# Forward And Reverse BIAS Diodes Computer Science Essay

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A diode is simply the main building block of semiconductors. Diodes allow electricity to flow in only one direction. The arrow of the circuit symbol shows the direction in which the current can flow. When forward biased the diode acts as a short circuit.Â  When reverse biased, it acts as an open circuit

Circuit symbol Physical symbol

There are two sides to a diode: the anode, the positive side and the cathode, the negative side.

## Forward bias

When the positive and negative terminal of the battery connect to the N-type the force exert on the depletion layer, when the depletion disappear then current starts flowing.Forward bias - all current, almost no volts.

## Reverse bias

When the positive terminal of the battery connects to N-type and negative terminal of the battery connects to P-type, then no force exert on the depletion layer. Distance starts increasing and no current flows. Reverse bias - all volts, almost no current.

N-type - Is when there is more negative charges then positive charges.

P-type - Is the opposite of N-type, is when there are more positive charges then negative.

PN junction - it is when the P-type and N-type materials are crossing over the centre.

Conductor - it allows current and electrons to flow. e.g. metal

It has a electron free in both types so its and conductor.

Insulators - it will not allow current and electrons to flow and does not have any electrons free.

It has no electron free and has a strong bond so this is a insulator.

Semiconductor - It under certain conditions it behaves like insulator or conductor. In a p-n diode, conventional current can flow from the p-type side (the anode) to the n-type side (the cathode), but cannot flow in the opposite direction.

The PN junction

The area between two regions in a semiconductor which have been treated so that one is a P-type semiconductor and the other is an N-type semiconductor.

## Resistor

A resistor is an passive electrical component which controls the flow of current. There are two types of resistor one is called fixed resistor and

the other is called variable resistor. The resistance of an resistor is measured in ohms and it can be measured by colour coding band and measurement.

This is an symbol for resistor

## The unit for resistance:

An ohmmeter (ohm) is used to measure resistance.

u Ohm = 1/1000000 micro

m Ohm = 1/1000

k Ohm = 1000 kilo

M Ohm = million (mega)

## Variable Resistor

The variable resistor can be set at any value between its limits. A variable resistor is a potentiometer (pot) with only two connecting wires instead of three. The pot allows us to control the potential passed through a circuit. The variable resistor allows the resistance to be adjusted in the circuit between two points in a circuit and it can be set at any points between its limits.

Voltmeter: A voltmeter is an instrument used for measuring the voltage, between two points in an electrical or electronic circuit. Some voltmeters are intended for use in direct current (DC) circuits; others are designed for alternating current (AC) circuits.

Physical symbol circuit Symbol

Ammeter: An ammeter is a measuring instrument used to measure the flow of electric current in a circuit. Electric currents are measured in amperes

Symbol for ammeter

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Battery - Supplies electrical energy. A battery is more than one cell. The larger terminal on the left is positive (+) and smaller terminal on the right is negative (-).

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## Transistor

A transistor is a device, which is used to amply voltage, current or sometimes as an on/off switch. The transistor acts as a simple electronic switch, either preventing or allowing current to flow through. When a transistor is saturated the collector current is strong-minded by the supply voltage and the outer resistance in the collector circuit,

## DC biasing of transistor

DC biasing of transistor - use resistors and a battery to make transistor forward bias for both junction is called DC biasing transistor

## Base Bias:

The simplest transistor biasing circuit is referred to as base bias (or fixed bias). The name stems from the fact that the current through the base circuit remains relatively fixed from one transistor to another.

## Emitter Bias:

Emitter bias consists of a dual-polarity power supply and a grounded base resistor. The name emitter bias stems from the fact that a dc power supply is used to set the value.

## Feedback-Bias Circuits:

The term feedback is used to describe a circuit configuration. There are two types of feedback that are used to bias transistors: collector-feedback bias and emitter-feedback bias. Collector-feedback bias is designed so that the collector voltage directly affects the value of.

Types of standard transistors

The above symbols are the two type's standard transistors, NPN and PNP, they have different circuit symbols but they look similar. NPN is most used today because they are easiest type to make silicon. The letters refer to semiconductors are used.

Transistor circuit symbols

Base (B) = it controls the flow of charge particle

Collector (C) = - it collects the charge particle.

Emitter (E) = it emits charge particle

The function of NPN

The base-emitter junction behaves like a diode. A base current flows only when the voltage across the base-emitter junction is 0.7V or more. The small base current controls the large collector current. The collector-emitter resistance is controlled by the base current.

1) BJT (bipolar junction transistor - This transistor current flow due to both positive and negative charge particle.

2) FET (Field Effect Transistor) - This transistor allows current to flow due to positive or negative charge particle.

## Construction of transistors

This is a P-N junction.

It consists of - two P-N junctions transistor back to back like this:

The arrows are there to represent the conventional flowing of current from positive to negative. Some semiconductors have an excess of electrons, n-type material that is due to doping of the semiconductors, while others have a deficiency of electrons called holes, they are found in P-type materials.

## Digital logic experiment

NOT gate - The NOT gate can have only 1 input. The (o) on the diagram below means 'not'. The out is opposite of input. So the output is 1 when input is 0. A NOT gate is also called an inverter.

## Boolean equation: A` (input is A and output

Truth table

A (input)

X (output)

0

1

1

0

OR Gate - The OR gate does not have less than two inputs but it can have more inputs. The output is 1 when at least one input is 1.

## Circuit symbol of OR gate

Physical symbol of OR gate

## Boolean equation: X = A+B (input is A&B and output is x)

Truth table

A (input)

B (input)

X (output)

0

0

0

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1

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1

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1

NOR gate - A NOR gate can have two or more inputs. The 'o' on the diagram means 'not' showing that it is a Not OR gate. The output is 1 when none of its inputs is 1.

## Boolean equation: X = (A+B)` (input is A&B and output is x)

Truth table

A (input)

B (input)

X (output)

0

0

1

0

1

0

1

0

0

1

1

0

AND gate - An AND gate can have two or more inputs it can't have one inputs. The output is 1 when all its inputs are 1.

## Circuit symbol of AND gate

Physical symbol of AND gate

## Boolean equation: X = A.B (input is A&B and output is x, '.' Means times)

Truth table

A (input)

B (input)

X (output)

0

0

0

0

1

1

1

0

1

1

1

1

NAND gate - A NAND gate can have two or more inputs. The 'o' on the output means 'not' showing that it is a Not AND gate. The output is 1 if not all its inputs are 1.

Truth table

A (input)

B (input)

X (output)

0

0

1

0

1

1

1

0

1

1

1

0

## The truth table

The truth table describes a logic function by the output respond with various input combinations. The table uses 1 & 0 only, there is no other numbers. The input columns are usually put in the binary order.

## The Boolean equation

The Boolean is algebra which consists of values 1 & 0 with three operations OR, NOT and AND gates. It describes how to determine Boolean value outputs and inputs.

Circuit 1

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Circuit 2

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Get all the equipments together

Put all components together in the breadboard

Connect it to the power

Switch the power on

Put voltage on 9 volts

use the keys A, B and c to work out (0 or 1)

Check with the truth table for correction.

Do the same method for both circuits

Turn off power

The Boolean equation for circuit 1

X = (A+B) + (C+B)

## X

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The Boolean equations for circuit 2

X = (A+B) + (C+B)

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## Multisim method

open up multisim

Put grids on the page

Take out all the component

Put source on 9v

Connect all the component together

Put ground on

Run it or put the power on

â€¦.by using the A,B and C to turn the led on

check the results with the truth table

Turn off power

## X

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The Boolean equations for circuit 2

X = (A+B) + (C+B)

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## Comparison

I have compared both of the experiments, breadboard and multisim for the experiment digital logic gate. The experiments have very similar results. They kind of don't have any differences. I used same equipments for both experiment but I think that multisim is more accurate because it was done in the computer.

The advantages of doing the experiment on the computer using software called multisim are: that it is easier and faster, if you make a mistake or you have damaged a components you can get another one easily and also there will be no human errors because you are doing the experiment on the computer and the technology of the program multisim will be accurate, But on the hand there are a few disadvantages the radiation of the screen can damage your eye sight.

The advantages of doing the experiment on the breadboard are that you can physically feel the components and sometime when you are doing the on yourself you will understand more because you will be physically doing it.

## Learning out come:

In this experiment I learnt how logic gates works and at the end of the course work I could explain logic gate. I also learnt some of the gates such as OR gate, AND gate and NOR gate. I could explain what the function of each one is. If I wanted to improve my course work I will write more about logic gates and try doing more research and add some good information.

## Conclusion

In this experiment I did two different tests one was on breadboard which was a bit hard and confusing putting all the components together but I work out well at end. Then I checked the results if it's correct. The multisim was easy but I did struggle to find some of the component on the software, after I had the component in place I put them together which was really easy and fun. Overall I think this experiment I was a really good experience for me because I learnt a lot of new things.

## Operational amplifier:

An operational amplifier is usually said as an op-amp. It's a DC coupled high-gain electronic voltage amplifier, with deferential inputs but it's usually a single output controlled by negative feedback which determines magnitudes of its output voltage gain, and has high input impedance, low output impedance. Used with split supply, usually +/- 15V.

The most common and most famous op-amp is the mA741C or just 741, which is packaged in an 8-pin mini-DIP. The integrated circuit contains 20 transistors and 11 resistors

## Pin descriptions:

Cexta - External Capacitor

IN - - Op-Amp Negative Input

In + - Op-Amp Positive Input

V - - Negative Supply

Cretn - Capacitor Return Path

OUTPUT - Op-Amp Output

V + - Positive Supply

Cextb - External Capacitor

## Transistor

A transistor is a device, which is used to amply voltage, current or sometimes as an on/off switch. The transistor acts as a simple electronic switch, either preventing or allowing current to flow through. When a transistor is saturated the collector current is strong-minded by the supply voltage and the outer resistance in the collector circuit,

Circuit symbol physical symbol

## LDR (light dependent resistor):

LDR are used in light-measuring or light-sensing instruments where light is converted to a digital signal and in switches

## Cell:

The cell supplies electrical energy. The larger terminal (on the left) is positive (+). A single cell is often called a battery, but strictly a battery is two or more cells joined together.

Circuit symbol physical symbol

## Buzzer:

A buzzer is electrical device which converts electrical energy to sound.

Circuit symbol physical symbol

## Diode:

The diode is a device which only allows current to flow in one direction.

Circuit symbol Physical symbol

## Switch:

SPST = Single Pole, Single Throw.

An on-off switch allows current to flow only when it is in the closed (on) position.

circuit symbol physical symbol

## Relay

An electrically operated switch, for example a 9V battery circuit connected to the coil can switch a 230V AC mains circuit.

NOÂ =Â NormallyÂ Open, COMÂ =Â Common, NCÂ =Â Normally Closed.

circuit symbol physical symbol