if we know the gain of an Amplifier and the magnitude of the input signal, we can calculate the magnitude of the output. For example, if an Amplifier with an AC current gain of 3.5 is given an AC input signal of 28 mA RMS, the output will be 3.5 times 28 mA, or 98 mA:

Ioutput = (AV)(Vinput)

Ioutput = (3.5)(28 mA)

Ioutput = 98 mA

In the last two examples I specifically identified the gains and signal magnitudes in terms of “AC.” This was intentional, and illustrates an important concept: electronic Amplifiers often respond differently to AC and DC input signals, and may amplify them to different extents. Another way of saying this is that Amplifiers often amplify changes or variations in input signal magnitude (AC) at a different ratio than steady input signal magnitudes (DC). The specifc reasons for this are too complex to explain at this time, but the fact of the matter is worth mentioning. If gain calculations are to be carried out, it must first be understood what type of signals and gains are being dealt with, AC or DC.

Electrical Amplifier gains may be expressed in terms of voltage, current, and/or power, in both AC and DC. A summary of gain defnitions is as follows. The triangle-shaped “delta” symbol (¢) represents change in mathematics, so “¢Voutput / ¢Vinput” means “change in output voltage divided by change in input voltage,” or more simply, “AC output voltage divided by AC input voltage”:

DC gains AC gains

If multiple Amplifiers are staged, their respective gains form an overall gain equal to the product (multiplication) of the individual gains:

Input signal Amplifier Output signal   => Amplifier gain = 3 => gain = 5 Overall gain = (3)(5) = 15

1.1 From electric to electronic

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 °ow of electrons occurs. Electronic circuits add a new dimension to electric circuits in that some means of  control is exerted over the °ow of electrons by another electrical signal, either a voltage or a current. In and of itself, the control of electron °ow is nothing new to the student of electric circuits.  Switches control the °ow 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 °ow of electrons is controlled rather than whether or not any form of control exists in a circuit. Switches and rheostats control the °ow 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 °ow of electrons according to another °ow of electrons, or by the application of a static voltage. In other words, in an electronic circuit, electricity is able to control electricity. Historically, the era of electronics began with the invention of the  Audion tube , a device controlling the °ow of an electron stream through a vacuum by the application of a small  oltage 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 e®ect as the Audion tube, but in a vastly smaller amount of space and with less material. Transistors control the °ow of electrons through solid semiconductor  substances rather than  hrough a vacuum, and so transistor technology is often referred to as  solid-state  electronics.

1.2 Active versus passive devices

1.3 Amplifiers

The practical benefit of active devices is their  amplifying  ability. Whether the device in question be voltage-controlled or current-controlled, the amount of power required of the controlling signal is typically far less than the amount of power available in the controlled current. In other words, an active device doesn’t just allow electricity to control electricity; it allows a  small  amount of electricity to control a  large  amount of electricity. Because of this disparity between  controlling and controlled   powers, active devices may be em-ployed to govern a large amount of power controlled) by the application of a small amount of power (controlling). This behavior is known as amplication.

It is a fundamental rule of physics that energy can neither be created nor destroyed. Stated formally, this rule is known as the Law of Conservation of Energy, and no exceptions to it have been discovered to date. If this Law is true { and an overwhelming mass of experimental data suggests that it is { then it is impossible to build a device capable of taking a small amount of energy and magically transforming it into a large amount of energy. All machines, electric and electronic circuits included, have an upper e±ciency limit of 100 percent. At best, power out equals power in:  

Efficiency =

Usually, machines fail even to meet this limit, losing some of their input energy in the form of heat which is radiated into surrounding space and therefore not part of the output energy stream.

Efficiency =

Many people have attempted, without success, to design and build machines that output more power than they take in. Not only would such a

Efficiency =

Despite much e®ort and many unscrupulous claims of “free energy” or

In other words, the current-controlling behavior of active devices is employed to 

Amplifers, like all machines, are limited in efciency to a maximum of 100 percent. Usually, electronic amplifers are far less efcient than that, dissipating considerable amounts of energy in the form of waste heat. Because the efciency of an amplifer is always 100 percent or less, one can never be made to function as a “perpetual motion” device.

The requirement of an external source of power is common to all types of amplifers, electrical and non-electrical. A common example of a non-electrical amplifcation system would be power steering in an automobile, amplifying the power of the driver’s arms in turning the steering wheel to move the front wheels of the car. The source of power necessary for the amplifcation comes from the engine. The active  evice controlling the driver’s “input signal” is a hydraulic valve shuttling fuid power from a pump attached to the engine to a hydraulic piston assisting wheel motion. If the engine stops running, the amplifcation  system fails to amplify the driver’s arm power and the car becomes very difcult to turn.

shape DC power from the external power source into the same waveform as the input signal, producing an output signal of like shape but di®erent (greater) power magnitude. The transistor or other active device within an amplifer merely forms a larger  copy  of the input signal waveform out of the “raw” DC power provided by a battery or other power source. over-unity machines, not one has ever passed the simple test of powering itself with its own energy output and generating energy to spare. There does exist, however, a class of machines known as amplifers, which are able to take in small-power signals and output signals of much greater power. The key to understanding how amplifers can exist without violating the Law of Energy Conservation lies in the behavior of active devices. Because active devices have the ability to control a large amount of electrical power with a small amount of electrical power, they may be arranged in circuit so as to duplicate the form of the input signal power from a larger amount of power supplied by an external power source. The result is a device that appears to magically magnify the power of a small electrical signal (usually an AC voltage waveform) into an identically-shaped waveform of larger magnitude. The Law of Energy Conservation is not violated because the additional power is supplied by an external source, usually a DC battery or equivalent. The amplifer neither creates nor destroys energy, but merely reshapes it into the waveform desired: Poutput / Pinput > 1 = more than 100%perpetual motion machine prove that the Law of Energy  conservation was not a Law after all, but it would usher in a technological revolution such as the world has never seen, for it could power itself in a circular loop and generate excess power for “free:” Poutput / Pinput < 1 =  less than 100% Poutput / Pinput = 1 = 100%