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department 

of 

electrical engineering

 Mini project 

on

Application of diodes as gates

Submitted by: 

 Manu raj b090696ee 

Sunil kumar banra b090 

Sushant nayak b090 

Vikash kumar b090706ee 

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Diodes : 

Fig. Typical diode packages in same alignment as diode symbol. Thin bar depicts the  cathode . 

In electronics, a diode is a two-terminal electronic component that conducts electric current in

only one direction. The term usually refers to a semiconductor diode, the most common type

today. This is a crystalline piece of  semiconductor material connected to two electrical terminals.A vacuum tube diode (now little used except in some high-power technologies) is a vacuum

tube with two electrodes: a plate and a cathode. 

The most common function of a diode is to allow an electric current to pass in one direction

(called the diode's forward bias direction) while blocking current in the opposite direction (the

reverse direction). Thus, the diode can be thought of as an electronic version of a  check valve. This unidirectional behavior is called rectification, and is used to convert alternating current to

direct current, and to extract modulation from radio signals in radio receivers.

However, diodes can have more complicated behavior than this simple on-off action. This is due

to their complex non-linear electrical characteristics, which can be tailored by varying theconstruction of their P-N junction. These are exploited in special purpose diodes that performmany different functions. For example, specialized diodes are used to regulate voltage (Zener

diodes), to electronically tune radio and TV receivers (varactor diodes), to generate radio

frequency oscillations (tunnel diodes), and to produce light (light emitting diodes). Tunnel diodesexhibit negative resistance, which makes them useful in some types of circuits.

Current – voltage characteristic

A semiconductor diode’s behavior in a circuit is given by its current – voltage characteristic, orI – V graph (see graph below). The shape of the curve is determined by the transport of chargecarriers through the so-called depletion layer  or depletion region that exists at the p-n junction 

between differing semiconductors. When a p-n junction is first created, conduction band

(mobile) electrons from the N-doped region diffuse into the P-doped region where there is a

large population of holes (vacant places for electrons) with which the electrons ―recombine‖.When a mobile electron recombines with a hole, both hole and electron vanish, leaving behind

an immobile positively charged donor (dopant) on the N-side and negatively charged acceptor

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(dopant) on the P-side. The region around the p-n junction becomes depleted of  charge carriers 

and thus behaves as an insulator. 

However, the width of the depletion region (called the depletion width) cannot grow without

limit. For each electron-hole pair that recombines, a positively charged dopant ion is left behind

in the N-doped region, and a negatively charged dopant ion is left behind in the P-doped region.As recombination proceeds more ions are created, an increasing electric field develops through

the depletion zone which acts to slow and then finally stop recombination. At this point, there is

a ―built-in‖ potential across the depletion zone. 

If an external voltage is placed across the diode with the same polarity as the built-in potential,the depletion zone continues to act as an insulator, preventing any significant electric current

flow (unless electron/hole pairs are actively being created in the junction by, for instance, light.

see photodiode). This is the reverse bias phenomenon. However, if the polarity of the external

voltage opposes the built-in potential, recombination can once again proceed, resulting insubstantial electric current through the p-n junction (i.e. substantial numbers of electrons and

holes recombine at the junction). For silicon diodes, the built-in potential is approximately 0.7 V(0.3 V for Germanium and 0.2 V for Schottky). Thus, if an external current is passed through thediode, about 0.7 V will be developed across the diode such that the P-doped region is positive

with respect to the N-doped region and the diode is said to be ―turned on‖ as it has a  forward 

bias. 

A diode’s ' I  – V characteristic' can be approximated by four regions of operation.

Figure : I – V characteristics of a P-N junction diode (not to scale).

At very large reverse bias, beyond the peak inverse voltage or PIV, a process called reverse

breakdown occurs which causes a large increase in current (i.e. a large number of electrons andholes are created at, and move away from the pn junction) that usually damages the device

permanently. The avalanche diode is deliberately designed for use in the avalanche region. In the

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zener diode, the concept of PIV is not applicable. A zener diode contains a heavily doped p-n

 junction allowing electrons to tunnel from the valence band of the p-type material to theconduction band of the n-type material, such that the reverse voltage is ―clamped‖ to a knownvalue (called the zener voltage), and avalanche does not occur. Both devices, however, do have a

limit to the maximum current and power in the clamped reverse voltage region. Also, following

the end of forward conduction in any diode, there is reverse current for a short time. The devicedoes not attain its full blocking capability until the reverse current ceases.

The second region, at reverse biases more positive than the PIV, has only a very small reverse

saturation current. In the reverse bias region for a normal P-N rectifier diode, the current through

the device is very low (in the µA range). However, this is temperature dependent, and atsuffiently high temperatures, a substantial amount of reverse current can be observed (mA or

more).

The third region is forward but small bias, where only a small forward current is conducted.

As the potential difference is increased above an arbitrarily defined ―cut-in voltage‖ or ―on-voltage‖ or ―diode forward voltage drop (Vd)‖, the diode current becomes appreciable (the level

of current considered ―appreciable‖ and the value of cut-in voltage depends on the application),

and the diode presents a very low resistance. The current – voltage curve is exponential. In a

normal silicon diode at rated currents, the arbitrary ―cut-in‖ voltage is defined as 0.6 to 0.7 volts. The value is different for other diode types —  Schottky diodes can be rated as low as 0.2 V,

Germanium diodes 0.25-0.3 V, and red or blue light-emitting diodes (LEDs) can have values of 

1.4 V and 4.0 V respectively.

I. Diode and Diode Application1.  Diodes

2.  Rectification

3.  Power supply filter

4.  Applications of diodes

  Signal rectifier

  Diode gate

  Diode clamps

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Diodes can perform switching and digital logic operations. 

Logic Gates

Logic gates are devices that can combine multiple inputs at independent logic

levels and come up with an output accordingly. There are many kinds of logic gate,and the distinction lies in that each kind processes the inputs differently, and may

give different ouputs for the same inputs.

The way the logic gate processes different inputs is given in a truth table for that

gate, which lists all the possible combinations of inputs next to their outputs. Anexample is given for a simple one-input gate with the function of giving the

opposite logic level at the output to the one at the input. The inputs are given on the

left, and the outputs are on the right. Generally, the inputs are called A, B, C, etc., and the output

is labelled Q. In this case, there are only two possible inputs, 1 or 0, but logic gates can have anynumber of inputs.

Logic

Diodes can perform digital logic functions: AND, and OR. Diode logic was used in early digital

computers. It only finds limited application today. Sometimes it is convenient to fashion a single

logic gate from a few diodes.

 Diode AND gate 

An AND gate is shown in Figure above. Logic gates have inputs and an output (Y) which is afunction of the inputs. The inputs to the gate are high (logic 1), say 10 V, or low, 0 V (logic 0).

In the figure, the logic levels are generated by switches. If a switch is up, the input is effectively

high (1). If the switch is down, it connects a diode cathode to ground, which is low (0). Theoutput depends on the combination of inputs at A and B. The inputs and output are customarily

recorded in a ―truth table‖ at (c) to describe the logic of a gate. At (a) all inputs are high (1). Thisis recorded in the last line of the truth table at (c). The output, Y, is high (1) due to the V

+on the

top of the resistor. It is unaffected by open switches. At (b) switch A pulls the cathode of theconnected diode low, pulling output Y low (0.7 V). This is recorded in the third line of the truth

table. The second line of the truth table describes the output with the switches reversed from (b).

Switch B pulls the diode and output low. The first line of the truth table recordes the Output=0

Input Output 

A  Q  

0  1 

1  0 

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for both input low (0). The truth table describes a logical AND function. Summary: both inputs

A and B high yields a high (1) out.

A two input OR gate composed of a pair of diodes is shown in Figure below. If both inputs are

logic low at (a) as simulated by both switches ―downward,‖ the output Y is pulled low by the

resistor. This logic zero is recorded in the first line of the truth table at (c). If one of the inputs ishigh as at (b), or the other input is high, or both inputs high, the diode(s) conduct(s), pulling the

output Y high. These results are reordered in the second through fourth lines of the truth table.

Summary: any input ―high‖ is a high out at Y.

OR gate: (a) First line, truth table (TT). (b) Third line TT. (d) Logical OR of power line supply

and back-up battery. 

A backup battery may be OR-wired with a line operated DC power supply in Figure above (d) to

power a load, even during a power failure. With AC power present, the line supply powers the

load, assuming that it is a higher voltage than the battery. In the event of a power failure, the linesupply voltage drops to 0 V; the battery powers the load. The diodes must be in series with the

power sources to prevent a failed line supply from draining the battery, and to prevent it fromover charging the battery when line power is available. Does your PC computer retain its BIOS

setting when powered off? Does your VCR (video cassette recorder) retain the clock setting after

a power failure? (PC Yes, old VCR no, new VCR yes.)

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AIM: Application of diode as gates

MATERIALS REQUIRED:

1.  Resistance 470ῼ 2 in no.

2.  Si diodes 4 in no.

3.  PCB

CIRCUIT DIAGRAM:

THEORY AND PROCEDURE: 

To the left (above) you see a basic Diode Logic OR gate. We'll assume that a logic 1 is

represented by +5 volts, and a logic 0 is represented by ground, or zero volts. In this figure, if 

both inputs are left unconnected or are both at logic 0, output Z will also be held at zero volts bythe resistor, and will thus be a logic 0 as well. However, if either input is raised to +5 volts, its

diode will become forward biased and will therefore conduct. This in turn will force the outputup to logic 1. If both inputs are logic 1, the output will still be logic 1. Hence, this gate correctly

performs a logical OR function.

"To the right (above) is the equivalent AND gate. We use the same logic levels, but the diodesare reversed and the resistor is set to pull the output voltage up to a logic 1 state. For this

example, +V = +5 volts, although other voltages can just as easily be used. Now, if both inputs

are unconnected or if they are both at logic 1, output Z will be at logic 1. If either input is

grounded (logic 0), that diode will conduct and will pull the output down to logic 0 as well. Both

inputs must be logic 1 in order for the output to be logic 1, so this circuit performs the logicalAND function.

RESULT: Diode application as gates was verified successfully.