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A brief description of a diode and its inner workings.


v-i relation and schematic symbol for the diode. Here, the diode parameters were room temperature and I 0 1  μA .

The resistor, capacitor, and inductor are linear circuit elements in that their v-i relations are linear in the mathematical sense. Voltage and current sourcesare (technically) nonlinear devices: stated simply, doubling the current through a voltage source does not double the voltage. Amore blatant, and very useful, nonlinear circuit element is the diode ( learn more ). Its input-output relation has an exponential form.

i t I 0 q k T v t 1
Here, the quantity q represents the charge of a single electron in coulombs, k is Boltzmann's constant, and T is the diode's temperature in K . At room temperature, the ratio k T q 25 mV. The constant I 0 is the leakage current, and is usually very small. Viewing this v-i relation in [link] , the nonlinearity becomes obvious. When the voltage is positive,current flows easily through the diode. This situation is known as forward biasing . When we apply a negative voltage, the current is quite small, and equals I 0 , known as the leakage or reverse-bias current. A less detailed model for the diode has any positive current flowing through the diode when it is forward biased, andno current when negative biased. Note that the diode's schematic symbol looks like an arrowhead; the direction of current flowcorresponds to the direction the arrowhead points.

Diode circuit

Because of the diode's nonlinear nature, we cannot use impedances nor series/parallel combination rules to analyze circuits containing them. The reliablenode method can always be used; it only relies on KVL for its application, and KVL is a statement about voltage drops around aclosed path regardless of whether the elements are linear or not. Thus, for this simple circuit wehave

v out R I 0 q k T v in v out 1
This equation cannot be solved in closed form. We must understand what is going on from basic principles,using computational and graphical aids. As an approximation, when v in is positive, current flows through the diode so long as the voltage v out is smaller than v in (so the diode is forward biased). If the source is negative or v out "tries" to be bigger than v in , the diode is reverse-biased, and the reverse-bias current flowsthrough the diode. Thus, at this level of analysis, positive input voltages result in positive output voltages with negativeones resulting in v out R I 0 .

Diode circuit

We need to detail the exponential nonlinearity to determine how the circuit distorts the input voltage waveform. We can ofcourse numerically solve [link] to determine the output voltage when the input is a sinusoid. Tolearn more, let's express this equation graphically. We plot each term as a function of v out for various values of the input voltage v in ; where they intersect gives us the output voltage. The left side, the current through the output resistor, does notvary itself with v in , and thus we have a fixed straight line. As for the right side,which expresses the diode's v-i relation, the point at which the curve crosses the v out axis gives us the value of v in . Clearly, the two curves will always intersect just once for anyvalue of v in , and for positive v in the intersection occurs at a value for v out smaller than v in . This reduction is smaller if the straight line has a shallowerslope, which corresponds to using a bigger output resistor. For negative v in , the diode is reverse-biased and the output voltage equals R I 0 .

What utility might this simple circuit have? The diode'snonlinearity cannot be escaped here, and the clearly evident distortion must have some practical application if the circuitwere to be useful. This circuit, known as a half-wave rectifier , is present in virtually every AM radio twice and each serves very different functions! We'll learn what functions later.

Diode circuit

Here is a circuit involving a diode that is actually simpler toanalyze than the previous one. We know that the current through the resistor must equal that through the diode. Thus, thediode's current is proportional to the input voltage. As the voltage across the diode is related to the logarithm of itscurrent, we see that the input-output relation is

v out k T q v in R I 0 1
Clearly, the name logarithmic amplifier is justified for this circuit.

Questions & Answers

what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Mueller Reply
Answers please
Nikki Reply

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Source:  OpenStax, Fundamentals of electrical engineering i. OpenStax CNX. Aug 06, 2008 Download for free at http://legacy.cnx.org/content/col10040/1.9
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