DOC PREVIEW
Digital vs. Analog Electronics

This preview shows page 1 out of 3 pages.

Save
View full document
View full document
Premium Document
Do you want full access? Go Premium and unlock all 3 pages.
Access to all documents
Download any document
Ad free experience
Premium Document
Do you want full access? Go Premium and unlock all 3 pages.
Access to all documents
Download any document
Ad free experience

Unformatted text preview:

Page 1Page 2Page 3Digital vs. Analog ElectronicsFrom its inception, electronics was centered around the vacuum tube’s ability to amplify signals applied atthe tube’s grid. A transducer (e.g. a microphone) first created an electrical copy or analog of another form of energy(e.g. sound waves). The copy was then amplified by the tube and finally sent to another transducer (e.g. a speaker).As a result, amplifiers connected to the right transducers made it possible to extend human senses. Circuits designed to generate output signals that follow the input signals ina continuous manner belong to a family known as “analog electronics”. Ananalog signal as displayed on an oscilloscope might look like fig. 1.A new use for electronics and an equally new approach were introduced after nearly three decades ofanalog electronics. As a result of the work of such mathematicians as Alan Turning who wrote “On ComputableNumbers” in 1937 and John Von Neumann, the “Analytical Engine” invented by Britisher Charles Babbage in thenineteenth century was transformed to its electronic equivalent by the 1940's. An army-sponsored project intendedfor use in calculating the trajectories of artillery shells led to the creation of the first digital computer - ENIAC(Electronic Numerical Integrator and Computer). The computer was composed of 17,468 tubes, weighed thirty tonsand drew 174 kW of power. What made ENIAC’s electronics radically different was not the components it used butrather the kind of signals it handled. Representing digits in binary form (i.e. in strings of 1's and 0's), tubes (andeventually transistors) were allowed to operate in only one of two states; conducting or not conducting. Intermediatevoltages and currents typical of analog signals became meaningless in circuits that were meant to be either “on” or“off”. The ENIAC circuits were the first to employ “digital electronics”.A typical digital signal as displayed on an oscilloscope might look likefig.2.Logic CircuitsThe two states of a digital signal can have various interpretations:On OffTrue FalseYes No10+V groundBasic to all sophisticated digital electronics are some elementary circuits known as logic circuits. Onefamily of logic circuits, called TTL’s (transistor-transistor logic), first appeared in the form of integrated circuits inthe early 1970's and is still popular today. Within this family, one circuit called a two-input NAND gate is used toLcreate the others. The schematic for a two-input NAND gate is shown in fig. 1. The resistor R is not part of theNAND gate but represents any load to which theoutput is connected including inputs to otherlogic circuits. Conspicuous by its two emitters is1the input transistor Q . Apart from each emitterforming its own depletion region with the base,1Q acts like a typical NPN transistor. Because each of the two inputs can be inone of two states (either 1 or 0), there are fourpossible combinations that can be displayed alongwith the resulting output in a “truth table”. Seefig. 2. ABOutput00100111Fig. 1 Fig. 2Let’s consider the state of the output for the various input states beginning with A = 1 and B = 0. In this1case, Q 's base is connected through the 4 K resistor to +V while A is at +V and B is at ground potential. As aresult, the depletion region between A’s emitter and the base remains, the depletion region between B’s emitter and11the base disappears and Q is turned on, meaning that it now conducts the ground potential at B through Q 's222collector and to the base of Q . The ground potential at the base of Q means that Q is turned off (i.e. does not3conduct). Therefore no current flows through the 1 K resistor. That puts the base of Q at ground potential and turns33off Q . So the output sees its only connection to ground through Q severed. At the same time, the 1.6 K resistor44forward biases Q which turns on allowing the +V of the power supply to be conducted to the output. So with Q3conducting and Q not conducting, the output is at +V or is in a state of 1.By symmetry, we see that reversing the states of A and B produces the same output. Also, if both A and B1are in a state of 0, the result is the same because this “turns on” Q and conducts ground (this time through both2emitters) to the base of Q just as before. So the outputs for the first three entries in our truth table are all 1's.As you might guess, the output does change to 0 when A and B are both 1's. In this case, the depletion12regions are present at the emitters of A and B, so Q no longer conducts ground potential to the base of Q . Instead,1the 4 k resistor forward biases the base-collector junction of Q which allows positive voltage to be applied to the22 2base of Q and Q turns on. With Q conducting, enough current now flows through both the 1.6 K and 1 K resistors34to produce voltage drops large enough to turn on Q which conducts the output to ground, and to turn off Q byreverse biasing its collector-base junction as well as restoring its emitter-base depletion region. The diode helps4assure that the emitter of Q stays at least 0.7 v above ground potential during this time when the output is at ground,4guaranteeing that Q stays off. So with the A and B at 1, the output is 0. This result also occurs when nothing isattached to A and B, because when nothing is attached to A and B, the depletion regions at their emitters remain inplace just as they are when A and B are deliberately set to 1's. So if nothing is attached to the inputs of a TTL, itassumes the equivalent of the state of 1. And consequently, if nothing is attached to A and B, the output of a NANDgate is automatically 0. The truth table for a two-input NAND gate is shown in fig. 3.ABOutput001101011110 Fig.


Digital vs. Analog Electronics

Download Digital vs. Analog Electronics
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view Digital vs. Analog Electronics and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Digital vs. Analog Electronics 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?